<76>

Dialogue the Second.

That Qualities, such as Light, Colours, and the like, are not really in Bodies. Metaphysical Doubts concerning our Sensations of them. Explication of the general Principles of Optics.

ALL the while we were at Dinner the Marchioness entertain'd herself with making the Globules of Light turn round, sometimes one Way, sometimes another, as the different Colours of the Objects before us requir'd, and look'd upon herself, as she said, to be the Empress and Arbitratress of Nature, since she was possess'd of Materials to diversify it as many Ways as she <77> pleas'd. As soon as Dinner was over, and we had return'd into the Garden, I am ready, said she, to deprive the Soup of all Taste, and willingly renounce every Colour, even what I had the greatest Fondness for. In short, I will be quite a Cartesian, provided you can furnish me with good Reasons. These Globules, 'tis true, lead me to strange Consequences, but perhaps they may furnish me with some Expedient to evade them. You treat Philosophy, answer'd I, as Attorneys do the Law. But there is no Expedient that will hold good at the severe Tribunal of Reason. Not all the Monarchs in the Universe, nor all the Beauties which are far more powerful, can influence the impartial Judgment of Philosophy, nor induce it to interpret the least Text in their Favour. This is a Trial, a Mortification that Des Cartes will make you undergo in your Novitiate of Philosophy. But are you terrified at so small a Hardship as this? Take Courage and fear nothing; you will at last add to the Pleasure, you receive from your Senses, <78> that of contending with them, and giving them no Credit.

Hitherto, said the Marchioness, I have only the Mortification of seeing that we are under a perpetual Delusion, since, if what you say be true, Things appear to us very different from what they really are. Bodies appear to us of a certain Colour, whereas there is really nothing else in them but a certain Disposition of Parts. They seem to us to be hot, cold, and yet they are possess'd of none of these Qualities. Really I cannot help thinking, that we are in a very strange Condition. It is certainly very strange answer'd I. Our Knowledge can make but very little Progress, unless it be conducted by the Sense. They continually make us believe Things which a more refin'd Sense, or our Reason, afterwards contradicts. You think, for Instance, that your Hands which have been the Subject of so many fine Verses, are smooth and polish'd; and possibly might be greatly offended, if any one should dare to dispute them this Quality. And yet if you were to look upon them <79> through a Microscope, you would be surprized to see a great Number of Pores that separate the Texture of them, and to find that they are cover'd with Scales like those of a Fish. You would discover in them Cavities, Promontories; Valleys and Hills, for the Abode of a Nation of little Animals, who perhaps spend their Life there. And to increase your Wonder, you would be presented with the Sight of Rivers and Seas. In short, you would not know them again, and you would be obliged to confess that they are very different from those which your Poets described. Nature, said the Marchioness, has done us a great Favour in not making our Senses too refined. It would be very bad for us, if our Touch was exquisite enough to feel all that the Microscope discovers to our Sight. We should certainly be extremely unhappy, answer'd I, if our Sensations were so perfect, that in handling the smoothest Surface our Touch should fail us at every Pore, and every little Eminence should make us shudder. It is to the Silence of our Reason, and <80> the want of more refined Senses, that we owe our Perceptions of Pleasure. And he gave a very just Definition of our Happiness, who affirm'd, that the most tranquil Possession of Pleasure consists in our being agreeably deceiv'd.

It must be confess'd, replied the Marchioness, that our Sex is greatly obliged to the Complaisance of Philosophers, who, notwithstanding they are so well acquainted with the Nature of our Superficies, are so genteel as to behave towards us like the rest of Mankind. But if I had a Mind to please any ignorant Person, the very first Think I would do, should be to forbid him the holding any Correspondence with those Gentlemen who deal in Microscopes; for these might do me a very great Prejudice. Not all the Microscopes nor all the Philosophy in the World, answered I, could ever hinder your appearing agreeable to the naked Eye, and even a Cleopatra might be contented with this. Virgil makes Corydon warn his Alexis not to confide in his beauteous Colour. But I may free <81> ly give you leave to confide in your Hands.

As our Sense are not microscopical, so neither are our Hearts philosophical. It would be very bad for us, if our Pleasure was in the Hands of Philosophers, and if Beauty, in order to prove its Existence, must stand out against all the Experiments of a Naturalist. This is just as if the Chastity of a Lady should depend upon the illgrounded Suspicion, and diligent Enquiry of a jealous Husband. These two Kinds of Men have this in common, that they both equally tend to destroy the most valuable Things in the World.

But Philosophers, said the Marchioness, destroy without Mercy, for they can leave but very little else to Bodies, after they have deprived them of Colour, Taste and those many other Qualities which they have taken from them. They leave them, answer'd I, in Possession of Extension (that is, Length, Breadth, and Depth) Impenetrability, Motion, Figure, and all the fine Things that <82> Mathematicians and Mechanics deduce from these Qualities, upon which I could produce you so many formidable Volumes, that all which has been written upon the Crusca, would seem compared to these no more than a King's Declaration of Love. Do not you think it enough for Bodies that they are no more than Bodies? Besides, what Philosophers do with regard to those Qualities we were speaking of, is not properly a Destruction: They take nothing away from Bodies, but what was falsely applied to them, and what they had long unjustly possess'd; and restore those Qualities to us, to whom they rightly and properly belong. Prescription has at present no Influence on Philosophy, as it formerly had. If a Lover for Example should say, that there was Hope in a certain favourable Glance, which had darted on him through a Fan, what Harm would a Philosopher do, who without destroying either the Hope or the Glance, should tell him that there was nothing in the Glance, but a particular Motion of the Eye, caused by <83> certain Muscles, either from a Principle of Pity, or (if we would trace the Thing to its Original) Coquetry. But that the Hope was intirely in himself, and excited by the Means of that Glance. Just in the same Manner, when we are pricked with a Needle, the Pain is intirely in ourselves; and there is nothing in the Needle, but a Motion by which it disjoins and lacerates the Fibres of our Body; this Separation is the Cause that we feel Pain. In short, Bodies are only Matter, and consequently can have no Properties but what depend on Matter; and these the Cartesians have confin'd to Extension, mutual Impenetrability, Diversity of Figure, and a different Disposition of Parts. And these are sufficient to give Bodies a Power of exciting different Ideas in us, as those of Light, Colours, Taste, and the like. It is not necessary, for Instance, that Colour should really be upon the Surface of a Body, in order to make me see that Colour, any more than it is necessary for Pain to be in a Needle, in order to <84> make me feel it when I am pricked. It is sufficient that as the Needles causes a certain Disposition in the Fibres of my Body, by the Means of which I feel the Pain, so that particular Rotation, which is in Globules repelled from the Surface of a Body, should cause another Motion upon the Nerves of the Retina, which carried from these to the Brain excites in me the Idea, or as they call it, the Sensation of Colour. Thus, if in any Body there be a certain Motion by which it pressed the Globules of the second Element, and these Globules be carried to our Eye, they will raise in us the Idea of Light. A certain Configuration of Particles, or perhaps certain little Animals which are in Bodies, by playing upon the Nerves of the Tongue in such a particular Manner, raise in us the Sensation of some Taste. These Sensations are generally raised in us by Means of certain Bodies, and because we see neither their Particles nor the little Animals which are in them, the Globules of the second Element, nor the Impression which is <85> made upon our Nerves, we ascribe to those Bodies both Light, Colour and Taste, which in Reality are only in ourselves. Reason at length convinces us of the Illusion which our Imagination continually puts upon us, and assures us, that the delightful and hitherto undefin'd Taste of the Pine-apple, the pleasing Verdure of a Meadow, and even the Light of the Sun which animates and revives the whole Universe, are all our own.

I understand you, said the Marchioness, we are inrich'd at another's Expence, and are like antient Rome, which founded its Grandeur on the Spoils of the whole Universe. Philosophy would be in a bad State, answer'd I, if its Rights had no better Foundation, than those of Policy and Ambition. I see you have not yet a right Notion of it. In order to convince yourself that Philosophy is no Usurper, but only takes its Due, press one Corner of your Eye with your Finger, and you will see on the opposite Part a round Flame of a reddish Colour. In this case there is <86> certainly neither Light nor Colour without your Eye. The only Reason of your seeing them is the Pressure which your Finger makes upon the Nerves of your Eye. The Globules of Light which flow from the Surface of Bodies, occasion the same Effect upon the Eye as your Finger, only their Operation is more imperceptible. The different Disposition and Configuration of the Parts of a Body, are the Reason why the Globules make different Impressions upon us. The Power of a Body's exciting in us the Idea of any particular Colour, consists alone in this Disposition, and the Configuration of its Parts. Is it not evident from hence, that if this Disposition be changed, the Colour is changed also? which could not happen if the Colour was really in the Parts of the Body itself. Coral, which is of a fine red, if it be ground to Powder turns pale. One liquid mix'd with another changes its Colour. The Reason of all this is, that the Disposition and Configuration of the Parts of these Bodies, are changed by being ground or mixt, <87> and from hence they transmit the Light to us in a different Manner, and consequently our Idea of the Colour is changed; from no other Reason proceed the venerable white Locks of old Age; the transient Whiteness of many Animals of the North in Winter. From hence too it is, that certain Roses in China are in the same Day both white and purple. From this Cause arises that surprising Variation of Colour which generally follows the Change of Passions in the Camelion, which has furnish'd the Moralists and Poets with so many Allusions, the Antients with so many Fables, and the Moderns with so many fine Observations. And what is it else but one Disposition which hinders us from seeing you Goddesses when you first rise, and another which gives you to our Sight and Adorations after you have spent two or three Hours in the sacred Rites of the Toilet?

I perceive, replied the Marchioness, that there is nothing secret to Philosophy; we may hide ourselves from Men, but not from Philosophers. And indeed <88> to what Purpose would it be for us to endeavour to conceal ourselves from a Set of People who are quick sighted enough to discover the Globules of Light indued with a certain Motion, and those Nerves and Fibres to which this Motion is communicated and thence convey'd to the Brain? A Sight which mortal Eyes have never yet penetrated into. But I must confess, I stand in Need of your Assistance to guide me through this obscure Labyrinth. I do not see what Relation all these Motions have to any Colour that I have a Conception of. This is a Thing which seems to me quite different from these Motions. Have you any better Conception, answer'd I, of the Relation between the Idea of Pain, and a Separation of the Fibres of your Hand? Or between the Idea of Hope and a certain Motion in the Muscles of an Eye? And yet you see that these Things are in Fact connected, and that the one is the Cause, or at least the Occasion of the other. You seek for more than it is possible to give you. <89> Unhappily for us, those Things which are of the greatest Importance to human Knowledge are the most doubtful. Who can tell you in what manner Objects occasion certain Ideas in the Soul, and how the Soul on the other Hand gives certain Motions to the Body? How the Soul which is unextended yet is present in every Part of our whole Machine, and though incapable of being seen or felt, yet sees and feels every Thing? Philosophers can with a great deal of Ease transmit the Motion of the Globules of Light, (or any other Motion) to the Nerves, and from these to the Brain, where they all terminate, either by Means of a Fluid that runs through them, or a certain Tremor raised in them. Nay, Philosophers will go yet farther, and transmit this Motion to certain Parts of the Brain which are imagined to be the Seat of the Soul. But how these Motions, when they are arrived to the Brain or Seat of the Soul, should produce in it different Ideas, is an absolute Mystery. This Passage, which in Appearance is so short, <90> is to Philosophers what the innavigable Ocean was to the Antients. What Communication, what Connexion can there be between Body and Soul, between Extension and Thought, Motion and Idea, Matter and Spirit? What sort of Communication these can have with one another, is beyond the Reach of our Imagination. The same, answered she smiling, that Æneas had with the Shade of his Father Anchises in the Elysian Fields. They mutually communicate the most agreeable Things in the World to one another. But when Æneas attempts to embrace the old Man, he vanishes away and is dissipated into Air.

We may draw a fine Allegory from this Passage, answered I, which would have done great Honour to a learned and musty Commentator of the last Age. Now in order to put your Allegory in a clear Light, and to let you see on the other Hand that nothing is able to discourage a Set of people brought up and educated in the midst of Difficulties, some of them will tell you, that there <91> is a certain Correspondence or pre-established Harmony betwixt Soul and Body, so that though they have no more Connexion with each other, than a Harlequin Dance in our Operas has with the Death of Dido, or the Fate of Rome; yet by Virtue of this pre-established Harmony, at the same Time that certain Motions happen in the one, certain Ideas and Desires arise in the other. In short, that they are like two Clocks independent of each other, whose Weights are adjusted in such a Manner, that when this strikes One, that shall always strike Two, and so on. Your Des Cartes will tell you that upon Occasion, when Bodies without us in the material World excite certain Motions in our Body, the Soul sees certain Ideas in the intellectual World. So that in the material World, you have nothing but Extension and certain Motions, and Configurations, and whatever other Qualities you possess, and which render you so agreeable and charming, exist only in the intellectual World. Others will tell you, that by <92> Means of certain Motions in the Body, God reveals and displays certain Ideas to the Soul. But they have so little Regard for any Connexion between these Motions and our Ideas, that they affirm, we might as well hear with our Eyes, or see with our Ears, provided the Laws of the Union of the Soul and Body were different from what they now are, which is not impossible, since these Laws are merely arbitrary. One of the Laws of this Union is, that when there are certain Motions impress'd on one of the Membranes of the Eye, the Idea of Light should be raised in us, and in the same Manner, when certain Motions are made upon a Membrane of the Ear, we should perceive the Idea of Sound. And as these Things are independent of one another, why might not the Idea of Light arise from certain Motions upon the Ear, and that of Sound from certain Motions impress'd on the Eye?

And why (said the Marchioness) may we not rather suppose that there are some secret Dependencies between these Things, which your Philosophers <93> may not have been acquainted with? The Vulgar hide their Ignorance under the Vail of Obstinacy, and do not the Learned endeavour to conceal theirs in Doubts and Questions? Your Suspicion at least is reasonable, answer'd I. Our Horizon is but faintly illuminated with the Beams of a glimmering Twilight, and we pretend to see as clearly as if it was full Day. We continually act (especially with regard to Metaphysics) as Columbus would have done, if he had pretended to write us a complete Description of America, and given an Account of its Inhabitants, Rivers, and Mountains, when he had only seen a little Tract of this Country, and did not know whether it was an Island or a Continent. We reason upon the Chimeras of our own Fancy, we destroy and build Systems, we raise doubts, and think to resolve them without agreeing upon so much as their first Ideas. One of the most elegant Genius's of England, who in our Days has revived the polite Court of Charles II. in that happy Country, in a little, but <94> very valuable Piece which he wrote against one of the most celebrated Metaphysicians of our Time, compares those Gentlemen to the fine Riders in a

"Manage, who show their Address

"and Dexterity by making their Horses

"go backwards or sideways, and all

"Ways, and at length after having

"laboured round and round for two or

"three Hours, get down just where

"they got up."

However it be with these metaphysical Jockies, it is certain, that some Things produce others very different from them. The Americans without doubt would be extremely surprized to hear that certain Cyphers, as the Letters of the Alphabet, could transmit the History of a Nation to Posterity, and furnish two People with Means of communicating their Thoughts, quarrelling or making Love at the Distance of four thousand Miles, just as well as if they were present with each other. And would not a Chinese be greatly astonish'd to see that certain Marks drawn upon Lines should pro <95> duce Sounds, Concords, and in short, a Concert of Music?

As I imitate these in their Surprize, replied the Marchioness, I will imitate them too in their Docility, which they discover in embracing whatever we teach them as reasonable even at the Expence of their Self-love. We must then solemnly abjure all those Charms which you call Roses and Lilies, and submit to that Philosophy which deprives us of them, perhaps to give us in Exchange some greater Good. I admire your Moderation, answer'd I, in agreeing to this Cartesian Philosophy, which, to say Truth, is somewhat injurious to fine Ladies. When the Philosophy of Aristotle was in Vogue, who asserted that Qualities were really in Bodies, the Ladies might be something vainer of their Beauty. But now they must renounce the very Things upon which that Vanity was principally founded. It is true, that with Globules alone, a bare Disposition of Parts, you will still continue to make the same Conquests as you did before with the Help of <96> Colour itself. But it is true on the other Hand, that it is for ever gone without any Hopes of being recovered again. However, if you are afraid that this System may do you any sort of Injury you need only name the Person whom you have a desire to please, and I will promise you never to let him into the Secrets of Philosophy.

Till there appears another System to deprive us of that Disposition of Parts which this leaves us, replied she, I do not see, that we have any Thing to fear, since after all, one certain Disposition has only one certain Idea affixed to it. So that the Disposition, which excites the Idea of a fine red in you, cannot produce that of a yellow or brown in another Person. And thus I think we are secure. Seriously (answered I) I do not at all doubt that Beauties are secure in any System of Philosophy whatever. But that a certain Disposition of the Parts of a Body should excite the same Idea in all Men, is what I cannot assure you of. Who can tell whether the Leaves of these Trees that <97> I see of one Colour which I call green, may not appear to your Sight of another Colour which I should call red or yellow, or perhaps of some other Colour, of which I have no Idea? You would render me too philosophical, said the Marchioness, and after this I shall be utterly at a Loss how to converse with Mankind. You have made me already rob Bodies of Light, Colours, Taste, Smell, and every other Quality which these have never made any Scruple to grant them, and would be greatly offended at any Attempt to take them away. But not content with all this, you would have me confess that a Colour which appears green to some, should seems to others red, yellow, or some other Colour, perhaps of which I have no Idea. Is it possible to offer a greater affront to Mankind, than to contradict a Thing they are so certainly persuaded of, and assert that they do not all see Colours in the same Manner? I will venture to tell you still more, answer'd I, for to shew a Regard to Mankind whom you seem so fearful of offend <98> ing) in this Point, is not possible for any one who has ever converse with them. Who can tell whether these very Trees, which I see of one certain Height, may not appear to you of another? So that what I for Instance call ten Foot high may appear to your Eyes of a Height which I should call eight, or twenty Feet. You have a Mind to divert yourself at my Expence, said the Marchioness, interrupting me. We both agree in calling this Tree so many Foot hight, as well as in calling the Leaves green. How then can what you say be true? We agree answered I, in Words, but perhaps not in Things. Two People, one of which should give a chief Magistrate the Name of King, from whose good or ill Administration depend the Life and Properties of his Subjects, and the other should give the Name of King to a chief Magistrate, who is only the Ratifier and Guardian of the Laws of Nature, to which he, as well as the rest is subjected. These two People would both agree in the Sound, by which they <99> denote their chief Magistrate, but not in the Idea which they annex to this Sound. Both you and I had a certain Measure at first shewn us, which tho' it appears to you of a different Size from what it does to me, yet both of us agree in calling it a Foot, because we were told that Mankind distinguished such a Measure by that Name. According to this, which is the Rule of our Mensurations, we both say that this Tree is so many Foot high, thought it may appear to me of a greater or less Height than it does to you, and so every other Thing in Proportion to the different Idea we may possibly have of a Foot. Who can tell then, but you may appear to yourself, and I to you, like one of Gulliver's Brobdingnagians, on the contrary each of us may appear to my Sight as small as a Lilliputian does to yours, and who knows too but you may see the whole World after the Proportion of my Brobdingnagian, and I of the same Size as your Lilliputian; so that if it were possible for us to see with each other's Eyes (which would be a <100> good exchange for me) you would despise the diminutive Size of my Colossuses, and I should tremble at the Gigantic Stature of your Pigmies. We may easily transfer the same Way of reasoning to Colours. We here too agree upon Names, but may very probably differ in Things. Each of us for instance calls the Leaves of this Tree green, because we were at first told that the Colour of Leaves was green; but it is possible, that if Things could appear to your Eyes as they do to mine, you would be surprized to see these Trees and the whole Country clothed in a Colour which you perhaps might call Purple or some other. Because we see that all Men resemble one another in the make of their Body, when they have all two Eyes, one Mouth, two Legs, and two Hands, we are led to imagine from thence that they must all resemble each other in their Ideas, and from hence arise many Inconveniences in Society which would not have happened, had Men been a little more philosophical than they are. From <101> hence it is, that a Politician, when you are thinking on something quite different from his Projects, will plague you with a long Account of the End and Intentions of all the Privy Councils in Europe, and the Division which he has already made of Italy, for he thinks it impossible that a Man, who resembles him in his outward Appearance, should not equally interest himself in his visionary Schemes. From the same Cause a Lover will talk you dead with the History of his continual Sighs and hopeless Passion. In short, this mistaken Notion gives Birth to numberless other Inconveniences in Society. None greater, said the Marchioness, than the Philosophers who endeavour to reverse the Ideas that Mankind have form'd to themselves, and make us believe that we do not all see the same Thing of the same Size and Colour. Cannot you find some Method to explain to me whether the World really appears so different to different Persons, as you say it does?

It is not possible, answer'd I, to find such a Method as you require, un <102> less there could be any one Measure which all Men were certain of seeing absolutely of the same Size, and certain Colours which, in the same manner they could be assured, appear the same to all Eyes, and to these they might refer all other Colours, as well as all other Sizes to the Measure. As those two People who make use of the same Word King to signify their chief Magistrate, though the one be in Effect very different from the other, can never come to a clear Explanation of the different Ideas they would annex to the same Word, unless they define and compare it with other Words, and more simple Ideas, such as both Parties are agreed upon. Now red, yellow, and the smallest imaginable Measure, are in themselves such simple Ideas, that they can neither be defined nor compared with other Ideas more simple. Therefore we have no way of knowing whether all Men have the same Conceptions of them, or not, so that Mankind are much to blame in being so confident that the World ap <103> pears in the same manner to all, for it is a great Chance but they are mistaken in this Affair.

But what ill Consequences can there possibly follow from our saying that the World appears to every single Man different from what it does to all the rest? Nay, if we should go farther, and say that even the World itself does not exist, and that all these Bodies, this Sun, these Stars, and these fine Ladies, are nothing else but Dreams and Appearances. There is one Philosopher who affirm'd that a Person need only to have slept once in his Life-time, to be convinced of this. So that while some are disputing about the Manner in which the World exists, others absolutely deny that it exists at all. But though I have slept more than once in my Life, I will not preach up a System to you which would mutually destroy us both. I will rather assure you, that though we really should see the World in different Manners, yet I am willing for my own Interest to consult your Preservation. They will all agree in saying, that this Tree <104> is so many Feet high, and the Leaves green, and that you are of a just Height and a fine Complexion; and does not this Difference of Ideas diffuse an infinite Variety over the whole System of Nature, which seems even in the minutest Things to take a Pleasure in diversifying herself a thousand Ways? But what a Pleasure must you find in imagining yourself to appear to some under the Height of a wax Baby, and to others as tall as the Image of Flora at Farnese. To some of an azure Complexion, with the green Locks of a Nereid, and to others of a Vermilion Dye, and adorn'd with the rosy Tresses of Aurora, and under these different Aspects, to be agreeable to all, and adored under various Forms, as the Goddesses formerly were among the Antients. I must confess this Imagination, that every single Man sees the Face of the World in a manner different from all the rest, though (if you will have it so) it be a doubtful Point, gives me so much Pleasure, that I make no Scruple of carrying it be <105> yond Size and Colours, to Taste, Smell, and all other Qualities. I said, if you will have it so, only out of Complaisance to you, for if it be considered how very different the Nature of Things is from what it appears to our Sight, since we reckon for Instance those Bodies to be smooth and solid, which are in Reality full of Pores, Cavities and Risings, and imagine them to be indued with Colour, Taste, and other Qualities which exist only in ourselves: When we consider too that the same Bodies have a different Appearance according to their Distance and the other Circumstances in which they are seen; when all this, I say, is well considered, I do not know whether we may not affirm that every single Man sees them in a different Manner from all the rest, and that our Judgment is as much deceived in supposing that the same Things raise the same Ideas in different Persons, as it evidently is in the other Respect; at least we may reasonably doubt whether it be not so. You will say perhaps, that this is ra <106> sing Doubts and Questions to hide our Ignorance. But it is however one of the Parts of a Philosopher to search for Motives upon which he may form rational Doubts upon Things, or rather (such is our Misfortune) this is the best Part of Philosophy. However, we every Day clearly see that the same objects do in Effect appear differently to different Persons. Not to say any Thing of the more important Affairs of Morality, Law, and Politics, where what is esteemed in one Nation an Object of Veneration and Respect, is reckoned scandalous and detestable in another. Did not the Ladies in one Age drive all the Colour out of their Cheeks, and affect a pale languid Look, which were capable of inspiring the most lively Sentiments, at a Time when a painted Face would have been as shocking as a Fury. But in the next Age this very Fury becomes a Venus, and instead of Sighs and fine Speeches, the pale Beauties are recommended to the Care of a Physician, or the use of Spanish Wool. Were not the very same Gras <107> hoppers, that weary us with their troublesome chirping, called by an ancient Poet as the Sweet Harbingers of the Summer? There are whole Nations who esteem black Teeth a singular Beauty, and others who paint one Eye white, and the other red or yellow. In some other Countries, a Beau scarifies and gashes his Face to appear more agreeable to the Eyes of a brutish Creature, who is alone the Mistress of his Heart. An olive Complexion joined to a long Head, a Pair of deep sunk black Eyes, a flat Nose, and the Feet of a Baby, are Charms that make great Havock in the Hearts of the Chinese, and occasion whole Volumes of gallant Verses and Love Epistles. Our Galatea's and Venus's would not get so much as one Billet Doux, or a single Ode there, but would be looked upon as mere Caricatures. In the same Country Learning is a Step to the highest Honours of State, and there is more Ceremony in making a Doctor there, than the Polanders use in electing a Kin. Are not Music and Dancing, which are with us (as an <108> tiently among the Greeks) an Exercise for Persons of the first Rank, looked upon in Persia (as they formerly were at Rome) as scandalous Employments? And would not the same Ladies, who cause so many Commotions and Disturbances in Europe, be close confined in a Seraglio and guarded by Eunuchs in the Eastern Countries? If you will not consent to admit a different Appearance of Things between Men, yet you must allow it to be so with regard to Nations, (as for Instance, between us and the Orientals) unless you will except some particular Follies which seem to have usurped a more extensive and universal Right over Mankind. The antient Greeks, the Romans, Orientals and Americans, tho' separated from each other by such vast Tracts of Land and so many Seas, yet all agreed in the ridiculous Notion, that when the Moon was in an Eclipse (which is occasioned by the Shadow of the Earth, that deprives it of the Sun's Light) she was in great Danger, and laboured extremely hard, and imagined they could be of Service <109> to her by howling, rattling with their Timbrels, and making the most horrible Outcries and Noises they could possibly invent.

I find, said the Marchioness, you begin to grow a little more moderate after this philosophical Enthusiasm which had carried you so far, that you endeavour'd to reverse the whole Order of Things. But you have now consented to grant, that we think alike in these Opinions which you call ridiculous. As to all the rest, I am very well satisfied, if you place this Difference of Ideas at so great a Distance as is between us and the Oriental Countries.

In order to make you still easier, answered I, we will at present place these different Ways of Conception at a Distance still greater, and in Proportion as you grow a greater Proficient in Philosophy, we will bring them gradually nearer to us, till at last we will agree to put some Difference between your Ideas and mine, and from thence between the two Eyes of some Persons to whom the same Object appears <110> bigger when seen through one Eye, than it does when looked upon by the other.

How is this possible, said the Marchioness? There is no End to your visionary Fancies, and you seem resolved to put me to the utmost Proof of my Credulity. Not contented to make a Difference of Ideas between different Persons, you carry your Notions so far as to make this Difference between the two Eyes of the same Person. I must confess, I think this is a very daring Way of proceeding. Did not Gassendus, (answered I) one of the celebrated Philosophers of the last Age, affirm that he saw the Characters of a Book larger through one of his Eyes, than the other? You see the Fault is not to be thrown upon me, but upon the Eyes of Gassendus. You would find many other Persons with these sort of Eyes, if they were but as curious in examining their Sense as they are diligent in making use of them. To some Persons an Object is said to appear green, when looked at through <111> one Eye, and yellow or blue, when seen by the other. But do not we see every Day, that what one Person esteems cold, another calls hot? Or rather do not we our selves think the same Thing to be cold or hot, according to our different Dispositions? Would not the very same Thing that Milo might have thought smooth as a Mirror, appear rough as a Nettle to that luxurious Youth whose Bed was strowed with roses, and who could not Sleep for a whole Night, because a single Leaf happened to be doubled? And do not these different Sensations which are so extremely opposite, as hot and cold, smooth and rough, proceed from a different Disposition of the Sensitive Organs; from a different Affection of the Nerves, or the more or less delicate Texture of the Parts appointed to carry these Sensations to the Brain? And is it not very probably too, that these Differences may be in that Membrane of the Eye, upon which the Images of Objects are depictured, and in the Filaments of the Optic Nerve which transmit these <112> Images to the Brain? Hence it would follow, that as we receive different Sensations of hot, cold, smooth, and rough, we should find the same Difference in our Sensations of colours, and the like.

In order for me to enter into your Sentiments, said the Marchioness, you must explain what you mean by saying that the Images of Objects are depictured upon the Membrane of the Eye; and that the Optic Nerve transmits those Images to the Brain? Do you know, answered I, that an Explication of this will be no less than an Explication of a Vision it self? So much the better, said she: Indeed it seemed pretty strange to me, that after you had spoke so much upon the different Ways in which it is possible for us to see, you should be silent upon the Manner in which we really do see. I will not defer this Explication any longer, answered I, and I shall be extremely happy if my shewing you in what Manner you see me, may induce you to look upon me in a <113> different Way from what you have hitherto done.

Light is principally subjected to the two Accidents of Reflexion and Refraction. Reflexion, according to the Cartesians, happens when by a Collision of the Globules of Light with the solid Parts of Bodies, these Globules are repelled back again just as a Ball rebounds when it is struck against the Earth. And it is by this reflected Light, that we see all Bodies, the Moon, the Planets, Heavens and every Thing else, except the Sun, Stars, Fire, and all those other Bodies here below, which shine by their own Light. Refraction is caused when the Globules of Light in passing through Air, Water, Glass &c. meet with the Pores and Cavities of those Bodies, so that the Ray, which is only a Chain or Series of Globules, breaks and is turned out of its proper Path, and takes a different Direction in its Passage from what it had before. Pellucid or transparent Bodies which suffer the Light to pass through them, such as Water, Air, Diamond, and <114> Glass, are called Mediums. Hence Refraction is said to happen when the Light passes from one Medium to another. And this Refraction is greater or less (that is, the Rays are more or less broken and turned aside from their Path) in Proportion to the different Densities of the Mediums through which the Light successively passes. Thus for Example, the Rays are more broken in passing from Air into Glass, than in passing from Air to Water, because Glass is much more dense than Water, and for the same Reason they will be more broken in passing from Air to Diamond.

If this was a proper Time, (said the Marchioness) to make Criticisms upon Poets, it might be said, that Tasso has not expressed himself very accurately, when speaking of Armida, he says,

As limpid Streams transmit the unbroken Ray, &c.

Poetry in these Verses, does not seem to agree with Optics, which will not allow <115> that a Ray can be transmitted unbroken Tasso perhaps (answered I smiling) would be understood to speak of those Rays which fall perpendicularly upon Water or Crystal, that is, without being inclined, (with Regard to the Surfaces of those Mediums) either to one Side or the other. As a Thread would fall upon the Ground if it had a Weight fastned to it; for in this Case the Rays pass on without being broken, and continue to proceed in the same Path as they first set out in: But the Truth is, that poets do not address themselves to Philosophers nor to you, who have nothing but Refractions in your Mind. But they write for the People, and consequently must often make use of vulgar Prejudices and Opinions. And provided the Images be lively, the Passions strong, and the Numbers harmonious, we may pardon them a Mistake in Optics. What do you think of Ovid, who has perhaps stretch the poetic Licence too far, and made the Sun in a Day run through all the Signs of the Zodiac; whereas according to the exact Rules of Astro <116> nomy, his diurnal Course is confined to about the thirtieth Part of only one Sign? In the second Book of the Æneis, that Master-piece of sublime Poetry, there is a very fine Image, which if examined by the Laws of Optics, would lose all its Justness. Æneas, after he had been assured by Hector in a Dream of the irreparable Ruin of his Country, ascends a Turret, and there discovers the Treachery of the Greeks, whose dreadful Effects appeared from every Quarter. The Palace of Deiphobus already levelled to the Ground, his next Neighbour Ucalegon on Fire, and the Flames of that City which a ten Years Siege had attacked in vain, dreadfully reflected by the Waves of the Sea. Now in the Situation in which Æneas stood this could not possibly be; for the Opticians will tell you, that in order for him to see the flames of the City shine upon the Sea, the Sea must have been placed between him and those Flames, which it was not. But who would not excuse this Error, which can be seen only by a very few, for the sake <117> of those fine Verses which all the World admires?

But to return from Poetry to Physics, (a Transition which you have rendered very familiar to me) the manner in which the Rays of Light are broken in passing from a rare to a dense Medium, as from Air to Glass, is different from what it is when they are transmitted from a Dense to a rare Medium, as from Glass to Air. I would be understood, always to speak of the Rays which fall upon these Mediums obliquely, and with some Inclination; for as I mentioned to you before, those Rays which fall perpendicularly do not suffer any Deviation. If you suppose then, that a Ray of Light coming from the Air should fall upon the Surface of a Glass, it will be broken in such a Manner, that after its Trajection, it will be less inclined to the surface of the Glass, and immerging will approach nearer a Perpendicular. After the same Manner a Ray of Light proceeding from your Eye, would strike the Middle of this Bason, provided it were dry. But supposing it filled with <118> Water, as it now is, the Ray could not continue its Course directly to that Point as at first, but in its Transmission thro' the Water would be bent in such a Manner that it would fall on one Side, and strike the Bason in a Point nearer to us. These are all the Lines and Figures that I will draw you, to explain the present Subject.

What need is there of Lines and Figures, replied the Marchioness, to understand that a Ray of Light passing from Air into Water or Glass, will be bent in going towards it, and approach nearer to a Perpendicular? And does not the contrary happen, when the Ray passes from Glass into Air? Yes certainly, answered I; the Ray in this Case is more inclined after its Trajection to the Surface of the Air, (which immediately touches the Glass) it becomes more unlike a Perpendicular, and places it self as it were behind the Surface of the Air

These Refractions of the Rays of Light which were known though very imperfectly to the Antients, and to the <119> Consideration of which we in great Measure owe the Perfection of Astronomy, are the Cause of an infinite Number of strange and amusing Phænomena, which we every Day observe; such as Objects appearing out of their Place when viewed through a Prism. An Oar broken in the Water, and the Surprize of seeing our selves deformed and crooked when in a Bath. --- This is the very Thing, said she, interrupting me, that I lately observed when I was in the Bath, and I was extremely surprized and puzzled to find out the Reason of it. It is nothing else, answer'd I, but the Refraction which the Rays suffer in passing from Air into Water. These Refractions, besides what we have already mentioned, are the Cause too why we see the Bottom of Vessels ad Rivers much deeper than they really are, and that Sailors after a long and tedious Voyage, have the Pleasure of seeing and saluting the Land, much sooner than they would otherwise do. This too is the Reason that the Sun and Full Moon appear to our Sight of an <120> oval Figure when near the Horizon, and many other Things of the like Nature, which proceed from hence, that the Rays in their Passage from these Objects to our Eyes are refracted, and come from Places different from those where the Objects themselves are. The Eye which is not sensible of these Refractions, always refers and transports the Objects to those Places from whence the Rays appear to proceed, or in other Words, it seems them in the Direction of the Rays which penetrate and strike it. Hence it is, that the Figure and Situation of Things which are seen by refracted Rays, come to be changed. If, without knowing any Thing of the Science of Optics, the first Time I had the Honour of seeing you, a Prism had been placed before my Eyes, which by refracting the Rays which proceed from you to me, had given them the same Direction which they would have had if they had come from the Sky, you would certainly have appeared to me to have been transported into the World of Chimeras, and incompass'd with <121> an infinite Variety of Colours, and I should have intreated you to descend, as Endymion did the Moon, and addressed my self to you in some florid Description of a shady Grove or lonely Vale, in order to tempt you from the Stars. And all this fine Delusion would have been occasioned by that Direction which the Prism had given to the Rays, which would have flowed from you to my Eyes.

I fancy, said the Marchioness, that Mankind always look upon those, who are in a Condition much superior to their own, through certain Prisms, which make them appear as if they were transported to Heaven, to revel upon Ambrosia, enjoy the Conversation of the Gods, and be surrounded with Glory and Happiness; whereas the more they are elevated above others upon Earth, the more subject are they to the Sport and Caprice of Fortune. This Comparison will appear still juster, answered I, upon this account, that as when we quit the Prism, we see the Objects again return to their proper <122> Place: So when we forsake the Opinions of the Vulgar, and substitute those of good Sense in their Room, these Demigods appear nothing more than other Men, and in a Condition not greatly to be envied. But to return to our Subject: A philosophical Eye every Day discovers an infinite Number of strange and diverting Phænomena arising from the Change of Direction produced in the Rays of Light, not only by Refractions but by Reflexion too. From hence proceed all the Wonders of Concave Glasses, by the Help of which, that Poet, who wrote a Dissertation on the Nature of Bees, could discern the small Members and diminutive Parts of that noble and industrious Insect, and magnified them to that Degree that each of them seemed as big as a Dragon. With these Glasses too the Vestals rekindled their sacred Fire, whenever it happened to be extinguished. From hence arose the Fables of Archimedes and Proclus, and Ignorance and Imposture have rendered these Glasses one of the Favourite Instru <123> ments of Magic. But among the Phænomena which arise from a Change made by Reflexion in the Rays of Light, you will perhaps be surprized to find one which is every Day present with you, and which perhaps you have never yet considered as a Phænomenon, much less a Matter of Wonder. What Phænomenon can this be, said the Marchioness, to which I have paid so little Regard? It is, answered I, the Image of your self which appears beyond the Looking glass every Morning, when you hold a Consultation with the Graces in what Manner it will be best to give an artificial Negligence to your Hair. This Representation of your self proceeds from hence, that all the Rays which flow from all the Points of your Face to the Looking-glass, are reflected in such a manner to your Eye, as if they proceeded from as many other Points as there are in your Face equi-distant from each other, and as far beyond the Glass, as you are of this Side of it; and consequently you see your Image at as great <124> a Distance from the Glass, as you your self are, and exactly like you; and from the Pleasure this beauteous Representation affords you, you easily conceive what Pleasure the Original must give to others. The celebrated Milton has in his sublime Poem finely described the Delight and Surprize of Eve the first Time she surveyed herself in a Fountain,

---- That stood unmov'd

Pure as the Expanse of Heav'n --.

And this Image of herself appeared so charming, that, like another Narcissus, she afterwards ingenuously confessed to Adam, that though she thought him fair, yet he seemed,

---- ---- less fair

Less winning soft, less amiably mild,

Than that smooth wat'ry Image ----

Does not this Passage of Milton convey some malicious Insinuation, said the Marchioness? And is not his real Mean <125> ing that the Sight of a Husband gives a Woman less Pleasure than even an Image or a Shadow? However I agree, that our first Parent was in the right to admire this fine Phænomenon, and I have been greatly to blame in my Neglect of it; but we are too early accustomed to the Sight of these Things, for them to make a strong Impression upon us. If any one had told me a few Days ago, that certain Rays flowing from my Face, would have been reflected from the Looking-glass, I should have believed it to be on of those usual Enigma's which Gallantry borrows from Tradition, or founded upon the Authority of some old Romance. But I confess that from this Time, I shall survey myself in the Glass with a sort of philosophical Pleasure.

There is no greater Pleasure, continued I, to Philosophers, than that of observing the various Sportings of the Rays of Light, in passing through a gibbous Glass, or one that is convex on both Sides, and which from its Resemblance to a Grain of Lentille, is <126> termed a Lens. And upon this depends the Explanation of Vision. If two Rays of Light mutually parallel, (that is to say, which always keep the same Distance from each other without approaching nearer or removing farther off, like the Espaliers of these Walks) fall upon a Lens, by Means of that Refraction which they suffer, they are united beyond it, into one Point that is called the Focus of the Lens, which is more or less distant in Proportion as the Lens is more or less convex. So that the greater the Convexity, the less will be the Distance of the Focus, and the less the Convexity is, the Distance of the Focus will be the greater. And this Distance of the Focus is what distinguishes the Lens: As for Instance, we say this Lens has so many Feet of Focus, and another so many, just as we say such a Machine can raise the Water to such a Height, by which we would signify the Force and Activity of it. I fancy, said the Marchioness, that the Reason why this Point is called a Focus, is because a Candle may be <127> set on Fire when placed there, as I once saw done by a Person who undertook to light a Candle without the help of Fire by the Sun. He might safely have engaged, answered I, to light the Candle not only without Fire, but even with Ice. For a Lens made of Ice in a little Space of Time produces the same Effect as one that is formed of Glass. How many Impertinences might this have furnished the Poets with in that Time when their Language was,

See guarded by the watchful Powers of Love,

Fair Delia slumbers in the peaceful Grove;

Struck with the Sight, let wond'ring Mortals own,

Amidst the gloomy Shades, a Radiant Sun!

But the Reason that you give is a very good one: The burning which follows in that Point where the Lens unites the Rays which were at first parallel, and forms them into a Flame, is the very Reason why it is called the <128> Focus. All the Rays, which are not mutually parallel, but in going from a Point keep continually removing from each other, and which are called diverging Rays, unite beyond the Lens in another Point, which is always more distant than the Focus of the Lens itself. Hence we say, that a convex Lens renders the Parallel and diverging Rays converging. For those Rays are called converging, which proceeding from various Parts, have a Tendency to unite themselves in one Point. Just as the Alleys of those Woods which are formed in the shape of Stars, continually approach to one another till they all meet in the Centre. These Walks, said the Marchioness, interrupting me, might be called diverging, which regard to one in the Centre of the Wood, from whence they proceed, always removing still farther from one another. You only want, Madam, answered I, to turn over Euclid and Apollonius a little, and sometimes put on an abstracted Look, and you will be a complete Geometrician.

<129>

But to follow the Track of these Rays as we have begun ---- the more that Point, from which the diverging Rays set out, is distant from the Lens, the nearer the Lens and its Focus is that Point where these Rays unite; and so on the contrary the nearer that Point from whence the diverging Rays proceed is to the Lens, the farther off from it and its Focus is that Point where they unite; provided however, that the Point from whence these Rays proceed be not as such a Distance, that instead of uniting they are thrown out of the Glass either diverging or parallel. Opticians in order to find out the innumerable Variations which these Rays may form, make use of a certain Science called Algebra, which after having extended its Empire over all the Regions of natural Philosophy, has since by the ingenious Contrivance of Interest been appropriated to civil Uses, to determine the Chances of those Games which are the most subject to the Caprices of Fortune, and has even insinuated itself into the litigious Provinces of Law and <130> Morality. By the help of this Science, they have always certain Letters at Hand, called Symbols, connected w each other by certain Signs: With these, provided they know the Quality of the Lens, that is the Distance of its focus and of the Point from whence the Rays proceed which fall upon the Lens, or the Distance of the Point to which the Rays tend if they should fall converging upon the Lens, Opticians can tell you in a Moment, whether the Rays will unite or not, whether they will go out of the Lens diverging or parallel, and in what Point they will unite. This looks like a Species of Magic, which perhaps would not have escaped unpunished in that Age, when it was a Crime to assert the Motion of the Earth and the Existence of the Antipodes.

The Uniting of the Rays diverging from several Points, into the like Number of Points beyond the Lens, which seems in itself a v indifferent Thing, supplies us with one of the finest Sights <131> you can possibly imagine. If to a Hole made in the Window-shut of a darkned Room, you apply a Lens, and over-against this at a proper Distance there be placed a Sheet of white Paper, you will see all the Objects which are without the Window (especially those which are directly opposite to the Lens) inverted and painted upon the Paper with a Beauty, Vivacity and Softness of Colours that would make a Landskip drawn by Claude Lorrain, or a Visto by Canalleto, appear faint and languid. You will perceive the Distance of the Objects exactly the same as you would do in a Picture that is the Smallness of those Objects which are far off from a little Confusion and Obscurity, from a certain Faintness of the Colours, and in short, from a most exact Perspective the grand Secret of that happy Art of Delusion, Painting, which accompanies and assists all I have been describing. It is impossible to express to you the Pleasure that results from the Motion and Life which animates this fine Piece: The Trees <132> are really agitated by the Wind, and their Shadow follows the Motion. The Flocks bound upon the Lawns, the Shepherd really walks, and the sun Beams play upon the Waters. Nature draws her own Picture inverted and in Miniature.

It is pity, said the Marchioness, that so fine a Picture drawn by the Hand of so excellent a Master, should be turned upside down, which I am as much at a Loss to find the Reason of, as I am of the Manner in which it is form'd. Let us suppose, answered I, without Side of the window over-against the Lens an Arrow to be placed horizontally, that is, even with the Bottom of the Window: Let the Point of this Arrow be on the Right-hand, and the Feathers on the Left. Suppose too that the Extremity of the Point emits Rays upon the Lens which intirely cover it. These Rays unite beyond the Lens itself in another Point, but in passing through the Lens, instead of being on the Right-hand as they were at first (as proceeding from the Point of the Arrow which <133> we supposed to be on the Right-hand) they change their Situation, and are placed on the Left. In the same Manner the extreme Point of the Feathers throws Rays upon the Lens which unite in another Point, and after their Passage through the Lens are turned from the Left to the Right-hand. Just in the same Manner as if a Person held two Sticks, one in each Hand, and should cross them together; that which before the crossing was on the Right will afterwards be on the Left-hand, and on the contrary that which was on the Left will be on the Right. Now the Rays that fall upon the Lens cross each other, just as these two Sticks do in the Point where they touch. The same may be said, if the Arrow should be set upright. Those Rays which proceed from the Top of it, after being crossed and passing through the Lens remain at Bottom, and those which came from the Bottom at Top. Thus you see the whole Situation of the Rays is changed. That which was at Top is placed at <134> Bottom, and what was at Bottom, appears at Top. That on the Right-hand is turned to the Left, and that on the Left to the Right. If a Sheet of Paper be then placed behind the Lens in the Place where these Rays unite, they will draw you an Image of the Arrow in which the Point shall be on the Left-hand, and the Feathers on the Right, or in other Words, the Image will be the reverse of the Object. You may easily transfer what I have said of the Arrow, to a Landskip, a Piazza or any other Object, with this Difference however, that all the Parts of a Landskip or Piazza cannot be equally distant in the Picture as those of the Arrow are, because the Rays unite at different Distances from the Lens, in Proportion to the different Distance of the Points from whence they flow. If, for Instance, an Object in the middle of this Walk is seen distinctly upon the Picture, as it will be if the Paper be set in a Place where the Rays which come from it unite, those Objects which are nearer cannot be distinct, because the <135> Point where their Rays unite is at a greater Distance; neither will those Objects which are farther off be distinct, because the Point where their Rays unite is nearer the Lens, and consequently the Rays, as well of the one as the other, fall upon the Paper disjoined, and only form an Image there which will be very dim and languid, or in other Words confused; so that for those Objects which are far off, we must place the Paper nearer the Lens, and set it at a greater Distance when we would see those which are near.

It will now be necessary, said the Marchioness, that you should provide your self with a Lens, and give me a Sight of these fine Landskips all round us upon a Sheet of Paper. For I must confess, I have great Curiosity for this, both as a Woman, and as a Woman whom you have rendered half a Philosopher. I wish, answered I, that I had one with me to satisfy your Curiosity this Moment, which by what you say must be extremely strong. But I <136> will satisfie you as soon as I am able with a View of this Camera Obscura. But what you will imagine if I say to you when we are in it, suppose your self to be placed in one of your Eyes, and to see every Thing that passes there?

The Camera Obscura, represents the Inside of our Eye, which is nearly of the Shape of a Ball: The Hole in the Window is the Pupil which is in the Fore-part of the Eye, and appears in all as a dark Hole, sometimes greater, sometimes less. The Lens is the Crystalline Humour which is exactly of that Figure, and is placed over-against the Pupil, and suspended by certain little Fibres called the Ciliar Processes, which proceeding from a Coat or very thin Skin which incompasses the Inside of the Eye, are fixed in the Edge of it: The Paper on which the Image of Objects is depictured, is the Retina, composed of the Filaments and Medullary Substance of the Optic Nerve, which is fastned to the Eye behind, and is the great Channel of Communi <137> cation between that and the Brain. The Spaces which are between the Fore-part of the Eye and the Crystalline Humour, and between this and the Retina are filled with two Humours less dense than the Crystalline, but denser than the Air. By the Help of all this Apparatus, external Objects are pictured upon the Retina in Miniature just as in the Camera Obscura, and thus we see.

Really I did not think, said the Marchioness, that I should be transported thus in an Instant from the Camera Obscura, to the Inside of my Eye, nor that the fine Picture, you before described, had so much Relation to Vision. Many must have observed this, answered I, before you, without suspecting any such Relation. If there be a Hole made in any Room which is otherwise dark, and this Hole does not exceed a certain Bigness, this will be sufficient to shew you those Objects which are over-against the Hole, painted upon the opposite Wall or the Floor of the Chamber. Is there no need of the <138> Lens then, said the Marchioness, in order to the Production of this Picture? It is necessary, answered I, to give it in some Measure the finishing Stroke. But even without the Lens if the Hole be small enough, and the opposite Wall or the Floor not very distant, the Rays which pass through the Hole are near enough not to appear confused, and may draw a tolerable Picture of the External Objects upon the Wall or the Floor.

If the Crystalline Humour becomes opaque, which is what forms a Cataract, there is no other Remedy in this Case to recover the Sight, than by depressing the Crystalline Humour and cutting away the Fibres which hold it suspended, and then some faint Representation of the Objects may be drawn on the Retina of those unhappy Persons. But as the Picture in the dark Room is much weaker and more confused if there be not a Lens applied to the Hole, so is that which is made upon the Retina of these Persons, when the Crystalline Humour which is the <139> Lens of the Eye is no longer fixed over-against the Pupil. It is true, those two Humours which remain (the glassy and aqueous) help the Rays to unite, and a Convex-glass may in some Measure supply the Defect of the Crystalline Humour. It would be well if this Convex-glass could assist the Eyes under a much more terrible Distemper in which though they seem well and sound, the Retina or the Optic Nerve being weakned and obstructed cannot transmit any Sensation to the Brain of the Images of Objects, though they are clearly and distinctly drawn upon it. This Distemper, which is called a Gutta Serena, occasioned the blindness, if not of the Greek at least of the British Homer, which he interweaves in his Poem among the Beauties of Paradise Lost, the Battles of Angels, and the pregnant Abyss.

This Picture then of the Camera Obscura, said the Marchioness, which seemed of no other use than to imploy idle People, or such as have a Taste for Painting, is in Reality of very great <140> Service to us, and in some Cases even restores Sight to the Blind. Are we not obliged to Des Cartes for having rendered it so useful to us? Des Cartes, answered I, is very happy, to whom you would willingly be obliged for every Thing. But in this Case your Acknowledgments are due to an industrious German, who laid the Foundation of many Things which others have since brought to Perfection. He was the first who gave us a true Explication of Vision, which has always been a Subject of Speculation among philosophers; and consequently has had its share of ridiculous Notions. For some among the Ancients supposed certain Rays which extending themselves from the Inside of the Eye to its Superficies, pressed the Air as far as the Object to be seen, and this Air finding some Resistance from the Object, made it perceptible to the Sight, others affirmed that Vision was form'd by the Reflexion of the Sight; that is, because Rays flowed from the Eye to the Object, and were from thence reflect <141> ed back to the Eye; so that these gave it an exact Information what the Object was. Nor were there wanting some who affirmed that certain Effluvia go from the Eye, and meeting in their Way with other Effluvia of Bodies, they link themselves with these, and turning back again with them to the Eye, give the Soul a Perception of Objects. And the most rational among them asserted that extremely fine Membranes formed of Particles and Atoms are thrown off from the Surfaces of Bodies, and have mutually the same Disposition and Order, as there is in the Surfaces of the Bodies themselves from whence they proceed, and that these Membranes, which they call Simulachra or Images exactly resembling the Bodies from whence they are sent, enter into the Eye, and this is the Cause of our seeing. And it is surprizing to think that in such an Age as this, such a Country as England, there should be found any Person[1] who shutting his <142> Eyes against the Light of Things, would be again immersed in the profound Darkness of unintelligible Words, and assert that Vision is formed by Means of the different Degrees of the expansive Force communicated from Bodies to the Eye, through a Plenum, and that the different Modifications of it, as Clearness, Weakness, and Confu <143> sion in the Sight, arise from the Proportion which these expansive Forces have with the contractile one's of the Optic Nerves. However, all the Moderns except this (who like him that wrote in these latter Ages against the Circulation of the Blood was necessary to give us a Specimen of the infinite Varieties and Extravagancies of the human Mind) have rejected these chimerical Explications, the Offspring of Pride and Ignorance; neither have they greatly esteemed the Reasonings of those who thought that the Effluvia proceeded rather from the Eye, than from the Objects; since it was more reasonable they should proceed from an animate than an inanimate Substance; that the Ears, the Mouth, and the Nose, were made concave to take these Effluvia in, whereas the Eye was made convex, and therefore proper to send them out. Notwithstanding all these fine Reasons, Opticians have reduced the Eye to a perfect Camera Obscura, rejecting and extinguishing that Light which the greater Part of the Ancients supposed to pro <144> ceed from it. Indeed the august Eyes of Tiberius must perhaps be excepted, who, as 'tis said, when he waked in the Night, could for sometime see as well as in clear Day-light, which is said to arise from his emitting certain Sparks from them. You may say the same of any other Person, who is considerable enough to deserve that an Exception should be made in his Favour.

It will be necessary for us, said the Marchioness, to look upon Cats as considerable Persons, and make an Exception in their Favour too. We shall willingly grant them that Honour, answered I, only they must not take it ill if we say that the Light which seems to proceed from their Eyes in the Dark, serve only to give Light to Objects, and by this Means the Image may be drawn upon their Retina: For Vision, as well as innumerable other Things, is performed in the same Manner in Men as in Brutes; or rather we may acknowledge ourselves obliged to those for that Evidence which we have of the Manner of its Operation: For <145> in order to demonstrate it, we make use of the Eye of an Ox, or some other Animal, at the Bottom of which when the Coats are taken away, if we place a very thin and transparent Paper, we shall see the Image of those Objects to which the Eye is turned drawn upon it and inverted, just the same as in a Camera Obscura.

This shews how very capricious our Senses are. For Instance, we say that there is Heat in the Fire no less than in our Hands. Thus we confound one Motion which is in the Fire, and another that it raises in our Hands, with the Sensation of Heat, which Sensation is neither in the former nor latter. But we do not say that Colour is in our Eye as it is in Objects, though without Dispute the Colours raise some Vibration and Motion upon the Retina. and are painted upon it as strong and lively as they are upon the Objects themselves. Thus we confound two Things in the Perception of Heat, and only one in that of Colours.

<146>

It appears, said the Marchioness, that we are much obliged to our Senses in this Point, for exempting us from one Illusion at least. But do not they amply repay themselves by those many others to which they have subjected our Sight? We see only one Object, though it be looked at with both Eyes, and see it upright, though it be drawn inverted upon the Eye. You are a little too much prejudiced against the Senses, answered I, and I must for this Time undertake their Defence. Is not the Reason of all this Violence which you express against Vision, because you had not the Explanation of it from Des Cartes? Defend it, if you please, said she, without accusing me, and rescue it, if you can, from the Charge of these two Illusions which I allege against it. Would they not rather be Illusions, answered I, if we were to see an Object double which we know to be single, and that to be inverted which we know to be direct? To-morrow we will enter upon a discussion of these two Points, which Huygens, one of the great Pro <147> moters of Knowledge in the last Age, thought beyond the Reach of human Understanding. To-morrow perhaps you may know more than that great Man, but it is impossible for that Acquisition to render you more charming than you are to Day.

[1] Dr. Robert Green, Fellow of Clare-hall, Cam <142> bridge, published in 1712. a Book intitled the Principles of Natural Philosophy, in which is shewn the Insufficiency of the present Systems to give us any just Account of that Science, and the Necessity there is of some new Principles in order to furnish us with a true and real Knowledge of Nature. In this Book he undertakes to shew the Unreasonableness of the greatest part of that Philosophy hitherto received under the Name of the Corpuscularian, and then proceeds to lad down the Principles upon which alone he thinks it possible for Nature to be explained. He farther endeavours to evince the Incompetency of the present Mathematics to furnish us with any just or adequate Reasonings upon Nature, and the Necessity there is of some new Principles in that Science, which he has in some Measure explained in the Geometria Solidorum annexed to this Book, and from which he has been long assured that the squaring of the Circle is not impossible. ---- The celebrated Mr. Cotes Professor of Astronomy used to say that this Book shewed the Author to have had an extraordinary a Genius as Sir Isaac Newton's, since it must have been the Effect of Design to guard so effectually as he did against saying any one right Thing throughout so large a Treatise.

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