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pains and practice, acquired the habit of diftinguishing the appearance of objects to the eye, from the judgment which we form by fight of their colour, distance, magnitude, and figure. He enters into a long detail, in order to thew, that the visible appearance of an object is extremely different from the notion of it which experience teaches us to form by sight; and to enable the Reader to attend to the visible appearance of colour, figure, and extension, in visible things, which is no common object of thought, but must be carefully attended to by those who would enter into the philosophy of this sense, or would comprehend what is said upon it. To a man newly made to fee, he obferves, the visible appearance of objects would be the same as to us; but he would see nothing at all of their teal dimensions, as we do. He could form no conjecture, by means of his light 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 discern, 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 so perfect, would at first give him scarce any information of things without him. They would indeed present the same appearances to him as they do to us, and speak the same language; but to him it is an unknown language; and therefore he would attend only to the signs, without knowing the signification of them : whereas to us it is a language perfectly familiar; and, therefore, we take no notice of the signs, but attend only to the things signified by them.

By colour, says Dr. Reid, all men who have not been tutored by modern philosophy, understand, not a sensation of the mind, which can have no existence when it is not perceived, but a quality or modification of bodies, which continues to be the fame, whether it is seen or not. The scarlet rose, which is before me, is still a scarlet rose when I shut my eyes, and was so at midnight when no eye saw it. The colour remains, when the appearance ceases; it remains the same when the appearance changes. For when I view this scarlet rose through a pair of green spectacles, the appearance is changed, but I do not conceive the colour of the rose changed. To a person in the jaundice, it has still 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 least 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. But all men believe, that

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

The common language of mankind shews evidently, that we ought to distinguish 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 cause, which, by the mediation of various kinds or degrees of light, and of various transparent bodies interposed, produces all this variety of appearances. When a coloured body is presented, 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 propriery. 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 constitution of our nature, we are led to conceive this idea as a sign of foniething external, and are impatient till we learn its meaning. A thcufand experiments for this purpose are made every day by children, even before they coine to the use of reason. They look at things, they handle them, they put them in various positions, at different distances, and in different lights. The ideas of fight, by these means, come to be asiociated with, and readily to suggest, things external, and altogether unlike them. In particular, that idea which we have called the appearance of colour, suggests the conception and belief of some unknown quality in the body, which occasions 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 distinguished when we think or speak of them, by being afsociated with the ideas which they excite.

In like manner, gravity, magnetism, and electricity, although all unknown qualities, are diftinguished by their different effects. As we grow up, the mind acquires a habit of passing fo rapidly from the ideas of right to the external things suggeited by them, that the ideas are not in the least attended to, nor have they names given them in common language.

< When we think or speak of any particular colour, however simple the notion may seem to be, which is presented to the imagination, it is really in some fort compounded. It involves an unknown cause, and a known effect. The name of colour belongs, indeed, to the cause only, and not to the effect. But

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as the cause is unknown, we can form no distind conception of it, but by its relation to the known effect. And therefore both go together in the imagination, and are so closely united, that they are mistaken for one simple object of thought. When I would conceive those 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 must, therefore, for the sake of distinction, join to each of them, in my imagination, fome effect or some relation that is peculiar. And the most obvious distinction 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 scarlet colour, that they are apt to be mistaken for one and the same thing, although they are in reality so different and so unlike, that one is an idea in the mind, the other is a quality of body.

I conclude then, that colour is not a sensation, but a few condary quality of bodies, in the sense 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 sometimes given to the sensation which indicates it, and is occasioned by it, we never, as far as I can judge, give the name of colour to the sensation, but to the quality only. Perr haps the reason of this may be, that the appearances of the fame colour are so 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 so little interesting, that they are never attended to, but ferve only as signs to introduce the things fignified by them. Nor ought it to appear incredible, that appearances fo frequent and so familiar should have no names, nor be made objects of thought; since we have before shewn, that this is true of many sensations of touch, which are no less frequent, nor less familiar.'

After drawing some inferences from what he has advanced upon Colour, and making some reflections upon the spirit of the ancient and modern philosophy concernirg Sensation, our Author proceeds to treat of visible Figure and Extension.--Althougla there is no resemblance, 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 otherwise, we are told, with regard to its figure and magnitude. There is certainly a resemblance, Dr. Reid says, and a necessary connection, between the visible figure and magnitude of a body, and its real figure and magnitude; no man can give a reason why a scarlet colour affects the eye in the manner it does; no man can be sure, that it affects his eye in the same manner as it affects the eye of another, and that it has the same

appearance to him, as it has to another man : but we can allign a reason why a circle placed obliquely to the eye, should appear in the form of an ellipse. The visible figure, magnitude, and position, may, by mathematical reasoning, be deduced from the real; and it may be demonstrated, that every eye that sees distinctly and perfe&ly, muft, in the same fituation, see it under this form, and no other. Nay, it may be affirmed, that a man born blind, if he were instructed in the mathematics, would be able to determine the visible figure of a body, when its real figure, distance, and position are given.

If it be asked, whether there be any sensation proper to visible figure, by which it is suggested in vision? Or by what means it is presented to the mind ? our Author thinks there is no sensation appropriated to it, but that it is fuggested immediately by the material impression upon the organ, of which we are not conscious. And why, says he, may not a material impression upon the Retina suggest visible figure, as well as the material impression made upon the hand, when we grasp a ball, suggests a real figure? One and the same material impression, in one case, suggests both colour and visible figure; and in the other case, one and the same material impression fuggests hardness, heat, or cold, and real figure, all at the same time.'

Nothing shews more clearly, he says, our indisposition to at. tend to visible figure and visible extension than this, that although mathematical reasoning is no less applicable to them, than to tangible figure and extension, 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 system of science drawn out of them; not a single proposition do we find with regard to the figure and extension which are the immediate objects of sight!

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 process of reasoning, and demonstrates the relations of the several parts of his figure ; he does not consider, that the visible figure presented to his eye, is only the representative of a tangible figure, upon which all his attention is fixed ; he does not consider, that these two figures have really different properties; and that what he demonstrates to be true of the one, is not true of the other.

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

Having explained visible figure, and shewn its connection with the things signified by it, he proceeds to consider some phenomena of the eyes, and of vision, which have commonly been referred to custom, to anatomical, or to mechanical causes; but which, he conceives, must be resolved into original powers and principles of the human mind; and therefore belong properly to the subject of his enquiry. These several phenomena are the parallel motion of the eyes--our seeing objects erect by inverted images-seeing objects single with two eyes--the laws of vision in brute animals

squinting, and facts relating to itthe effect of custom in seeing objects single-Dr. Porterfield's account of single and double vifion-Dr. Briggs's theory, and Sir Isaac Newton's conjecture on this subject.

These several points he confiders at full length, and the conclusion from all he has advanced upon our seeing objects single with two eyes, is this, that, by an original property of human eyes, objects painted upon the centers of the two retinæ, or upon points similarly situate with regard to the centers, appear in the same visible place; that the most plausible attempts to account for this property of the eyes have been unsuccessful; and, therefore, that it must be either a primary law of our constitution, or the consequence of some more general law, which is not yet discovered.

Our Author proceeds now to treat of Perception in general, of the progress of nature in perception, and of the signs by which we learn to perceive distance from the eye. The ingenious Reader will find many just and curious remarks on these subjects; but it is impossible for us to give a distinct view of what is said upon them, without transgressing the bounds we must alfign to this article.

He goes on to observe, that if we compare the general principles of our constitution, 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 senses, we fhall find them to be very fimilar in their nature, and manner of operation.

• When we begin to learn our mother-tongue, says he, we perceive by the help of natural language, that they who speak to us, use certain sounds to express certain things: we imitate, the same sounds when we would express the same things, and find that we are understood,

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