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pains and practice, acquired the habit of distinguishing the appearance of objects to the eye, from the judgment which we form by fight of their colour, diftance, magnitude, and figure. He enters into a long detail, in order to fhew, that the visible appearance of an object is extremely different from the notion of it which experience teaches us to form by fight; and to enable the Reader to attend to the vifible appearance of colour, figure, and extenfion, in vifible things, which is no common object of thought, but must be carefully attended to by those who would enter into the philofophy of this fenfe, or would comprehend what is faid upon it. To a man newly made to fee, he obferves, the vifible appearance of objects would be the fame as to us; but he would fee nothing at all of their real dimenfions, as we do. He could form no conjecture, by means of his fight only, how many inches or feet they were in length, breadth, or thickness. He could perceive little or nothing of their real figure; nor could he difcern, that this was a cube, that a sphere; that this was a cone, and that a cylinder. His eye could not inform him, that this object was near, and that more remote. In a word, his eyes, though ever fo perfect, would at first give him scarce any information of things without him. They would indeed present the fame appearances to him as they do to us, and fpeak the fame language; but to him it is an unknown language; and therefore he would attend only to the figns, without knowing the fignification of them: whereas to us it is a language perfectly familiar; and, therefore, we take no notice of the figns, but attend only to the things fignified by them.

By colour, fays Dr. Reid, all men who have not been tutored by modern philofophy, understand, not a sensation of the mind, which can have no exiftence when it is not perceived, but a quality or modification of bodies, which continues to be the fame, whether it is feen or not. The scarlet rofe, which is before me, is ftill a fcarlet rofe when I fhut my eyes, and was fo at midnight when no eye faw it. The colour remains, when the appearance ceases; it remains the fame when the appearance changes. For when I view this fcarlet rofe through a pair of green fpectacles, the appearance is changed, but I do not conceive the colour of the rofe changed. To a perfon in the jaundice, it has ftill another appearance; but he is easily convinced, that the change is in his eye, and not in the colour of the object. Every different degree of light makes it have a different appearance, and total darkness takes away all appearance, but makes not the leaft change in the colour of the body. We may, by a variety of optical experiments, change the appearance of figure and magnitude in a body, as well as that of colour; we may make one body appear to be ten. Ball men believe, that

as

as a multiplying glafs does not really produce ten guineas out of one, nor a microscope turn a guinea into a ten pound peice; fo neither does a coloured glafs change the real colour of the object feen through it, when it changes the appearance of that co

lour.

The common language of mankind fhews evidently, that we ought to diftinguifh between the colour of a body, which is conceived to be a fixed and permanent quality in the body, and the appearance of that colour to the eye, which may be varied a thousand ways, by a variation of the light, of the medium, of the eye itself. The permanent colour of the body is the caufe, which, by the mediation of various kinds or degrees of light, and of various tranfparent bodies interpofed, produces all this variety of appearances. When a coloured body is prefented, there is a certain apparition to the eye, or to the mind, which we have called the appearance of colour. Mr. Locke calls it an idea; and, indeed, it may be called fo with the greatest propriety. This idea can have no existence but when it is perceived. It is a kind of thought, and can only be the act of a percipient or thinking being. By the conftitution of our nature, we are led to conceive this idea as a fign of fomething external, and are impatient till we learn its meaning. A thoufand experiments for this purpose are made every day by children, even before they come to the ufe of reafon. They look at things, they handle them, they put them in various pofitions, at different diftances, and in different lights. The ideas of fight, by these means, come to be affociated with, and readily to fuggeft, things external, and altogether unlike them. In particular, that idea which we have called the appearance of colour, fuggefts the conception and belief of fome unknown quality in the body, which occafions the idea; and it is to this quality, and not to the idea, that we give the name of colour. The various colours, although in their nature equally unknown, are eafily diftinguished when we think or speak of them, by being af fociated with the ideas which they excite. In like manner, gravity, magnetifim, and electricity, although all unknown qualities, are diftinguished by their different effects. As we grow up, the mind acquires a habit of paffing fo rapidly from the ideas of fight to the external things fuggefted by them, that the ideas are not in the leaft attended to, nor have they names given them in common language.

• When we think or speak of any particular colour, however fimple the notion may feem to be, which is prefented to the imagination, it is really in fome fort compounded. It involves an unknown caufe, and a known effect. The name of colour belongs, indeed, to the caufe only, and not to the effect. But

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as the caufe is unknown, we can form no diftin& conception of it, but by its relation to the known effect. And therefore both go together in the imagination, and are fo clofely united, that they are mistaken for one fimple object of thought. When I would conceive thofe colours of bodies which we call scarlet and blue; if I conceive them only as unknown qualities, I could perceive no distinction between the one and the other. I muft, therefore, for the fake of diftinction, join to each of them, in my imagination, fome effect or fome relation that is peculiar. And the moft obvious diftinction is, the appearance which one and the other makes to the eye. Hence the appearance is, in the imagination, so closely united with the quality called a fearlet colour, that they are apt to be mistaken for one and the fame thing, although they are in reality fo different and fo unlike, that one is an idea in the mind, the other is a quality of body.

I conclude then, that colour is not a fenfation, but a fecondary quality of bodies, in the fense we have already explained; that it is a certain power or virtue in bodies, that in fair day-light exhibits to the eye an appearance, which is very familiar to us, although it hath no name. Colour differs from other fecondary qualities in this, that whereas the name of the quality is fometimes given to the fenfation which indicates it, and is occafioned by it, we never, as far as I can judge, give the name of colour to the fenfation, but to the quality only. Perhaps the reafon of this may be, that the appearances of the fame colour are fo various and changeable, according to the different modifications of the light, of the medium, and of the eye, that language could not afford names for them. And, indeed, they are fo little interefting, that they are never attended to, but ferve only as figns to introduce the things fignified by them. Nor ought it to appear incredible, that appearances fo frequent and fo familiar fhould have no names, nor be made objects of thought; fince we have before fhewn, that this is true of many fenfations of touch, which are no lefs frequent, nor lefs familiar.'

After drawing fome inferences from what he has advanced upon Colour, and making fome reflections upon the spirit of the ancient and modern philofophy concerning Senfation, our Author proceeds to treat of visible Figure and Extenfion.-Although there is no refemblance, nor, as far as we know, any necefiary connection, between that quality in a body which we call its colour, and the appearance which that colour makes to the eye; it is quite otherwife, we are told, with regard to its figure and magnitude. There is certainly a refemblance, Dr. Reid fays, and a neceffary connection, between the vifible figure and mag

nitude of a body, and its real figure and magnitude; no man can give a reason why a fcarlet colour affects the eye in the manner it does; no man can be fure, that it affects his eye in the fame manner as it affects the eye of another, and that it has the fame appearance to him, as it has to another man: but we can affign a reason why a circle placed obliquely to the eye, should appear in the form of an ellipfe. The vifible figure, magnitude, and position, may, by mathematical reasoning, be deduced from the real; and it may be demonftrated, that every eye that fees diftinctly and perfectly, muft, in the fame fituation, fee it under this form, and no other. Nay, it may be affirmed, that a man born blind, if he were inftructed in the mathematics, would be able to determine the vifible figure of a body, when its real figure, diftance, and pofition are given.

If it be asked, whether there be any fenfation proper to vifible figure, by which it is fuggefted in vifion? Or by what means it is presented to the mind? our Author thinks there is no fenfation appropriated to it, but that it is fuggefted immediately by the material impreffion upon the organ, of which we are not confcious. And why, fays he, may not a material impreffion upon the Retina fuggeft vifible figure, as well as the material impreffion made upon the hand, when we grasp a ball, fuggefts a real figure? One and the fame material impreffion, in one cafe, fuggefts both colour and vifible figure; and in the other cafe, one and the fame material impreffion fuggefts hardnefs, heat, or cold, and real figure, all at the fame time.'

Nothing fhews more clearly, he fays, our indifpofition to attend to vifible figure and visible extenfion than this, that although mathematical reafoning is no lefs applicable to them, than to tangible figure and extenfion, yet they have entirely escaped the notice of Mathematicians. While that figure, and that extenfion which are objects of touch, have been tortured ten thousand ways for twenty centuries, and a very noble fyftem of science drawn out of them; not a fingle propofition do we find with regard to the figure and extenfion which are the immediate objects of fight!

When the Geometrician draws a diagram with the most perfect accuracy; when he keeps his eye fixed upon it, while he goes through a long procefs of reafoning, and demonftrates the relations of the feveral parts of his figure; he does not confider, that the vifible figure prefented to his eye, is only the representative of a tangible figure, upon which all his attention is fixed; he does not confider, that these two figures have really different properties; and that what he demonftrates to be true of the one, is not true of the other.

This leads our Author to enter a little into the mathematical confideration of visible figure, which he calls the Geometry of Vi fibles; but for what he fays upon this point, we refer our Readers to the book.

Having explained vifible figure, and fhewn its connection with the things fignified by it, he proceeds to confider fome phenomena of the eyes, and of vifion, which have commonly been referred to cuftom, to anatomical, or to mechanical caufes; but which, he conceives, muft be refolved into original powers and principles of the human mind; and therefore belong properly to the fubject of his enquiry. Thefe feveral phenomena are the parallel motion of the eyes-our feeing objects erect by inverted images-feeing objects fingle with two eyes-the laws of vifion in brute animals-fquinting, and facts relating to itthe effect of cuftom in feeing objects fingle-Dr. Porterfield's account of fingle and double vifion-Dr. Briggs's theory, and Sir Ifaac Newton's conjecture on this fubject.

These several points he confiders at full length, and the conclufion from all he has advanced upon our feeing objects fingle with two eyes, is this, that, by an original property of human eyes, objects painted upon the centers of the two retina, or upon points fimilarly fituate with regard to the centers, appear in the fame vifible place; that the most plaufible attempts to account for this property of the eyes have been unsuccessful; and, therefore, that it must be either a primary law of our conftitution, or the confequence of fome more general law, which is not yet discovered.

Our Author proceeds now to treat of Perception in general, of the progrefs of nature in perception, and of the figns by which we learn to perceive distance from the eye. The ingenious Reader will find many juft and curious remarks on these fubjects; but it is impoffible for us to give a distinct view of what is faid upon them, without tranfgreffing the bounds we must affign to this article.

He goes on to obferve, that if we compare the general principles of our conftitution, which fit us for receiving information from our fellow creatures by language, with the general principles which fit us for acquiring the perception of things by our fenfes, we fhall find them to be very fimilar in their nature, and manner of operation.

When we begin to learn our mother-tongue, fays he, we perceive by the help of natural language, that they who speak to us, ufe certain founds to exprefs certain things: we imitate the fame founds when we would exprefs the fame things, and find that we are understood.

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