The humours are inclosed in dense membranes, which preserve the eye in a spherical form, the convexity of the fore part of which is such, that parallel rays of light en tering it are refracted to a focus exactly at the back part. Kepler demonstrated this to be the case, by removing a portion of the membrane from the back part of the eye, and covering it with oiled silk ; we then perceive a small inverted picture of the object towards which the eye is directed. The principal refraction takes place when the rays first enter the cornea ; it increases until they arrive at the centre of the crystalline, and afterwards is dimi nished a little when they pass into the vitreous humour, in proportion to the respective densities of these bodies.
When we consider the situation, the structure, and the appendages of the crystalline, we find that it is the most elaborate part of the eye, and that to which the others stem to be subservient. It was, on this account, sup posed by the earlier anatomists to be the immediate seat of vision, until Kepler discovered its refractive power, and showed that it possessed all the physical properties of a lens of the same form and density. Yet we con ceive that its use, as forming a part of the eye, is still somewhat obscure. After the lens has been removed, in consequence of disease, if there be no displacement of the other parts, and no circumstance occurs to prevent the healing process, the eye retains the faculty of vision, almost as perfectly as before the loss of the part. Yet it would be quite inconsistent with our notions of the animal econoi: y, to suppose that there should not be some spe cific use for an organ that holds so conspicuous a situa tion ; and, accordingly, three different purposes have been assigned to it, although, perhaps, rather upon theoretical grounds, than as the result of observation or experiment.
The first of these uses that has been proposed for the crystalline, is to correct the spherical aberration of the eye. When rays of light pass through a sphere, and form a focus behind it, provided the sphere be of uniform den sity in all its parts, the focus will be imperfect ; but if we suppose the sphere to consist of concentric layers, which gradually increase in density as we approach the centre, this imperfection or aberration in the focus will be corrected, and this structure has been supposed to exist in the crystalline. The same structure has been applied to correct what has been termed the Newtonian aberra tion, the defect which would ensue from rays passing through a sphere, in consequence of the different re frangibility of the component parts of the entire ray The different refractive powers of the different layers of the crystalline, it is said, will render the eye an achromatic and thus prevent the mixture and confusion of colours which would take place without this con trivance. These remarks are founded upon correct ma thematical principles, yet it has been ques•ioned, both on theoretical and on practical grounds, how far they apply to the actual condition of the eye. It has been calculated,
that no perceptible degree of aberration could take place in an instrument similar to the eye ; and in those persons who have had the crystalline removed, it is stated that the defects from the two kinds of aberration have not been observed.
The third use that has been assigned to the crystalline, is the adaptation of the eye to distinct vision at different distances. In the natural state of the organ, and when its structure is perfect, we know that those rays alone can form an accurate image which enter the cornea in nearly a parallel direction, yet we have the power of seeing ob jects distinctly which are so near to us, that the rays pro ceeding from them must enter the eye in a diverging state. If we attend accurately to our sensations when we view near objects, we shall perceive that we exercise a voluntary power, by which the conformation or shape of the eye is altered, and we shall find that a specific effort is necessary for the purpose, and that a certain length of time must elapse before the effect is produced.
The nature of this power, or the means by which the adjustment of the eye is effected, has been a very fertile field for controversy, and is a point which we can scarcely consider as even yet quite decided. Since the time of Porterfield, who made many experiments upon this sub ject, it is generally agreed that it must depend either upon a change in the general form of the eye, or in the relative position of its parts ; and the structure and situa tion of the crystalline immediately pointed it out as the pro bable agent by which the effect was produced. Leeuwen hock, by employing his microscope, thought that he had discovered muscular fibres in the lens, which, by their contraction, would render it more or less convex,and thus contribute to form an accurate image on the retina, whe ther the rays entered the cornea in a parallel or a divergent direction. Descartes adopted this opinion, and it has been more lately supported by Dr. Young. Porterfield thought there was a muscular ligament attached to the crystalline, which had the power of bringing it forwards, and thus in creasing its distance from the retina, when the ray enters the cornea from a very near object ; but Haller contro verts Porterfield's doctrine, upon the principle that the ligament in question is not muscular, and that its action, if it were so, from the general structure of the eye, would not permit this change in the disposition of the parts to take place. But to all these hypotheses an objection of very great weight has been started, that those persons who have lost the crystalline, still retain the power of al tering the focal distance of the eye, an objection which, if it could be maintained, would evidently be decisive against them.