<148>

Dialogue the Third.

Several Particulars relating to Vision, Discoveries in Optics, and a Confutation of the Cartesian System.

THE Marchioness felt the utmost impatience to be more learned than Huygens. She was not for losing a Moment's Time, but would have continued our Discourses upon Vision the next Morning as soon as ever we were up. I told her that we must prepare ourselves with somewhat more Ceremony for so high a Degree of Knowledge, and that it well deserved that e should at least wait till after Dinner. In the Interim not to lose Time, she might learn how it comes to pass that the Eye sees the Objects which are without it, but cannot at all see itself. And <149> from hence she might form a clearer Idea of certain Verses which no doubt must have been often addressed to her.

This was all the Optics she could get from me in the Morning. She waited for the Afternoon with as much Impatience as an Initiate full of the Expectation of being let into some profound Mystery. In order to understand why we see only one Object with two Eyes, would you be pleased, Madam, (said I) if I should tell you that in Reality we see only with one Eye, and that the other remains idle and at rest? You had better, said she, at once reduce us to a single Eye, and then you will have no farther Difficulty. I should be as well pleased if you was to say that we walk only with one Leg. You are more moderate, answered I, than a Latin Poet, who says that Woman would be as ill-pleased to have but one Lover, as but one Eye. But this strange Explication however was given by a grave Philosopher, and resembles the Pride of the Chinese, (who fancy that all Nations except their own see with but one <150> Eye) since both the one and the other are equally the Offspring of Ignorance. But this may serve to let you see what Difficulties there are in solving the Question we are now upon. Another made the Optic Nerves to resemble two Lutes composed of various Strings, which have an exact Correspondence with each other, so that the two Images of the Object falling upon Strings wound up of an equal Height or a Unison, the Object must appear single; but all these fine and ingenious Explications will not make you a better Philosopher than Huygens. I believe that the true Explanation of this difficult Phænomenon, like many others in Vision, depends upon Experiments. The Sense of Feeling and Sight lend each other a mutual Assistance in the Formation of our Ideas, just as our Eyes and Ears help each other when we learn a new Language. The Sense of feeling, which is must stronger than the Sight, has constantly informed us that in the ordinary way of seeing the Object is but one, and by a long Habitude we join the <151> Idea of one single Object with the two Sensations of it. In the same Manner an Object that is felt with two Hands or two Fingers at a Time, notwithstanding the two Sensations which we have of it, seems to be only one, and this is occasioned by those other Ideas which we had conceived of it when we touched it only with one Hand or one Finger. If a Button or a Ball of Wax be pressed with two Fingers at a Time in an unusual Manner, by crossing the Fingers together, it will appear double, just as Objects do when we squint upon them. In both Cases the antecedent Ideas of feeling are not so strongly united by a long Habitude with these unusual Sensations, as to make us join them with the Idea of one single Object.

Do you believe then (said the Marchioness with an Air of Surprize) that if a Person had been long accustomed to press a Button with two Fingers crossed together, he would no longer feel it double? No certainly, answered I, for the very same Reason that Ob <152> jects do not appear double to Persons who are naturally squint-eyed. These by a long use acquire the same Habit in their manner of seeing as we do in ours. We have a very singular and curious Observation to this Purpose, upon a Person who by some Misfortune had distorted and dislocated one of his Eyes. At first all Objects appeared double to him, till at Length by little and little those which were the most familiar to him, (that is, those which he had the most Experience of by feeling,) became single, and in Time all the rest, though the Dislocation still continued. I may venture to affirm that by Virtue of this Experience, Argos with all his hundred Eyes did not see the fine Heifer committed to his Charge by the jealous Juno, at all more multiplied than Polipheme did his Galatea with only one Eye.

You seem to exult mightily, said the Marchioness, in this Experience which we gain from our Touch, will it give you Confidence enough to undertake by its Assistance the Solution of that <153> Question I proposed to you yesterday: How it comes to pass that Objects, which are drawn inverted upon the Eye, appear direct in the Mind? These Experiments, answered I, extend farther than you perhaps imagine, and the Ideas of Sight considered with Regard to those which we receive from Touch, are no more than four Strokes of a Pen compared to a fine Relief. We have the Example of a Statuary, who tho' he was blind, yet by the Help of his Feeling made tolerably good Likenesses: One of the greatest Mathematicians in England, (that Land of Phænomena) and who could give you a much better Explanation of Optics than I can, was yet deprived of his Sight so young, that he may be affirmed to have been born blind. This Person is certainly much more wonderful than that learned Frenchman, who without either Voice or Ear, undertook to learn Music, and very much improved the greatest and most curious Mysteries in that Art. Feeling furnishes the Imagination of this great Philosopher with much clearer <154> and more distinct Ideas, than others receive from Sight. What a Pleasure would he have in making use of your fine Fingers to explain the converging and diverging of the Rays? On the other Hand what could we learn, or what could we do without our Feeling? We should be incapable to judge of the Situation, Distance or Figure of Objects, as Berkley had prophesied, who perhaps considered the Metaphysics of vision more than any one ever did; and Experience has verified this in some Persons, who, after being cured of Cataracts born with them, could not form any Judgment till the Touch lent them its Assistance; without this our Eyes would continually tantalize us with a View of Knowledge and Pleasure which we could never enjoy. The daily Experiments then, that we make with our Feeling, inform us that Objects are direct, (in the same Manner as they teach us they are single) that they are placed in certain Situations, at certain Distances, and of certain Figures. I believe Des Cartes is the only one who <155> ever pretended to give an immediate Explication of this difficult Phænomenon by a Similitude. Suppose yourself, said he, to have two Sticks across each other, one in your Right, and the other in the Left-hand, and to walk with your Eyes shut about a Room with these Sticks before you. There is no Question but you will think those Things which you touch with the Stick in your Right-hand, and which by this Means will make a Pressure upon that Hand, to be on your Left; and in like manner whatever you touch with the other Stick in your Left-hand, you will affirm to be on the Right. After the same manner the Rays which proceed from Objects to the bottom of the Eye, crossing each other in the Crystalline Humour, those which press the Retina on the right Side will make you refer those Points from whence they proceed to the left Side, and those Rays which press it on the Left you will refer to the right Side; and thus those Rays which press upon the upper Parts of the Retina, will <156> make you refer the Points from whence they proceed to the lower Part, and those of the lower to the upper. And by this Means the Image, which is inverted upon the Retina, makes you see the Objects direct.

Indeed, said the Marchioness, this is very ingenious; why may not we confine ourselves to this without seeking any other, since it gives us an immediate Explication of our Phænomenon? Experience, answered I, unhappily informs us, that this is not an ingenious Explication: A Boy who stands upon his Head sees every Thing inverted, notwithstanding the Images of external Objects are pictures upon his Retina, in the very same manner when he is in this Situation as they are when he stands upon his Feet. He has no other Idea of high and low, than what regards his own Situation; and when he is in this inverted Position, he imagines the whole Universe to be so too. Besides, the Explication of Des Cartes supposes the Ideas of high and low, Right and Left, which we can only have from Feeling, to be antecedent.

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It is our Feeling, which by an Experiment every Moment repeated has constantly taught us to call the Earth, towards which we find ourselves continually carried by Gravity, low, those Things which are contagious to the Earth, as the Pedestal of a Pillar or our Feet, below, and those Things which are distant from the Earth above, as our Head or the Top of a Tree. The Sense of Feeling conveys these and the like Ideas into the Soul of a Man born blind, with as much Exactness as that of Sight does the Ideas of Colour into ours. Now, if we suppose that Vail which hides the visible World from his Sight, to be taken away all at once, and consider in what manner he would judge of the Situations of Objects, we might from thence arrive to a clear Knowledge of the Manner in which we ourselves judge of them, since we have the Ideas of high and low in common with him; he would certain be more surprized at first opening his Eyes, than the famous[1] Epi <158> menides was after his long Sleep of so many Years, who remembered nothing which he saw about him when he waked, not even the Place where he had been born and brought up. A new Scene of Ideas displays itself to him, a Torrent of new Perceptions rush upon him through this new Avenue by which Objects enter the Soul. Amazed and overwhelmed with them, he finds himself transported, without knowing how, into another World. What a Pleasure, what an Extasy must this be! said the Marchioness? If Novelty which always hovers about these Things of which we <159> have an Idea, and is nothing more than an unusual Combination of those Objects we are already acquainted with, affords so much Delight, what an infinitely greater Pleasure must this Man have in a World of Things really new, and a new Combination of those Ideas which he before had joined to those with which the Addition of another Sense abundantly Supplies him. But as human Happiness is too often attended with some Allay, is it not possible for him to see something that would make him with his Eyes shut again when they were hardly open? He would have great Reason to believe, that those Objects would appear pleasing to this new Sense, which had been so to the rest, and flatter this as agreeably as they had done those. But might not the Event happen quite contrary to his Expectation? And those Objects which had delighted his Touch and Hearing, prove very disagreeable to his Sight? So that instead of increasing the Number of his Pleasures, this new Sense would deprive him of the most <160> sensible of them, and perhaps cruelly dissolve some pleasing Tye which the others had closely bound.

It is too true, answered I, that these Senses do not always agree: How many Persons do we daily see who, convinced by sorrowful Experience, that the Reality of Things does not agree with their outward Appearance, find themselves too soon in Possession of what they once thought they could never obtain soon enough? A blind Man, answered the Marchioness, at least while he is in Love, should never desire to see; satisfied in the Judgment of those Senses which represent an Object agreeable to him, why should he ask that of another which perhaps would at once condemn his Choice, and, like, Reason, probably make him see his Misfortune without giving him any Assistance to avoid it? The only Consolation, answered I, that this miserable Person could have under the Misfortune of Sight is, that he would not be unhappy so soon as you perhaps imagine. How? said she, if the Joy of receiving <161> his Sight did not entirely deprive him of all Complaisance, would not the very first Thing he desired to see be that Person for whose Sake he principally wished for the use of his Eyes? And when he had seen her, he would immediately, if she should appear disagreeable, perceive his Unhappiness, if, with regard to Beauty, Love did not render him a second Time blind. He would enquire for her, answered I, would see her, and yet would not by that Means know her again. This would be a Miracle even beyond the Power of Love to effect, he would hear, if you will, the sound of those Words pleasing to his Ear, and still more pleasing to his Heart, but he would not know the Mouth from whence they proceeded. Yet more, he would be so far from distinguishing another, that he would not distinguish himself, his own Feet or Hands from any Assistance his new Sight could give him, since he must be utterly ignorant of the Connexion which the Perceptions of Sight have with those of the Touch, <162> yet this Connexion is absolutely necessary in order for his knowing those Objects again of which he had no Idea, but what he received from Senses of a Nature very different from that of Sight, and depends upon an Experiment which he has never yet made. His own Hands would be the first Object, he would learn to distinguish by touching and looking upon them at the same Time, and remembring that such an Idea of his Feeling, would agree with such another of his Sight. When he had learnt this short Lesson, Love would the more easily conduct him to those Experiments that would soon either to his Joy or Sorrow satisfy his Curiosity, and we will conduct him to those that may content ours, which is of a more philosophical Nature.

One of the first he would try, would be to life up that Hand which he has now no Trouble to distinguish, and in doing this, he would perceive some change in the Sensation which he received of it from his Sight, and the <163> Reason of this Change is because the Image changes its Situation in the Retina, in Proportion as the Hand is higher or lower. Guided by Nature herself, he would diligently observe what sort of Sensation he felt when he held his Hand up, and whenever he felt the same Sensation raised in him, either by the same or any other Objects whose Image would fall in the same Situation of the Retina, though unknown to him, he would conclude that Object to be high, or in that Situation in which his Hand at first was. By this Means connecting the old Ideas of Feeling, with the new ones of Sight, he judges of the Height or Lowness of the Object, of its being direct or inverted, and it is of no Importance whether the Image of this Object be pictured upon the Retina direct or inverted, or in any other Position.

External Objects are signified to him (if I may use that Expression) by certain Sensations of Light and Colours, as the Thoughts of the Soul are to <164> us by certain Characters, not by Virtue of any Resemblance between the one and the other, but by Means of an arbitrary, yet constant and perpetual, Connexion which we have observed between them. And as when we are accustomed to any particular manner of writing, it makes no Change in the Order of the Ideas which these Characters excite in us, whether they be written from Left to Right as ours are, from Right to Left like the Oriental, or from Top to Bottom after the Chinese Manner; so whether certain Images be drawn direct or inverted upon the Retina, it does not at all change our Judgment of their Situations.

The blind Person who has hitherto safely conducted us through this Labyrinth, resembles each of us. We enter into Light with our Eyes shut, and probably do not begin to see till we have for some Time felt. Thus, Madam, you are indebted to the predominant Sense of Feeling, for this new Explication too, and however little you may <165> imagine it, you will find that to this you have had the greatest Obligations through the whole Course of your Life.

I see very plainly, said she, that you interest your self for the Honour of the Touch, much more than for that of Des Cartes, and it is not possible to propose any Difficulty but you will be ready to give it a Solution by the help of this Sense. There are other Difficulties, answered I, which I will give you a Solution of without this, that you may see I have great Plenty of Explications. One of these perhaps may be to know what Change must be made in the Eye, in order for it to have a distinct View of Objects placed at different Distances. For as in the Camera Obscura, the Rays which flow from Objects that are near unite at a greater Distance from the Lens than the Rays of those which are more remote; so the very same Thing happens in the Eye where the Rays, which proceed from the Pillars of <166> this Gallery, unite at a greater Distance from the Crystalline Humour, than those that come from these Trees which are farther from it, what Change then must be made in the Eye, in order that when we look upon these Trees, after having looked upon the Pillars, the Rays which proceed from them may be united upon the Retina, or in other Words; that we may have a distinct View of them? The Retina, answered she, must be brought nearer the Crystalline Humour, just as the Paper in the Camera Obscura is brought nearer the Lens, in order to give us a distinct Image of the more remote Objects.

You have hit upon the true Explication, answered I, and some have affirmed that certain Muscles which incompass the Eye, are made use of to produce this Effect, and press the Retina farther back from the Crystalline Humour, or bring it more forward, as different Occasions require. These Muscles have besides another <167> Office, and help us to life up or depress the Eye, to turn it to the Right or Left, and give it a certain oblique Motion which Venus has the principal Care of regulating. Others have supposed that the Retina remains immoveable, and that the Crystalline Humour approaches nearer to it, and removes farther off, or that the Crystalline Humour only changes its Figure, growing more convex for near Objects and less so, for those which are distant; and others have affirmed that each of these happens at the same Time, which will produce the same Effect as if the Retina approached to or receded from the Crystalline Humour; but you had better suppose this latter, because it is easiest to imagine. For every Distance then, the is required a new Conformation in the Eye, and because this cannot be effected without Motion and a certain Effort, some are of Opinion that the Eye informs us of the various Distances of Objects by a certain natural Geometry. But this way of judging, <168> especially where very distant Objects are concerned, is extremely uncertain, as indeed almost all the rest are which have ever appeared upon the Stage of Philosophy.

But however this be, there are some Persons who cannot bring their Retina so near to the Crystalline Humour as is necessary in order to give them a distinct View of distant Objects. There are others who, on the contrary, cannot press it back far enough to make Objects which are near appear distinct. The first of these, who are vulgarly called Short-sighted, are by Opticians named Myopes, and the second, commonly called Long-sighted, Presbitæ; these may be considered as the Extremes betwixt which that Sight, which is called just and perfect, stands. Notwithstanding the Honour of being distinguished by Names drawn from Greek, these Persons could not help perceiving they had a Defect in their Eyes, which led them to seek for some Remedy. The Myopes, in order to have a distinct View of distant <169> Objects, and the Presbytæ of those at Hand. These last found Relief in Convex-glasses, which applied to the Eyes, removed their Defect; for these making those Rays to become converging, which would without their Assistance have fell diverging upon the Crystalline Humour, unite them at a less Distance that they would otherwise have done, and the image of those Objects which are opposite to the Lens is drawn distinctly upon the Retina The Myopes found their Remedy in Concave glasses which disperse the Rays and make them diverge; contrary to the Convex ones which make the diverging Rays become converging. These Concave-glasses then give a Disposition to the Rays as if they came from a nearer Object than they really do, and applied to the Eye of a Myop, they in a certain Manner transport the distant Object to a nearer Situation, and thus a distinct Image of it is drawn upon his Retina; for there is nothing more required to give Short-sighted Persons a distinct <170> View of any Thing, than to bring it nearer their Eye.

It was very happy for them, replied she, to find these Glasses as safe and easy a Remedy perhaps as ever the Art of Physic itself invented. But what did they do before they were found out? Till the thirteenth Century (answered I) when these Glasses are supposed to have been invented, the Myopes were obliged to approach near to distant Objects in order to see them distinctly, hoping perhaps that an advanced Age (when according to the common Opinion the Retina approaches nearer to the Crystalline Humour) might give them some Relief from this Inconvenience; but indeed this is a Remedy much worse than the Disease. The Presbytæ on the other Hand were obliged to remove a great way off, without any hope of ever seeing the Objects situated near them distinctly, if they should at any Time have a Curiosity for it, and they continually tormented their Eyes with Plaisters and Collyria without <171> gaining the least Advantage to their Sight. I think the Condition of these last, said the Marchioness, much more to be pitied than that of the first, both because they cannot flatter themselves with the least Hope of growing better, and because they lose a great deal more in the Conversation of Ladies than the Myopes do. How miserable must have been the State of a poor Presbyta, who could never have a distinct View of his Mistress, unless he breathed his Sighs ten Foot distant from her!

These are not so much to pitied, answered I, as you imagine, since this Defect generally happens in an Age when Hope and the Conversation of Ladies naturally desert us. For this is a Defect of old Men, as the very Word Presbyta imports. But there are other Defects and Infirmities of the Sight incident to all Ages, which are never placed in the Number of Defects, for the very same Reason that the Folly of being more solicitous for the future than the present, and by that Means conti <172> nually procrastinating our Happiness is never reckoned among the List of Follies, namely, because it is common and universal. Philosophers, who have a quicker Perception of Things, have discovered and sought Remedies for these Defects, one of which is, that very small Objects are invisible, however near they may be placed to the Eye, the other, that very distant Objects cannot be discerned, even if they are of a very great Magnitude. These are Inconveniences which you see are not at all felt by the Vulgar, but reserved for the Curiosity and nice Perception of Philosophers. The Reason of both Defects is, because the Image of very small or very distant Objects, painted upon the Retina, is not big enough to be perceived by the Eye, notwithstanding any Proximity of the one, or extraordinary Magnitude of the other. The Remedies invented by Philosophers to supply them are certain Instruments, whose only Business is to magnify this Image and render it <173> perceptible to the Eye, by Means of various Combinations of Glasses, or by one alone. Those which are made use of to discern distant Objects, are called Telescopes, and those which help us to a Sight of exceeding small Objects, Microscopes. We are obliged to each of these for an infinite Number of Discoveries, which could never have been made without them. The Heavens are the principal Object of the Telescope, from whence it has furnished Philosophers with more Curiosities than ever Columbus could bring from America to inrich the Cabinets of the Naturalists. For not to speak of the Hills and Vallies which they have discovered in the Moon, the Satellites of Jupiter so very useful to Geography, those of Saturn with his Ring, it is to those Telescopes that we owe our Discovery of the Spots in the Sun, Jupiter and Mars, which are necessary in order to determine the Periods of their Revolution round themselves, and by the Assistance of these Astronomers have given us so exact a <174> Map of Venus, that they are as well acquainted with her Mountains in Heaven as the Geographers are with those upon Earth. They have discovered that this Planet has an Increase and Decrease, that it is at one Time half, at another full, in short, that its Appearance and Phases exactly resemble those of the Moon, as it had been conjectured by the celebrated Copernicus before the Invention of the Telescope. These give the Celestial Bodies their proper Distances, and have shewn us an infinite Number of Stars unknown to Antients, discovering so great a Quantity in the Milky-way as is sufficient to supply ten or twelve Worlds besides our own. In short, they have given us a true System of our World by extending its Confines to Infinity, so that if a Poet to flatter a Nation which had made a greater Progress in the Conquest of the World, than in the Knowledge of its Frame, could say that when Jupiter turned his Eyes upon our Earth, he could see nothing in it that was not <175> subject to the Roman Empire, we may affirm with greater truth, that if he looked upon the Heavens, or at least the Solar Vortex, he could not see any Thing in it but what is the Discovery and Conquest of the Telescope.

You represent these Telescopes to me (replied the Marchioness) under such sublime Images, that I am afraid the Microscopes will make but a very insignificant Figure when compared to them. There is, answered I, a very remarkable Difference between them, in which I believe the last have the Advantage. The Telescopes, 'tis true, by discovering to us the Hills and Vallies in the Planets, their Stations, Revolutions round their own Axis, (that is to say, their Day and Night) the Moons that from Time to Time supply the Absence of the Sun; in short, by representing them to be of the same Nature as our Earth, have furnished us with Materials to people these vast and immense Bodies which were before uninhabited, stood neglected in a Corner of the Universe, <176> and were believed to exist for no other End than to please our Eyes. But Microscopes have made us in Reality see an infinite Number of Animals of which we had not the least Knowledge before, in Things which were not looked upon as very proper to afford them a Habitation. Not to say any Thing of the Discoveries in Anatomy and Natural History, which we owe to these Glasses; Aromatic Infusions, a Drop of Vinegar, are peopled by so prodigious a Number of little Animals, that Switzerland and China would appear empty and uninhabited when compared to them. The Microscope, said she, is the Compass of Philosophy; each of these have given their Assistance to the Discovery of new Worlds; the only Difference is, that the Microscope has lent its Art to people, and the Compass to destroy.

It is very wonderful, answered I, into what innumerable animal Worlds Philosophers have penetrated under the Guidance of this Compass. It is an <177> amazing Thing to reflect upon the Minuteness, Art, and Curiosity of the Joynts, Bones, Muscles, Tendons, and Nerves necessary to perform the swift Motions of the smallest microscopical Animals.

These Discoveries shew us in how little a Compass all Art and Curiosity may be comprised, even in a Body less than a small Grain of Sand, and yet as compleat, as exquisitely formed, and as finely adorned as that of the largest Animal. Their Multiplicity is no less surprizing than their extreme Smallness. A Drop of the green Scum upon Water no bigger than a Pin's Head, will contain not fewer than an hundred; which far from being confined in that narrow Extent, play about in it with all the Freedom imaginable. The Eye of a Butter-fly will contain more than twenty-four Millions; and the Wonder is still greater when we come to consider the Organization of their fine and minute Parts. If the Eye of a Fly, which seems to be a <178> little misformed Protuberance, be looked on through a Microscope, it appears to be only a Composition of thousands of little Eyes, just as some nubilous Stars on being viewed with a Telescope appear to be a Cluster of innumerable other Stars. In some Insects there have been counted no less than thirty-four thousand of these little Eyes, which notwithstanding their extreme Smallness had each of them a Crystalline Humour as perfect as ours.

Why are not our Eyes, said the Marchioness, of so fine a Texture? This Question, said I, has been already answered,

For this plain Reason, Man is not a Fly

Say to what Use were finer Optics given,

To inspect a Mite, not comprehend the Heav'n.

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But in Fact there are some Insects, that with these microscopic Eyes can see as far as the greatest Part of Men. The Bees, an industrious Species of Flies, from whose Labour we reap so great an Advantage, can direct themselves safely to their Hives though at a Mile Distance, when they return laden with the sweet Treasures of the Spring. It appears that what Nature has given us in Reason, she has denied us in Exquisiteness of Sense. Pigeons, the Couriers of the East, such as that which brought News from Egypt to Jerusalem when it was besieged of a quick and powerful Succour, or that beautiful one presented by Venus to Anacreon in exchange for one of his Odes, and which, for having so often carried his Letters to Bathyllus, deserved to sleep, and be sung upon that Lyre which could resound nothing but Love; these flying Couriers, I say, being let loose by the Person who has a Mind to send home News of himself, ascend a prodigious Height, and from thence, thought at a very great Distance, can see their native <180> Country, and safely direct their Flight to it without the help of either Stars or Compass. Moles seem to be quite contrary to these sharp sighted Couriers. Nature, said the Marchioness, has perhaps made them amends some other Way. It is probably that she has constituted the Condition of Animals pretty near as equal as that of Men. Their Eyes, answered I, are certainly not to be envied: They are so small and covered with Hair, that it seems as if Nature had given these Tenants of Darkness Eyes to see the Light, for no other End than to fly from it. These Animals are not formed to contemplate the Wonders of the Microscope, nor see in one Drop of Water so many thousands of Animals, organized with all that Exactness which is necessary to enable them to see, move, and nourish both themselves and other little Animals, which repay them the Mischief they do to us, and to contain within them an infinite Number of still other little Animals of their own Species, much less than they, and which only wait to <181> unfold themselves to make their Appearance in the Microscope. These Observations open a Scene of innumerable other Worlds of Animals unknown before, which notwithstanding their extreme and surprizing Smallness, have their greater and less, their Elephants and Ants just as ours has; the only Difference is, that our Ants become Elephants when compared with their largest Animals, or rather are as the immense Distance of Saturn from us, is to the Extent of a Grain of Sand.

Indeed, said the Marchioness, this new Scene of Pigmy Worlds gives me as much Pleasure as that other immense and Gigantic Scene of Vortices or Suns diffused over the whole Universe. The little has it Beauties as well as the great, or rather, answered I, there is no great or little, but with regard to ourselves. Gulliver, who could destroy the Liliputians like so many Fleas, was among the Brobdinagians kept in a Cage like a Canary Bird, or for an Ornament upon the Chimney like a Chinese Pagod. It <182> is principally the Microscope and that infinite Number of Pigmy Worlds discovered by it, which has rectified our Ideas of great and little, so much that I am persuaded, that the Consideration of this incredible and surprizing Smallness, which it has rendered perceptible to our Senses, has served to soften and familiarize to Mankind another Consideration which is the Master-piece of human Understanding, and directly leads us to the Subversion of great and small. This is the Consideration of infinitely small Quantities which has made so great a Noise in the learned World, and which you perhaps may have heard of. The meaning of this Expression is, that there are Parts and Quantities in Extension, so exceedingly small, that they may be reckoned as nothing when compared with our Measures, as the Fathom, Foot, Ounce, and the like. So that if one of these Quantities was added to the Extremity of a Line (for Example of a Foot,) it would not increase the Length of it, nor decrease it if it was to be taken away. And the Mathematicians affirm, that in these <183> Quantities, infinitely small with regard to the ordinary Measures called Differences, there are innumerable Orders and Gradations, so that a Quantity which is infinitely small compared with the Order of our common Measures, is infinitely great when compared with an inferior Order of infinitely small Quantities, and so of the rest. The most enormous Sizes we have may become infinitely small when compared with an Order of Greatness infinitely superior. To how small a Size is reduced the Collossus of Nero, or that of Rhodes when compared to Mount Athos, carved in the Shape of a Man, and holding a City in one Hand, and pouring out a River from the other? Compared to Milton's Satan, Virgil's Fame, the formidable Shade of[2]Camoens, that Polypheme of the <184> Ocean which appeared to the Portuguese Sailors, hid its Head in the Clouds, and its Feet in the unfathomable Abyss of the Sea; or in short, what would all this appear to that Angel seen by Mahomet in his mysterious Night, whose Eyes were seventy thousand Days Journey distant from each other? It is computed that if he were of a human Shape, there must be the Distance of forty thousand Years Journey from his Head to his Feet.

Probably, said the Marchioness, there must be a great Number of Telescopes and Speaking-trumpets in the Turkish Paradise, in order for the Mahometans to be able to see and converse with these diabolically great Angels of theirs.

There are, answered I, the same Orders of Infinities in the Succession of Time, as there are in Extension. An- <185> Hour, a Minute, a Second, are of an infinite Duration compared with Periods of Time infinitely shorter. How enormous must the Duration of the Roman Empire seem to an Animal, which in the Space of five or six Hours is born, grows up, produces one like itself, becomes old and dies? What we should call the Flight of Time, would seem to this Insect an Eternity. But what are these Durations of Empires, this long Succession of Kings, Emperors, Consuls, and these tedious Sieges when compared with Eternity? Is it more than a Point in which we live, fight, raise such great Commotions, and make so much Noise? The Orientals say, there is a God that governs this World, who dies at the End of a hundred thousand Years, and this Space another superior God esteems but as a Minute. And yet all these Examples give us but a very imperfect Idea of Infinity. This Consideration, the utmost Stretch of the human Mind which we owe to Sir Isaac Newton, and which entirely overthrows all the Ideas of absolutely great or little, <186> was the Foundation of the famous Arithmetic of Fluxions, or infinitely small Quantities, which transplanted Geometry into a Province intirely new. Here it made so rapid and great a Progress, that all it had done before seems nothing ,and here by the Assistance of new Discoveries it produced such strange Paradoxes, that they have in some Measure clothed Truth in the agreeably surprizing Dress of Fiction: And what is the most remarkable in the new Geometry is, that by considering the Properties, Relations, and Habitudes between infinitely small Quantities, it arrives at the Discovery of common and finite Measures, which are the Object of our Enquiries.

If the Sagacity which we so much admire, said she, consists principally in uniting those Things in the Mind, and finding their Relation, which seem to be in their own Nature disjoined and separated, what an unlimited Understanding must Sir Isaac Newton have had to find the Relation, and in some Measure unite these Quantities, disjoined <187> and separated from each other by the immense Tracts of Infinity, where the human Imagination quite loses itself. And, continued I, the Consideration of these infinitely small Quantities that we neither see nor can conceive, which appeared only fit to perplex Geometry, have in Fact served to render it more easy, and reduced it at the same Time to such general Rules that the most sublime and abstruse Truths in this Science are at present nothing but one of the infinite Consequences which is lost among the Crowd of those that are deduced from the Stroke of a Pen; and if you please in a circle of Ladies; Truths that once required an Archimedes with all that Attention of Thought which was necessary to make a Person insensible of the Noise of a Town taken by Storm, and be knocked on the Head without perceiving it.

This Consideration then of infinitely small Quantities (said the Marchioness) and the Observations of the Microscope which have rendered it more familiar and common, have given a very strange <188> Turn to Geometry. It now treats of Quantities, which from their extreme Smallness were once utterly unknown, and does not at present disdain to enter in to the Company of Ladies. A Province with which I believe it was once as little acquainted, as with that of the infinitely small Quantities. It is true, answered I, that Geometry is rendered so very familiar, as sometimes to suffer itself to be treated by a Hand as beauteous as that of Venus of Medicis: But 'tis true likewise, that it sometimes resumes its fierce and savage Disposition, especially when it is attended by that Train of Consequences, deduced from the Stroke of a Pen, as I before mentioned to you, and goes back into Solitude and Retirement.

Mankind however (said she) ought to think themselves greatly obliged to the Microscope for having contributed to soften and familiarize a Thing whose very Name alone inspired so much Terror. Mankind, answered I, are not very often guilty of the Sin of Gratitude, and, as that Polite Philosopher, <189> who is to instruct you in the Motion of the Earth, observes, there are some who make no Scruple to treat the Study of Anatomy (which perhaps may have saved their Lives) as a useless Thing. You may judge from hence, whether it be probably that Mankind will be grateful at the Expence of so much Consideration as is necessary in order to know whether Microscopes have contributed any Thing to familiarize the Calculation of infinitely small Quantities, what this Calculation is, and what Uses it may have; all which Things are necessary to form a well-grounded and rational Gratitude. An English Frier called Roger Bacon, who lived in the thirteenth Century, and had a general Knowledge of the Effect of the Refractions of Light by a Lens, and was besides acquainted with many other Things which are commonly believed to be the Production of much later Ages, such as the Invention of Gunpowder, the Reformation necessary to the Calendar, and was sensible of the false Method of Study at that Time in Fa <190> shion; this very Man, worthy of a Statue and immortal Honours, was ill treated, persecuted, kept Prisoner for many Years, accused as a Conjurer and Wizard, and of holding Intelligence with the Devil, in order to effect what required only superior Parts, and a free use of Reason; and all the Honour these Inventions, which we at present so greatly admire, met with at that Time was, that the Inventor was judged worthy to be burnt alive. It is true, that at present the Learned cannot enough admire the Depth of Understanding and the quick Penetration of of a Man, who, in so barbarous an Age as the thirteenth Century, thought in a Manner that very few of his Species do even in this Age, as inlightened as it is. But what Gratitude is this to persecute, imprison, and almost burn him when living, and at the End of five Centuries, to republish and give him the highest Encomiums? Is not this like deifying Homer after his Death, when he had been suffered to starve with Hunger while living? The Telescopes <191> which have been the Cause of so many fine Discoveries, had no less Reason, in the Beginning of the last Century, to complain of the Ingratitude of Mankind. The Spots in the Sun, the Satellites of Jupiter, and the various Appearances of Venus were imputed to them as so many Deceptions of the Sight, of which they alone were guilty. There was no Calumny so black that was not thrown upon them, and he who (I will not say invented them) but at least made so many fine Discoveries in the Heavens by their Assistance, did not by that Means at all advance his Fortune here upon Earth.

It is surprizing, said the Marchioness, to see the Folly and Caprice of Mankind. In some Cases they are so extremely fond of Novelty, as to adopt the most extravagant Things merely on that Account. This, we see, happens, every Day in the Fashion of Dress, fitting, taking Snuff, and even Sneezing. At another Time, Novelty is an Objection to the most useful and well con <192> trived Schemes. Are our Judgments never to be guided by Reason?

The wise Men of past Ages, answered I, appear to us like the Moon just on the edge of the Horizon, and those of the present Time, like the same Planet, when it is a great way above it. The Image of the Moon painted on ourRetina, when she is at the Horizon, is less than when she is elevated a great way above it, at the Meridian, (for Example;) and this is occasioned by the distance of the Moon from us, which is greater in the first Case than the second. Yet notwithstanding this, we imagine her to be much bigger at the Horizon than at the Meridian. This Mistake proceeds from the Interposition of other Objects, as Trees, Houses, Tracts of Land, Sea and Sky which are betwixt us and the Moon, when at the Horizon, but not when she is at the Meridian, for in that Situation she is left intirely to herself. Now as the Objects placed betwixt us and the Moon, make us imagine that she is more distant from us at the Ho <193> rizon than the Meridian, they are the Reasons too why we imagine her to be bigger, because the apparent Bigness of an Object depends on the Size of its Image on the Retina, joined to the Judgment we form of its Distance; so that the Image being always of the same Size the Object must appear so much the greater, as it is judged to be more distant. Hence it is that Actors, when they come from the Bottom of the Theatre, appear to us like Giants, the Perspective and the Delusion of the Scene making them appear a great way off. Why should those Objects placed betwixt us and the Moon, when at the Horizon (said the Marchioness interrupting me) make us imagine her to be farther off from us than when she is at the Meridian? I should think they would rather make her seem nearer, for in that Situation she appears to touch them, and it seems probably she should appear at the same Distance from us as the Objects themselves: whereas at a greater Altitude we see her placed in the Sky, and consequently judge her <194> to be at a great Distance. We know, answered I, that the Moon in both Cases is in the Heavens, or rather that the Heavens themselves are an immense Vault to which our Imagination always refers the heavenly Bodies. But the Sky itself seems much more distant from us at the Horizon, than when we look directly above our Heads, so that it appears to us as a compressed Vault. Between us and that part of the Heavens which is over our Heads, there is nothing to regulate our Judgment of its Distance, whereas at the Horizon the long Series of intermediate Objects helps us to form the Distance, and makes us judge it very great. Hence it comes to pass, that Distances appear much greater upon a Plain than a Mountain; because the Equality of the Plain lets us see every Thing that is placed between us and the distant Object, which the Inequality of the Mountain will not suffer us to do.

In the famous Picture of Correggio at Parma (so ill copied by the Chizel of Agostius Caracci, who was otherwise <195> a great Man in his Profession) an artful Series of Hands, Heads and Feet place a Distance between St. Catharine and the Head of the Madona, so sensible that you would imagine it might be measured by the Touch, and which, added to the other Beauties and Graces of Art all united there, render it a Master-piece of Painting.

Now (to finish our Optic Comparison) the Antients appear to us through a long Series of Emperors, Kings, Archons, Consuls, and many other Objects, which greatly magnify them: But we see the Moderns alone, separated, and left entirely to themselves like the Moon at the Meridian. Hence it is, that the Manner in which the Antients buttoned their Coats will be a Subject of Admiration to the Learned; whereas there will be only two or three Men of good Sense to applaud any useful Invention of a Modern, who has the Misfortune to be born in the same Age with ourselves, and not to be distinguished by a Name with a Greek Termination. And this is the Way in <196> which a great part of those who value themselves upon their Learning, think. Horace very finely satyrized this Folly even in the Time of Augustus. So true is it, that a wrong Turn of thinking is the Growth of every Age.

But would not the Chinese (said the Marchioness) be Gainers by the immense Distance between them and us? And may not a Million of Miles produce the same Effect as many Successions of Kings and Consuls? They are certainly no Losers by it, answered I, but however those very Persons who most idolize this Nation (which in the midst of Observators and Astronomers could not produce a tolerable Almanack) agree that we are superior to them. This Confession perhaps is the Effect of National Self-Love. The Chinese form a Nation entirely separated and different from us; whereas the Antients are as it were of the same Family with ourselves, and we regard them as our Ancestors. And after all, some few sorry thousand Miles can never be equivalent to a List of <197> Archons or Consular Fasti. In short, it is here, as in the Compositions of the Theatre, in which People suffer themselves to be much more easily deceived in what regards the Customs and Manners of the antient Greeks and Romans, than in those of the Turks or Japonese.

Another Instance in which this Comparison holds good between the Antients and the Moon at the Horizon is, that she appears the greater to us upon the Account of her being less resplendent there, than when she is at the Meridian. Those Objects which are the farthest off, are the least illuminated. So that if two Objects are of an equal Size, the least illuminated will be thought the most distant, and consequently the biggest. Hence Trees and Houses appear greater to Travellers in the Twilight, than in full Day, the Sun seems bigger when seen through a Cloud, and Objects must generally appear greater in England, than they do in Italy. The Sun, after the Death of Julius Cæsar, continued pale and languid for the Space of a <198> Year, and according to the Expression of an elegant Poet, threatned that guilty Age with an eternal Night. If the Romans had dealt in Observations, I do not doubt but they would have informed us, that he appeared likewise bigger than ordinary. Objects then (said she) are magnified by the Mists of Antiquity, many of those great Philosophers, whose Names now pass for a Proverb, were perhaps no more in their own Time than the Regent of a College, or the Lector of a University. Those, answered I, who are the most devoted to them, are the most likely to see them greatly magnified: For (as the finest and most judicious Verses in the World inform us) Fools admire, but Men of Sense approve, and every Thing appears magnified to Dullness, as Objects do when seen through a Mist. I should not at all wonder if some profound Admirer of the Greeks should prefer the Epicurean Explication of Vision to that of the Moderns, for this only Reason, that one is more antient than the other.

<199>

What Explication is this (said the Marchioness) for I do not remember that you have mentioned it to me before? It was the last I spoke of, answered I, when we were talking yesterday about the Explications which the Antients have given of Sight. This supposes that certain Shadows or Images fly off from Bodies, by whose Means we see. Though this may seem reasonable enough to some Persons, yet there arises a great Difficulty to explain how it happens, that when we are in the Dark we see Objects that are placed in the Light; but when we are in the Light, we cannot discern Objects placed in the Dark: Since according to this Explication of the Epicureans in both Cases, there are Shadows which fly off from the Objects and raise in us the Sensation of Vision. Lucretius calls a certain lucid and subtile Air to their Assistance, which, entering into the Eyes when they are in the Dark, disengages them from the more thick and black Air that obscured them, and by this Means opens a Passage to the Shadows <200> which from Objects placed in the Light proceed to the Eye. When Objects on the contrary are placed in the Dark, the thick black Air fills the Eyes, and by this Means denies a Passage to the Shadows which are transmitted to the Eye from those Objects.

The Image of any Object, said the Marchioness, cannot be drawn upon the Retina, unless there are Rays transmitted from the Object to the Crystalline Humour, just as in the Camera Obscura, an Object, in order to have its Image painted upon the Paper, must transmit Rays to the Lens. If then the Object be placed in the Light and we in the Dark, its Image will be drawn upon the Retina and we shall discern it, but if the Object be placed in the Dark, it cannot transmit any Rays to the Crystalline Humour, there can be no Image drawn upon the Retina, and consequently the Object cannot appear to our Sight. But I do not see what Relation there is between the thick or subtile Air of Lucretius and these Images. It is true, answered I, that <201> this Air has nothing to do with the Image on which Vision depends, but is has a great Relation to these Shadows on which depends the Honour of the Lucretian Philosophy. And what is there in the World that a Philosopher, embarrassed in the Explanation of a Phænomenon, does not lay hold on? But since you have so well explained this, I will venture to propose to you another, which you must often have observed, it is, that in going from a very light Place to one which is much less so, and may even be called dark when compared with the other, the Objects in this Place are at first not at all discernible, but by Degrees they begin to appear, and after some Time are seen very distinctly. This often occasions great Mistakes in Society, which are very soon found out and repented of. Any one for Instance going into the Chamber of a Lady, who, either because she is indisposed or fancies herself to be so, likes to sit in the Dark, may take one Person for another, and a fine Compliment be wrong addressed, and the Error after <202> wards appear to the great Confusion of the Person who had been at so great an Expence of Wit in forming it.

This Phænomenon, said the Marchioness smiling, has very important Consequences and merits the utmost Attention. But I must confess it appears to me a little more perplexed than the first, and I do not know how many Degrees of Subtilty in the Air Lucretius would require to explain it by. Yet the Explication of this Phænomenon (answered I) depends intirely upon a Thing which you must very often have observed with the utmost Diligence. Have you never remarked that there are no Eyes, not even your own, but appear much finer by Night than in the Day? I agree to this, said the Marchioness, that we may not spoil our Observations by Compliments: But does not it proceed from hence, that the Night does not shew the Defects of the Face so much as the Day, and therefore the Eyes too must be Gainers? The true Reason of this Phænomenon, answered I, is, that in the Dark the <203> Pupil is more open and dilated, which makes the Eyes look blacker and brighter in the Night than in the Day when the Pupil is more contracted. How many Eyes have triumphed in the Evening and gained Conquests which they lost the very next Morning at the approach of the Sun! The Pupil is contracted in very light Places in order that it may not admit too great a Quantity of Rays which would only serve to do it Hurt. On the contrary it is dilated in the Dark enough to admit such a Quantity of Rays as are sufficient to cause Vision. The Reason perhaps why some Animals never creep out of their Holes till Evening is, because they are not able to contract their Pupil so much as is necessary to hinder the Light of the Sun from injuring their Eyes. When we go therefore from a light Place to one which may comparatively be called dark, the Pupil, being at first very much contracted, does not admit such a Quantity of Rays into the Eye as is sufficient to raise the Idea of Vision. The Pupil afterwards begins to dilate <204> itself, and we begin to see. And as this Dilatation is made by Degrees, so we discern the Objects by Degrees still clearer and clearer, till at last when the Pupil desists from dilating itself at a certain Point, we afterwards continue to discern the Objects with the same Degree of Clearness.

You have not given me the least Time to think on this, said she, who can tell whether I might not have found out this Explication, which now at least does not appear at all difficult to me? It is sufficient for you, Madam, said I, to have explained one Phænomenon, and seen the Difficulty of another. A very great Exploit truly, (said the Marchioness with some Emotion) to see Difficulties and not be able to resolve them. It is a very great Honour indeed for a General to besiege a Town and not take it? No, said I, but it is sometimes an Honour for him not to undertake the Siege at all. The first Step to Wisdom is, to cease from Folly, and the first Point of Learning not to be too arrogant, but perceive our own <205> Weakness. This is a Point of Modesty very little practised by those Gentlemen who with the Vulgar gain the Reputation of Philosophers, merely by their declaiming in Assemblies and Coffeehouses, against the antient Philosophy which they know nothing of but the Name, stigmatizing those who profess it with the Title of Ergotists, and having read perhaps some Preface or Literary Journal. Such sort of Persons never doubt of their Knowledge, but explain and pass a decisive Judgment upon every Thing. These are blind Men who would walk in a Garden with the same Freedom as other People, but the first Bason they come to, fall in. There is an Observation which the more 'tis examined the truer it will appear, that nothing in the World is so difficult to be met with as common Sense.

I perceive, said the Marchioness, that I have some Right to call myself a Philosopher. I have my Head full of Vortices; I form Light by the Pressure alone of the Globules of the second Element, and Colours by their Rotation. I have <206> renounced the most agreeable Qualities, and retain nothing but a little Extension, and infinitely small Quantities. I am not certain whether the World appears the same to all Eyes: I explain one Phænomenon, and see the Difficulty at least of another; I think I have Contempt enough for the antient Philosophy; and after all this I hope it will not be said that I am not wiser than I was. Do I want any Thing else to make me a complete Philosopher? -- Yes, Madam, answered I, perhaps you should have a little less Beauty, or make a better use of it. But you do not perceive that the Philosophy you are so fond of needs a Reformation, and I wish this Reformation might be the last.

What (said she with some Emotion) will you tell me that Vision is not performed in the manner you have hitherto explained it? This is plainly betraying me, and affirming upon your Word what afterwards appears not to be true. No, no, Madam, answered I, be not uneasy, I am not of a Character to <207> propose to you Things differently from what they are. Vision shall remain untouched; that Abjuration, which you have generously made of the Roses and Lilies in your Cheeks, shall stand good and be ratified in Form. The Doubts you are under concerning the different Appearance of Objects in different Persons, shall still continue to be reasonable, and your preferring the Moderns to the Antients shall still be compatible, and remain as it is founded on the very best Reasons. The Reformation I spoke of will affect nothing but the Globules of Light, and the manner in which they excite in us the Sensation of Colours. You may hereafter if you please regard the System of Vortices, as one of the finest and most entertaining philosophical Poems in the World, which is the Idea under which I at first proposed it to you. This is absolutely disconcerting my Ideas, said the Marchioness. I would willingly look upon this System as something more than a Fable, were it ever so fine a one. I cannot think of changing any Thing in the Globules of <208> Light, which with so very little Trouble supplied me with the most agreeable Colours. If I once quit these, who knows how much Pains, and how great an Apparatus it will cost me after this to make even a sorry Metzotinto?

It will cost you no more, answered I, than it did with your Globules. This Reformation was produced by Father Malebranche; one of the greatest and most illustrious Cartesians that ever appeared in the world. The Authority of so great a Name may let you see how very necessary this Reformation was, and you may moreover be sure that the Simplicity which has always formed the Delight of this Sect cannot fail here. This Simplicity is an Idol to which they sacrifice every Thing, even Truth itself sometimes, that Truth which an Antient called the Citizen of Heaven, and Companion of the Gods. But before I proceed to this Reformation, it will be proper to lay before you the great Difficulties which must make you forever renounce your Globules. This System, like Hercules in the Fable, <209> had very great Difficulties to encounter with from its very Birth; but perhaps it did not overcome them with equal Honour.

Some objected, and with great Reason, that according to those Laws assigned to the Vortices by their Inventor himself, as the Stars are not composed of the subtile Matter but of that of the third Element, instead of being luminous, they would be covered with an opaque Crust; and even if they were luminous, the contrary and equal Pressure of the Vortices would not suffer us to see them.

However weighty these Objections might be, they would not be able to shake the Faith of the steady Cartesians. But this, which I am now going to mention, appears an indissoluble Gordian Knot, even to the most zealous and ardent among them. You have this formidable Enemy of the Cartesian Philosophy in your own House, nay, in the very Gallery where we are, and you do not perceive it. This painted Wall is an Adversary that makes War against <210> the System you are fond of. Deliver me from this Perplexity, I intreat you, said the Marchioness, or I shall erase the Picture. You have a Mind to make me quarrel with my own House for presenting me with such detestable Objects. No, Madam, answered I smiling, that is not my Intention, I would rather have you persuaded that every Corner of it is since yesterday become philosophical.

Let us fix upon one Point in the Air to which your Eye and mine may be equally directed while we are both looking at the same Time upon different Parts and different Colours of this Wall. Do you, for Instance, place yourself at this Pilaster and look upon the red on the Vesture of Achilles. I will stand at the Window, and look upon the blue of the Sea, so that while you are looking at the red and I the blue, our Eyes may each be directed to the same Point of Air. It is certain that two Rays will pass through this Point, one from the Robe of Achilles, and the other from the Sea. These Rays you <211> know are nothing but two Series or Strings of Globules immediately touching each other, the one continued from the Robe of Achilles to your Eye, the other from the Sea to mine. These two Strings of Globules intersect each other in that Point which we have fixed in the Air, and consequently in this Point there will be one Globule common to both Strings. Do you understand all this? Yes, too well, said she, and I begin already to tremble. In order for these Globules to excite Vision in us, answered I, the Pressure of those contained in that String which comes from the Robe of Achilles, must be continued from thence to your Eye, and the Pressure of those contained in that String which comes from the Sea must from thence be continued to mine. Now that Globule, which happens to be in the Point of Air where the two Strings intersect, and which is common to each of them, must at the same Time press towards your Eye and mine, which is utterly impossible if the Globule be hard, as Des Cartes supposes it to be; <212> because the close Union of the Parts of such a Body can never suffer it to press at the same Time towards two different Sides. But this is not all. -- And yet this is sufficient, said the Marchioness, to demolish my Globules. The very same Globule, said I, as hard as it is, must have at the same Time two different Rotations, one to excite in you the Idea of a red Colour, which is communicated to the whole String that comes from the Robe of Achilles to your Eye, and the other to excite in me the Idea of an azure Colour, communicated to the whole String that comes from the Sea to my Eye. But the Difficulty will be still greater if we suppose other Spectators to be placed in this Gallery, who all direct their Eyes to the same Point in the Air as we have fixt upon, and other Rays to pass through this Point, some of which should convey the golden Colour of Achilles's Hair, that Minerva lays hold on in order to calm his destructive and impetuous Rage; others the green of this Meadow, and the innumerable <213> other Colours which variegate the Picture. You see then, that supposing your Globules, such as Des Cartes has made them, it would be impossible for us to see what, however, we really do see. I understand this but too well, answered she, but I beseech you, for the Love of Philosophy, never mention these Globules to me again. I am resolved to think no more of them since they have so shamefully yielded for the very first Difficulty. They seem to resemble those unexperienced Lovers, who at the first Repulse think of a Retreat. But pray let me see what your reformed Cartesian Malebranche substitutes in their Place; for I am persuaded he will be able to make a better Resistance.

Malebranche, answered I, entirely rejects these solid Globules which you have forbid me to mention, and substitutes in their Place certain exceeding small and fluid Vortices composed of an ethereal and very subtile Matter, which fills all the great Vortices, as the great Vortices which are the Seats of Stars and Light do <214> the Universe. These little Vortices, by that Power which they have of dilating themselves, keep a mutual Equilibrium in their respective Vortices, just as the great Vortices do in the Universe. In this System of Malebranche Light is nothing but the Undulation or Vibration of the Vortices, occasioned by the Vibration of the luminous Body which is always repelled in the same Moment that it impels, and the greater or less Force of the Light depends upon the greater or less Force of these Vibrations, just as Colour depends on their greater or less Velocity. For Instance, if these Vortices should raise fifty Vibrations upon the Optic Nerve or the Retina in any determinate space of Time, it would give us the Idea of a certain Colour; if in the same Time there should be a greater or less Number of Vibrations raised, we should have the Sensation of a different Colour. Malebranche ingenuously confesses however, that he was not able to assign what determinate Degrees of Quickness were necessary to form different Colours in particular. A Con <215> fession so much the more ingenuous as it is extraordinary in a Philosopher.

This System of Light and Colours has a very great Affinity with that of Sound, with this Difference only, that Air is the Vehicle or Channel of the one, and the ethereal Matter or the Vortices composed of that Matter, the Channel of the other. The Vibrations, which a sonorous Body when it is struck raises in the Air, and from thence in the auditory Nerve, excite in us the Idea of Sound. In the same manner the Vibrations which a luminous Body raises in the ethereal Matter, and from thence in the Optic Nerve, excite in us the Idea of Light.

If the Air be entirely excluded from any Place (which may be done by the help of a certain Machine) a sonorous Body put into that Place would not be heard to sound. In the same manner if it were possible to exclude the ethereal Matter from any Place, a luminous Body put into that Place would not appear resplendent.

<216>

The greater or less force of the Vibrations on the Air, or the auditory Nerve, produces a greater or less Intenseness of Sound. In the same manner the greater or less Force of the Vibrations on the ethereal Matter, or the auditory Nerve, produces a greater or less Intenseness of Light.

The different Quickness of the Vibrations on the Air, or the auditory Nerve, produces different Sounds, as Bass, Treble, and their different Degrees: So after the same manner the different Quickness of the Vibrations raised in the ethereal Matter or the Optic Nerve produce different Colours, as red, yellow, and the like, which may be considered as the Tones of Light.

I do not believe, said the Marchioness, that even our Preachers ever carried a Similitude farther than this. It may be carried still farther, answered I. As various and different Vibrations cross and intersect without destroying, or rather without giving each other the least Interruption, which we ever Day see in Consorts of Music, where the <219> Vibrations of the Strings of a Violin do not interrupt those of any other Instrument) so the different Vibrations which flow to our Eye from different Colours, do not interrupt each other, notwithstanding they cross and intersect. The Fluidity of these Vortices gives them a Power of transmitting the Vibrations, of different Colours to various Parts, (which the Solidity of the Globules would not suffer them to do). In the same manner as the Fluidity of the Air does the different Sounds in a Concert of Music. The Explication of this seems so very difficult to Malebranche, that he affirmed a System capable of effecting it, must be conformable to Truth.

Sound and Light, replied the Marchioness, appear to be as faithfully copied from each other, as Nature was by the Paintings of Apelles, which were so very accurately done, that an Astrologer could predict the future Fortune of the Persons whom they represented.

I have not yet finished the Comparison, answered I, an Object placed be <220> tween two Looking-glasses over-against each other, will be greatly multiplied. One Candle is changed into a thousand, and brings to mind the celebrated annual Feast of the Illumination of Candles among the Egyptians, from which some imagine the Chinese to have borrowed their Feast of Lanthorns. And does not the famous Echo of the Castle of la Simonetta not far from Milan, produce the very same Effect upon Sound? The Report of a Pistol in this Place is repeated more than forty Times, and a single Instrument of Music forms a Fulness of Sound superior to that of the most numerous Concert. Two great Wings of a Building opposite to each other with Windows, which are all false except one, and formed of a Matter very proper to throw back the Vibrations, reflect and multiply the Sound just as the two Glasses do the Candle.

The great Lord Bacon, the Harbinger of true Philosophy, has among innumerable other curious Things proposed the Examination of this Affinity betwixt Light and Sound to the Con <221> sideration of Naturalists. Perhaps he could not have wished to find a more close Union betwixt them, than what the System of Malebranche discovers. But he advised too that Philosophers should carefully examine in what Points they might disagree; and the greatest Difference betwixt them is, (as I first told you), that the Channel of Sound is the Air, and that of Light the ethereal Matter. Hence it follows that Sound must be propagated from a sonorous Body in Time, for the Spaces and Interstices between the Particles of Air make it absolutely necessary, that there should be some little Time before the Motion can be communicated from one Particle to another. Light on the contrary must be propagated in an Instant, or an exceedingly little Space of Time, because there are no Spaces betwixt the Vortices or ethereal Matter, since the whole Universe is every where filled with them. Light and Sound have the same Resemblance to each other as the Nereids engraved by Vulcan upon the Silver Gates of the Sun's Palace, in <222> Ovid's Metamorphoses. Their Features are not exactly the same, but so extremely like, that it is very easy to perceive they are Sisters.

Let us then become reformed Cartesians, said the Marchioness, by accepting a Reformation which not only explains all that the Globules did, but something more, and of much greater Importance, which they were not able to explain. Let us adopt Malebranche's System of Light and Sound, these two new Brothers in Natural Philosophy. I do not absolutely despair, answered I, but the Harpsicord of Colours, and the Music of the Eyes, which gives a still greater Confirmation to this new Alliance, may in Time meet with a favourable Reception from you.

What do you mean, replied the Marchioness, by this new invented Music and Harpsicord? Have you a Mind to ridicule the philosophical Similitude you have been entertaining me with? I should be very sorry, answered I, to have any Temptation to ridicule what you have adopted instead of your <223> Globules. This Harpsicord is indeed a new Invention, but is not therefore the less true or real. Upon moving the Keys of this Instrument, instead of hearing Sounds, you will see Colours and Mezzo Tintos appear, which will produce the same Harmony as Sounds do. The Sonatas of Rameaux or Corelli will give the same Pleasure to the Eyes when seen upon this philosophical Harpsicord, as they do to the Ear when they are played upon the common Sort. The Concords of a Piece of Purple and Scarlet will raise the Passions of Love, Pity, Courage, or Anger in our Souls: This surprizing Instrument is now making beyond the Mountains, and you may for the future expect your Silks and Ribbons in Music. The transient Pleasures of the Ear will be fixed in the Eye; you may continually enjoy the fine Airs of Farinelli wove in a piece of Tapestry.

The Inventor, answered she, probably took the first Idea of this, from the Dress of a Harlequin. However, we are greatly obliged to him, for we shall <224> have no occasion to embarrass ourselves for the future in adjusting the Colours of our Clothes. We need only consult the Thirds and Octaves of this Harpsicord, and we shall be sure never to make any Discord in the Shading. The Painters, answered I, may find Relief in their Indispositions from this new invented Music of Colours, as Singers and Musicians are said to do from that of Sounds.

Why will you restrain the Effects of so singular a Thing (said she) to the Painters only? It will help Physicians to increase their Prescriptions and prolong their Consultations. Physicians, answered I, must act with regard to their Patients, as Musicians do with weak Voices, and as those are careful not to discourage their scholars with difficult Notes, so these in certain Distempers, as the Bite of a Tarantula, which can be cured only by Pleasure, must take Care not to prescribe any Colours for which the Patient has an Aversion. But we must leave the Physicians to divert their Patients in this new Method as <225> they judge proper. This new Instrument will help to give us a Proof of the Justness of that fine Comparison made by an elegant Poet between the gradual languishing and dying Sounds in the Voice of our tuneful Orpheus, and the insensible fading and vanishing of the Colours in the Rainbow. Who knows replied the Marchioness, but we may some Time or other form a Dinner by the Assistance of a Harpsicord, and find Harmony in a Fricassy?

After this Discourse, as we were taking the Air in the Garden, the Marchioness suddenly cried out, -- What shall we do? I see a Gentleman in our Neighbourhood coming towards us, who in every Visit he makes does me the Favour to repeat Sonnets by the hundred, and yet always finds Time for some Ode. How shall we disengage ourselves from his troublesome Company? Will there be no Vortex so merciful as to snatch him away with itself, and remove him from our System? For want of a Vortex, answered I, we will serve him as I once <226> did a Mathematician, who happens to be very talkative, a Fault which those Sort of People are seldom guilty of. When you are walking with a Friend and discoursing about the Country of Kouli Kan or some such Trifle, he will entertain you with the most abstruse Points in Geometry. He assaulted me one Day as I was walking with some Company in a Garden, and we presently found by his Air that he was preparing to torment us with his Demonstrations and Corollaries. I and the others which were with me began to talk of Poetry, and repeat Verses, (a Language to which he was an utter Stranger) without suffering him once to open his Mouth. By this Means we compassed one of the most difficult Enterprizes, that of tiring the most tiresome Person in the World. Now we need only continue our Discourse upon Philosophy, and I will engage your Sonnetteer will suffer the same Punishment as my Mathematician. Thus we resolved to proceed.

After the first Compliments were over, the Poet, who knew nothing of <227> our Scheme, took occasion to inform us that the Muses had for a long Time used him ill, and that he was resolved to renounce them forever. After we had civilly contradicted him, he answered that he was ready to prove his Assertion by a few Sonnets which he had lately made, and which would shew us, how very little he was in their Favour. If they are really so perverse, said the Marchioness, taking him at his Word, you must intirely abandon, and never think of them again. We have just been discoursing on Philosophy and Optics, it will be taking an effectual Revenge if you enter into our Conversation, which is so very different from Poetry. He excused himself by saying that he had not a Capacity for such sublime Subjects, and though it was necessary sometimes to shew a little Resentment, he must take Care not to affront the Muses too much. He observed that a little Poetry would alleviate the Severity of our philosophical Discourses, and alledged the Example and Authority of Plato, who did not think it <228> beneath him to write Love Verses to Agatis, and engrave the three Graces on the Citadel at Athens, with the same Hand with which he had wrote the Institutions of his Republic and the Timeus, and in this manner divided his Time between Philosophy and the Arts of Apollo. But all these Arguments could not prevail on us to let him repeated his Sonnets, which was the chief Design both of his Visits, and all that Erudition which he had lavished upon us.

The Marchioness asked me several Questions, which our Poet did not think at all a Propos. Among the rest, she was very anxious to know whether she might rely upon Malebranche's System of Light and Colours; for the Destruction of the Globules made her fear every Thing, and her Suspicions were terribly increased by this Harpsicord. I answered, that such was the Condition of all human Things, that nothing must hope for a long Duration here below, a Truth which our Gentleman could evince by many fine Verses of the best <229> Poets, and perhaps by some of his own. I added, that I was extremely pleased to find the Example of the Globules had warned her not to place too much Confidence in the Reformation of Father Malebranche. But that the most fatal Blow to this Opinion was, the being obliged to forsake it upon the Account of that very Analogy and Correspondence betwixt Light and Sound, which at first seemed to give it so much Lustre.

This Analogy, continued I, fails in one of the Parts, and in that Part where it was most wanted. And this Circumstance is sufficient to overthrow Malebranche's reformed System. All those fine Resemblances, which you observed with so much Admiration, cannot save it from Destruction.

If an undulating Motion happens to meet with any Obstacle in its Way, this does not stop its Progress, for it bends on all Sides, and continues to propagate itself in Spite of the Obstacle that opposes it.

<230>

A very familiar Example will make you understand what I mean. If we were at the Bottom of this Hill, and some Person on the opposite Side of it should sound a French Horn to proclaim the Destruction of some innocent Tenant of the Woods, whose only Fault is the Pleasure we find in destroying him by the help of our Reason and Contrivance, we should hear this Sound notwithstanding the whole Hill is placed betwixt the Horn and our Ear. The Reason of this is, because the Undulations raised in the Air by the French Horn do not stop their Progress when they meet with the Hill, but giving Way on every Side and bending all round it, communicate the like Vibrations to the opposite Air. After the same manner if you throw a little Stone into this Bason, the Undulations formed in the Water will not stop when they meet with the Pipe, but giving way all round it, will communicate themselves indifferently to all the Water, and thus the Effects of it will be discovered in the whole Bason.

<231>

You see then that if Light were nothing but an Undulation of the ethereal Matter communicated to it by the Vibrations of the lucid Body, no interposed Body could ever deprive us of the Light of the Sun or any other luminous Object, or in other Words we should never have any Shadow, which (especially in this Season,) would be a terrible Inconvenience brought upon us by Malebranche's System. The Pressure of Des Cartes Globules could give us no Assistance in this Case.

Thus Sir Isaac Newton, the avowed Enemy to imaginary Systems, and to whom you are indebted for the true Idea of Philosophy, has at one Blow lopped off the two principal Heads of the reviving Cartesian Hydra.

Though the Marchioness perceived the Force of this Argument, she did not seem greatly dipleased with it. As she had renounced the Globules, she easily gave up the Vortices. But our Sonnetteer was not quite so well satisfied. As he could find no Opportu <232> nity of discharging his poetic Fury, he was obliged to go some where else, to find an Audience to a Satyr which it is very probable he had begun to compose against Philosophy.



End of Vol. I.

[1] A Cretan Philosopher when he was a Boy, being <158> sent by his Father into the Country to fetch a Sheep, he turned out of the Road at Noon and reposed himself in a Cave, where he slept 57 Years. After this Refreshment he awaked and looked about for the Sheep (imagining he had slept but a little while). Not finding it, he proceeded to his Father's Country Estate, where he saw every Thing altered and in Possession of another. He then returned to the City, and went to his Father's House, where his younger Brother now grown an old Man, at last knew him, and gave him an Account of all that had happened. He was held in great Veneration among the Greeks, who imagined him a peculiar Favourite of Heaven. He is said to have lived till 150 Years old, or, according to others, 297.

[2] Camoens, the famous Portuguese Poet in his Lusiada, the Subject of which is the Discovery of the East Indies by his Countrymen, conducts their Fleet round the Coast of Africa, and as it fails in sight of the Cape of Good Hope, he introduces a formidable Spectre walking in the Depth of the Sea, its Head reaching to the Clouds, its Arms extended over the Waves, and its whole Form surrounded with Clouds, Storms, Winds, Thunders, and Lightnings. This <184> Spectre is the Guardian of that foreign Ocean which no Ship has ever passed through before, complains of his being obliged to submit to Fate, and the bold Undertaking of the Portuguese, and foretels them the Misfortunes they must undergo in the Indies.

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