Proceeding, therefore, upon the principle, that our per ceptions of visible distances are all acquired, we find that there areanumber of circumstances, which assist us in j forming our judgments, some of them depending upon pecu liar sensations in the eye itself, and others upon the appear ance of the object. With respect to the peculiar sensation excited in the eye, we have already offered some remarks upon the mode in which the organ accommodates itself to distinct vision at short distances, and in these cases we learn to associate certain distances with the feelings that are excited in the eye by the voluntary efforts that we use for this purpose. When an object is viewed at a moderate distance, but not such as to render it necessary to alter the figure of the lens, the eyes are directed straight for wards, but are turned inwards, so that the centre of each eye, on the optic axis, may point exactly to the object ; in this case also a peculiar sensation is excited, connect ed with the relative direction of the optic axes, which by habit we learn to associate with certain distances. This appears to be the mode which we the most frequently em ploy in judging of the distances of objects that are within our reach, and, as a proof of this, we find that those indivi duals who are deprived of the sight of one eye possess this power of judging of distances in an imperfect degree only, The circumstances in the objects themselves, which enable us to judge of their distances, are the apparent size of the object compared with what we know of its real size, the clearness with which it is seen, the vividness of its colour, and the number of objects interposed between it and the eye.
It is generally admitted, that we judge of the magnitude of objects entirely from experience. We learn, from the principles of optics, that the farther an object is removed from the eye, the smaller is the image which it forms upon the retina, and it appears that our judgment is de rived entirely from our supposed knowledge of the real size obtained by other means, and not from the size of the image. We are perpetually liable to fall into the greatest errors about the actual magnitude of objects, when we are unacquainted with their distance, so that we are aware that we can only judge of objects that are beyond our reach by experience and association. The art of land scape and architectural painting, and the science of per spective, depend upon this principle, and we immediately refer every object to the size which it is intended to repre sent, without any regard to the space which it occupies upon the canvas.
The third point which we proposed to investigate is not so easily resolved,—the mode in which we acquire our ideas of the visible position of bodies. When the rays of light form the picture upon the retina, we know, from the laws of optics, that the image must be reversed, and it has been asked, why does the object, which is reversed upon the retina, appear to us in its natural position ? If a man were born blind, and were suddenly restored to sight, would he conceive of objects as being inverted or erect ? When we speak of two points, as being one above the other, do we employ these terms in consequence of some innate principle, or of something in the structure of the eye, which directly leads us to form the conclusion, or does it depend upon knowledge which is gradually acquir ed by experience and association ? Berkeley supposes that our ideas of visible position, like those of visible dis tance and magnitude, are acquired perceptions, and that the blind man, referred to above, would have no concep tion of the relative situation of the two points, until he had learned by means of the touch, or of information de rived from other sources, that one of them was more dis tant than the other from the surface of the earth, and had thus learned to associate visible with tangible position.
Porterfield and Reid, on the other hand, maintain that we possess ideas of visible position, independent of experience or of association with the touch, and they lay down certain general principles, which they consider to be laws of the animal economy, or original principles of our nature, by which objects are necessarily seen in certain situations with respect to each other.
We are disposed to think that no arguments have been yet brought forward in this controversy, which are deci sive in favour of either opinion. Berkeley's doctrine would appear to be more in conformity with the general principles of vision, but there are certain facts which seem rather incompatible with it. In those persons who haw?, been born blind, and have acquired their sight at a mature age, we have not perceived those erroneous conceptions respecting the position of objects, which might have been expected upon this view of the subject; while the effects of pressure upon the ball of the eye favour the opinion that there is some natural connexion between the position of objects and the part of the ball on which the impression is made. We conceive, however, that the greatest part of the difficulty has arisen from some inaccuracy in our mode of considering the subject, in consequence of our not discriminating between the impression made upon the retina and the idea conveyed to the mind. It is true that the image is inverted, but we do not see the image, and we can perceive no reason why the inversion of the pic ture should convey to the brain the impression of an in verted rather than of an erect object. We apprehend, therefore, that the question which has been so often ask ed, why do not objects appear to us inverted, might be answered by asking in return, why should we expect this to be the case ? The more general question, how do we acquire our ideas of the relative position of objects, as proposed by Berkeley, is of a different aspect, and highly deserving of attention, but we are disposed to regard it as one to which we are at present unable to return a satisfac tory answer.