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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 3rOn Light and ColoursCartesian, 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, 3vDialogue II.
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 4rOn Light and Colours4vDialogue II.
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 free5rOn Light and ColoursHands.
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 5vDialogue II.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 6rOn Light and ColoursCartesians 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 6vDialogue II.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 7rOn Light and Colours
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 7vDialogue II.8rOn Light and ColoursChina 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 8vDialogue II.9rOn Light and Colours9vDialogue II.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 10rOn Light and ColoursDido, 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 10vDialogue II.
And why (said the Marchioness) may we not rather suppose that there are some secret Dependencies between these Things, which your Philosophers 11rOn Light and ColoursColumbus 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 11vDialogue II.
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 pro12rOn Light and Colours
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 12vDialogue II.
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 1rOn Light and Colours1vDialogue II.2rOn Light and ColoursGulliver'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 2vDialogue II.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 3rOn Light and ColoursEurope, 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, un3vDialogue II.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 ap4rOn Light and Colours
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 4vDialogue II.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 be5rOn Light and Coloursif 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 ra5vDialogue II.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 Gras6rOn Light and ColoursSweet 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 an6vDialogue II.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 7rOn Light and Colours
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 7vDialogue II.
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 8rOn Light and ColoursMilo 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 8vDialogue II.
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 9rOn Light and Colours
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 9vDialogue II.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,
Poetry in these Verses, does not seem to agree with Optics, which will not allow 10rOn Light and ColoursTasso 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 Astro10vDialogue II.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 11rOn Light and Colours
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 11vDialogue II.
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 12rOn Light and ColoursPrism. 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 12vDialogue II.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 1rOn Light and ColoursEndymion 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 1vDialogue II.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 Instru2rOn Light and Colours2vDialogue II.Milton has in his sublime Poem finely described the Delight and Surprize of Eve the first Time she surveyed herself in a Fountain,
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,
Does not this Passage of Milton convey some malicious Insinuation, said the Marchioness? And is not his real Mean3rOn Light and Colours
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 3vDialogue II.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 4rOn Light and ColoursLens 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,
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 Focus.4vDialogue II.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.
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 5vDialogue II.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 6rOn Light and ColoursLens, 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 6vDialogue II.
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 7rOn Light and ColoursLens 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 7vDialogue II.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 8rOn Light and ColoursLens, 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 8vDialogue II.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 Communi9rOn Light and ColoursRetina 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 Lens9vDialogue II.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 Lens10rOn Light and ColoursLens 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 10vDialogue II.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 reflect11rOn Light and ColoursSimulachra 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 PersonRobert Green, Fellow of Clare-hall, Cambridgebridge, 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.11vDialogue II.Plenum, and that the different Modifications of it, as Clearness, Weakness, and Confu12rOn Light and ColoursCamera Obscura, rejecting and extinguishing that Light which the greater Part of the Ancients supposed to pro12vDialogue II.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 1rOn Light and ColoursCamera 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.
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 Pro2rOn Light and Coloursthat Acquisition to render you more charming than you are to Day.