If a nerve be confined by a ligature or firmly pressed upon, it loses the power of transmitting impressions, but regains it on removing the pressure, provided the struc ture of the part be not injured. Now we have no proof of the existence of any substance attached to the nerve, or proceeding from it, which is concerned in the produc tion of sensation, so that we are led to conjecture that some kind of change must take place in the relation of of the parts of the nerve to each other ; and as it appears to be propagated successively along the nerve, ive may farther conjecture, that this change consists in the motion of its particles. So far we may coincide with Hartley, that there is a certain degree of plausibility in the general doc trine of the propagation of impressions by motion, hut when he attempts to describe the kind of motion, and to explain how it operates, he goes farther than either the evidence of facts or sound reasoning will authorize.
We have hitherto spoken of the sensation of sight as produced solely by light falling upon the eyc; but although this is the appropriate cause, it appears that the sensation may be occasioned by other causes, as by galvanism and by external pressure. When a weak galvanic discharge is passed through the eye, a flash of light is perceived, and the same effect is produced by a smart blow or by friction. The sensation of light is also excited if the ball of the eye be firmly pressed upon, and it is observed, that the apparent situation of the light is on the side of the eye exactly opposite to that part upon which the pressure is made. It is difficult to conceive how a ray of light, the galvanic influence, and external pressure, can all produce the same effect, but it may be thought to indicate that they all operate by exciting some kind of motion in the nervous matter, although they afford us no indication of its nature.
There is a singular state of vision, in which the eye possesses a perfect sight, so far as respects the form and position of objects, but it has only an imperfect concep tion of colour. The defect exists in different degrees ; but in most cases of this description, the individual can perceive certain colours, but is entirely insensible to others. We have a minute account of this affection given us by i\lr. Dalton, as existing in his own person ; he informs us, that when he looks at the prismatic spectrum, he can dis tinguish only three colours, which would seem to be blue, yellow, and purple, while he is incapable of seeing either the green or the red rays. The cause of this defect is not well explained ; we are not acquainted with any physical state of the eye which should have this effect upon the rays of light, nor have we any analogies to guide us, de rived from the other senses.
A very common defect of the eve, and one which is easily explained upon optical principles, is the state that is named short-sightedness. here the refractive power of the eye is greater than ordinary, so that when parallel rays enter the cornea, they converge to a focus before they ar rive at the retina. It depends upon the form of the eye be
ing too convex, and is accordingly remedied by the use of a concave lens. We notice this state of the eye principally because it has been thought to throw some light upon the power which this organ possesses of accommodating it self to distinct vision at different distances. Numerous observations prove to us that short-sightedness is more frequently met with among persons of studious habits, or who are accustomed to examine minute objects. Young children appear to be seldom short-sighted, while the de fect commences about the period when they first begin to apply themselves to books. It prevails much more among the higher classes, which probably depends in part upon the frequent use of glasses, by which any tendency that the eye might have to assume this form is confirmed, the efforts being thus prevented, which would be otherwise necessary to produce distinct vision. Now it has been supposed that the state of the eye, in these cases, is simi lar to that which it acquires when we adapt it to near ob jects, and as in those who are short-sighted there is an obvious degree of increased convexity in the cornea, it has been concluded that there must be the same convex ity in the natural adjustment of the eye. There is some foundation for this reasoning, but it must be remarked, that the experiments of Dr. Young, to which we referred above, are in opposition to this conclusion, and seem to prove that the eye is accommodated to near objects by some change which does not affect its external figure.
We now proceed, in the second place, to consider the acquired perceptions of sight, and the associations which are formed between this sense and the other classes of the perceptions of impressions. One of the first inquiries that presents itself on this subject, is the mode by which our visible ideas of distance, magnitude, and position, are obtained. It was formerly supposed that the eye judged of distances by an original law of the constitution, until this opinion was controverted by Berkeley, who demon strated that our knowledge in this respect is derived solely from experience. This conclusion is sanctioned by many circumstances of daily occurrence, where we fall into the greatest errors, in attempting to form our judgment of distance by the mere action of the eye. Berkeley's doc trine was also remarkably confirmed by the well known case of Chcselden, where a young man that was born blind, was restored to sight by a surgical operation, when he was of an age to give an account of the impressions made upon him, after he had acquired the new sense. It ap pears that at first he had no ideas of distance, but that he was obliged to correct the mistakes of his bight by the touch, until he gradually gained more correct notions.