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Title: An Essay Towards a New Theory of Vision Author: George Berkeley Release Date: December, 2003 [Etext #4722] [Yes, we are more than one year ahead of schedule] [This file was first posted on March 7, 2002] Edition: 10 Language: English Character set encoding: ASCII The Project Gutenberg Etext of An Essay Towards a New Theory of Vision by George Berkeley This file should be named trvsn10.txt or trvsn10.zip Corrected EDITIONS of our etexts get a new NUMBER, trvsn11.txt VERSIONS based on separate sources get new LETTER, trvsn10a.txt Prepared by Col Choat colc@gutenberg.net.au
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An Essay Towards a New Theory of Vision by George Berkeley (1685-1753)
CONTENTS Sect. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Design Distance of itself invisible Remote distance perceived rather by experience than by sense Near distance thought to be perceived by the ANGLE of the OPTIC AXES Difference between this and the former manner of perceiving distance Also by diverging rays This depends not on experience These the common accounts, but not satisfactory Some IDEAS perceived by the mediation of others No IDEA which is not itself perceived, can be the means of perceiving another Distance perceived by means of some other IDEA Those lines and angles mentioned in optics, are not themselves perceived Hence the mind does not perceive distance by lines and angles Also because they have no real existence And because they are insufficient to explain the phenomena The IDEAS that suggest distance are, 1st, the sensation arising from the turn of the eyes Betwixt which and distance there is no necessary connection Scarce room for mistake in this matter No regard had to the angle of the OPTIC AXES Judgment of distance made with both eyes, the result of EXPERIENCE 2ndly, Confusedness of appearance This the occasion of those judgments attributed to diverging rays Objection answered What deceives the writers of optics in this matter
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The cause why one IDEA may suggest another This applied to confusion and distance Thirrdly, the straining of the eye The occasions which suggest distance have in their own nature no relation to it A difficult case proposed by Dr. Barrow as repugnant to all the known theories This case contradicts a received principle in catoptrics It is shown to agree with the principles we have laid down This phenomenon illustrated It confirms the truth of the principle whereby it is explained Vision when distinct, and when confused The different effects of parallel diverging and converging rays How converging and diverging rays come to suggest the same distance A person extreme purblind would judge aright in the forementioned case Lines and angles, why useful in optics The not understanding this, a cause of mistake A query proposed, by Mr. Molyneux in his DIOPTRICS, considered One born blind would not at first have any IDEA of distance by sight This not agreeable to the common principles The proper objects of sight, not without the mind, nor the images of any thing without the mind This more fully explained In what sense we must be understood to see distance and external things Distance, and things placed at a distance, not otherwise perceived by the eye than by the ear The IDEAS of sight more apt to be confounded with the IDEAS of touch than those of hearing are How this comes to pass Strictly speaking, we never see and feel the same thing Objects of SIGHT twofold, mediate and immediate These hard to separate in our thoughts The received accounts of our perceiving magnitude by sight, false Magnitude perceived as immediately as distance Two kinds of sensible extension, neither of which is infinitely divisible The tangible magnitude of an OBJECT steady, the visible not By what means tangible magnitude is perceived by sight This further enlarged on No necessary connection between confusion or faintness of appearance, and small or great magnitude The tangible magnitude of an OBJECT more heeded than the visible, and why An instance of this Men do not measure by visible feet or inches No necessary connection between visible and tangible extension Greater visible magnitude might signify lesser tangible magnitude The judgments we make of magnitude depend altogether on experience Distance and magnitude seen as shame or anger But we are prone to think otherwise, and why The moon seems greater in the horizon than in the meridian The cause of this phenomenon assigned The horizontal moon, why greater at one time than another. The account we have given proved to be true And confirmed by the moon's appearing greater in a mist Objection answered
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The way wherein faintness suggests greater magnitude illustrated Appearance of the horizontal moon, why thought difficult to explain Attempts towards the solution of it made by several, but in vain The opinion of Dr. Wallis It is shown to be unsatisfactory How lines and angles may be of use in computing apparent magnitudes One born blind, being made to see, what judgment he would make of magnitude The MINIMUM VISIBLE the same to all creatures Objection answered The eye at all times perceives the same number of visible points Two imperfections in the VISIVE FACULTY Answering to which, we may conceive two perfections In neither of these two ways do microscopes improve the sight The case of microscopical eyes, considered The sight, admirably adapted to the ends of seeing Difficulty concerning erect vision The common way of explaining it The same shown to be false Not distinguishing between IDEAS of sight and touch, cause of mistake in this matter The case of one born blind, proper to be considered Such a one might by touch attain to have IDEAS of UPPER and LOWER Which modes of situation he would attribute only to things tangible He would not at first sight think anything he saw, high or low, erect or inverted This illustrated by an example By what means he would come to denominate visible OBJECTS, high or low, etc. Why he should think those OBJECTS highest, which are painted on the lowest part of his eye, and VICE VERSA How he would perceive by sight, the situation of external objects Our propension to think the contrary, no argument against what has been said Objection Answer An object could not be known at first sight by the colour Nor by the magnitude thereof Nor by the figure In the first act of vision, no tangible thing would be suggested by sight Difficulty proposed concerning number Number of things visible, would not at first sight suggest the like number of things tangible Number the creature of the mind One born blind would not at first sight number visible things as others do The situation of any object determined with respect only to objects of the same sense No distance, great or small, between a visible and tangible thing The not observing this, cause of difficulty in erect vision Which otherwise includes nothing unaccountable What is meant by the picture being inverted Cause of mistake in this matter Images in the eye, not pictures of external objects In what sense they are pictures In this affair we must carefully distinguish between ideas of sight and touch
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Difficult to explain by words the true Theory of Vision The question, whether there is any IDEA common to sight and touch, stated Abstract extension inquired into It is incomprehensible Abstract extension not the OBJECT of geometry The general IDEA of a triangle, considered Vacuum, or pure space, not common to sight and touch There is no idea, or kind of idea, common to both senses First argument in proof hereof Second argument Visible figure and extension, not distinct IDEAS from colour Third argument Confirmation drawn from Mr. Molyneux's problem of a sphere and a cube, published by Mr. Locke Which is falsely solved, if the common supposition be true More might be said in proof of our tenet, but this suffices Further reflection on the foregoing problem The same thing doth not affect both sight and touch The same idea of motion not common to sight and touch The way wherein we apprehend motion by sight, easily collected from what hath been said QU. How visible and tangible IDEAS came to have the same name if not of the same kind This accounted for without supposing them of the same kind OBJ. That a tangible square is liker to a visible square than to a visible circle ANS. That a visible square is fitter than a visible circle, to represent a tangible square But it doth not hence follow, that a visible square is like a tangible square Why we are more apt to confound visible with tangible IDEAS, than other signs with the things signified Several other reasons hereof, assigned Reluctancy in rejecting any opinion, no argument of its truth Proper objects of vision the language of nature In it there is much admirable, and deserving our attention Question proposed, concerning the object of geometry At first view we are apt to think visible extension the object of geometry Visible extension shown not to be the object of geometry Words may as well be thought the object of geometry, as visible extension It is proposed to inquire, what progress an intelligence that could see, but not feel, might make in geometry He cannot understand those parts which relate to solids, and their surfaces, and lines generated by their section Nor even the elements of plane geometry The proper objects of sight incapable of being managed as geometrical figures The opinion of those who hold plane figures to be the immediate objects of sight, considered Planes no more the immediate objects of sight, than solids Difficult to enter precisely into the thoughts of the above-mentioned intelligence The object of geometry, its not being sufficiently understood, cause of difficulty, and useless labour in that science
AN ESSAY TOWARDS A NEW THEORY OF VISION 1. My design is to show the manner wherein we perceive by sight the distance, magnitude, and situation of OBJECTS. Also to consider the difference there is betwixt the IDEAS of sight and touch, and whether there be any IDEA common to both senses. 2. It is, I think, agreed by immediately, cannot be seen. to the eye, it projects only point remains invariably the shorter. all that DISTANCE, of itself and For DISTANCE being a Line directed end-wise one point in the fund of the eye, which same, whether the distance be longer or
3. I find it also acknowledged that the estimate we make of the distance of OBJECTS considerably remote is rather an act of judgment grounded on EXPERIENCE than of SENSE. For example, when I perceive a great number of intermediate OBJECTS, such as houses, fields, rivers, and the like, which I have experienced to take up a considerable space, I thence form a judgment or conclusion that the OBJECT I see beyond them is at a great distance. Again, when an OBJECT appears faint and small, which at a near distance I have experienced to make a vigorous and large appearance, I instantly conclude it to be far off: And this, it is evident, is the result of EXPERIENCE; without which, from the faintness and littleness I should not have inferred anything concerning the distance of OBJECTS. 4. But when an OBJECT is placed at so near a distance as that the interval between the eyes bears any sensible proportion to it, the opinion of speculative men is that the two OPTIC AXES (the fancy that we see only with one eye at once being exploded) concurring at the OBJECT do there make an ANGLE, by means of which, according as it is greater or lesser, the OBJECT is perceived to be nearer or farther off. 5. Betwixt which and the foregoing manner of estimating distance there is this remarkable difference: that whereas there was no apparent, necessary connection between small distance and a large and strong appearance, or between great distance and a little and faint appearance, there appears a very necessary connection between an obtuse angle and near distance, and an acute angle and farther distance. It does not in the least depend upon experience, but may be evidently known by anyone before he had experienced it, that the nearer the concurrence of the OPTIC AXES, the greater the ANGLE, and the remoter their concurrence is, the lesser will be the ANGLE comprehended by them. 6. There is another way mentioned by optic writers, whereby they will have us judge of those distances, in respect of which the breadth of the PUPIL hath any sensible bigness: And that is the greater or lesser divergency of the rays, which issuing from the visible point do fall on the PUPIL, that point being judged nearest which is seen by most diverging rays, and that remoter which is seen by less diverging rays: and so on, the apparent distance still increasing, as the divergency of the rays decreases, till at length it becomes infinite, when the rays that fall on the PUPIL are to sense parallel. And after this manner it is said we perceive distance when we look only with one eye. 7. In this case also it is plain we are not beholding to experience: it
being a certain, necessary truth that the nearer the direct rays falling on the eye approach to a PARALLELISM, the farther off is the point of their intersection, or the visible point from whence they flow. 8. I have here set down the common, current accounts that are given of our perceiving near distances by sight, which, though they are unquestionably received for true by MATHEMATICIANS, and accordingly made use of by them in determining the apparent places of OBJECTS, do, nevertheless seem to me very unsatisfactory: and that for these following reasons:-9. FIRST, It is evident that when the mind perceives any IDEA, not immediately and of itself, it must be by the means of some other IDEA. Thus, for instance, the passions which are in the mind of another are of themselves to me invisible. I may nevertheless perceive them by sight, though not immediately, yet by means of the colours they produce in the countenance. We often see shame or fear in the looks of a man, by perceiving the changes of his countenance to red or pale. 10. Moreover it is evident that no IDEA which is not itself perceived can be the means of perceiving any other IDEA. If I do not perceive the redness or paleness of a man's face themselves, it is impossible I should perceive by them the passions which are in his mind. 11. Now from sect. 2 it is plain that distance is in its own nature imperceptible, and yet it is perceived by sight. It remains, therefore, that it be brought into view by means of some other IDEA that is itself immediately perceived in the act of VISION. 12. But those LINES and ANGLES, by means whereof some MATHEMATICIANS pretend to explain the perception of distance, are themselves not at all perceived, nor are they in truth ever thought of by those unskilful in optics. I appeal to anyone's experience whether upon sight of an OBJECT he computes its distance by the bigness of the ANGLE made by the meeting of the two OPTIC AXES? Or whether he ever thinks of the greater or lesser divergency of the rays, which arrive from any point to his PUPIL? Everyone is himself the best judge of what he perceives, and what not. in vain shall all the MATHEMATICIANS in the world tell me, that I perceive certain LINES and ANGLES which introduce into my mind the various IDEAS of DISTANCE, so long as I myself am conscious of no such thing. 13. Since, therefore, those ANGLES and LINES are not themselves perceived by sight, it follows from sect. 10 that the mind doth not by them judge of the distance of OBJECTS. 14. Secondly, the truth of this assertion will be yet farther evident to anyone that considers those LINES and ANGLES have no real existence in nature, being only an HYPOTHESIS framed by the MATHEMATICIANS, and by them introduced into OPTICS, that they might treat of that science in a GEOMETRICAL way. 15. The third and last reason I shall give for rejecting that doctrine is, that though we should grant the real existence of those OPTIC ANGLES, etc., and that it was possible for the mind to perceive them, yet these principles would not be found sufficient to explain the PHENOMENA of DISTANCE, as shall be shown hereafter. 16. Now, it being already shown that distance is suggested to the mind by
the mediation of some other IDEA which is itself perceived in the act of seeing, it remains that we inquire what IDEAS or SENSATIONS there be that attend VISION, unto which we may suppose the IDEAS of distance are connected, and by which they are introduced into the mind. And FIRST, it is certain by experience that when we look at a near OBJECT with both eyes, according as it approaches or recedes from us, we alter the disposition of our eyes, by lessening or widening the interval between the PUPILS. This disposition or turn of the eyes is attended with a sensation, which seems to me to be that which in this case brings the IDEA of greater or lesser distance into the mind. 17. Not that there is any natural or necessary connection between the sensation we perceive by the turn of the eyes and greater or lesser distance, but because the mind has by constant EXPERIENCE found the different sensations corresponding to the different dispositions of the eyes to be attended each with a different degree of distance in the OBJECT: there has grown an habitual or customary connection between those two sorts of IDEAS, so that the mind no sooner perceives the sensation arising from the different turn it gives the eyes, In order to bring the PUPILS nearer or farther asunder, but it withal perceives the different IDEA of distance which was wont to be connected with that sensation; just as upon hearing a certain sound, the IDEA is immediately suggested to the understanding whi..."
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