Whichever view be adopted, the ciliary muscle takes an active part in the process. According to the observations of Hueck, the focal distance may be changed about three times in a second. The accommodation from a near to a distant object is effected much more rapidly than the converse process.
There are two well-known forms of defective vision in which this power of adapta tion is very much limited—viz., short-sightedness or myopia; and long-sightedness or presbyopia. The limitation, however, is not due to a defect in the muscular apparatus to which we have referred, but to an abnormality either in the curves or in the density of the refracting media. In short-sightedness from too great a refractive power from either cause, the rays from objects at the ordinary range of distinct vision are brought too soon to a focus, so as to cross one another, and begin to diverge before they fall on the retina; the eye in this case being able to bring to the proper focus on the retina only those rays which were previously diverging at a large angle from a very near object. The correction for this deficiency is accomplished by interposing between the eye and indistinctly-seen objects a concave lens, with a curvature just sufficient to throw the images of external objects at the ordinary distance of distinct vision backwards upon the retina. In long-sightedness, on the other hand, there is an abnormal diminution of the refractive power from too fiat a cornea, a deficient aqueous humor, or a flattening of the lens, so that the focus is behind the retina. This defect is corrected by convex lenses, which increase the convergence of the rays of light. Long-sightedness, as its name presbyopia indicates, usually comes on at a comparatively advanced period of life, while short-sightedness is most commonly met with in young persons; but both these rules present occasional exceptions; and the common belief that the latter affection naturally disappears after the middle period of life, is altogether erroneous.
We have already noticed the most essential use of the iris—viz., its power, under the influence of light upon the retina, of modifying the size of the pupil, so as to regulate the amount of light entering the eye. But this is not its only use; one of its
offices being to prevent the passage of rays through the circumferential part of the lens, and thus to obviate the indistinctness of vision which would arise from spherical aber ration (the unequal refraction of the rays passing through the center and near the mar gin of the lens), in the same manner as the diaphragms employed by the optician. But there are additionally two other means by which this spherical aberration is prevented, which so well illustrate the wondrous mechanism of the eye, that we cannot omit to notice them. They are described by prof. Wharton Jones as follows: (1.) "The surfaces of the dioptric parts of the eye are not spherical, but those of the cornea and posterior surface of the lens are hyperbolical, and that of the anterior surface of the lens elliptical—configurations found by thebry fitted to prevent spherical aber ration. This discovery was made at a time when it was not known but that the diop tric parts of the eye had spherical surfaces.
(2.) " The density of the lens diminishing [as we have already shown] from the center to its periphery, the circumferential rays are less refracted than they would have been by a homogeneous lens with similar surfaces. This elegantly simple con trivance has been hitherto inimitable by human art."—The Actonian Prize Treatise, 1851, p. 50.
Chromatic aberration, which is caused by the unequal refrangibility of the primitive rays of which white light is composed, when transmitted through an ordinary lens, whereby colored fringes are produced, is practically corrected in the eye, although it is doubtful whether it is entirely absent. The provision, however, on which the achrom atism depends has not been determined with certainty, probably because we do not yet know the relative refractive and dispersive powers of the cornea and humors of the eye. Sir David Brewster denies that the chromatic aberration receives any correction in the eye, and maintains that it is imperceptible only in consequence of its being extremely slight.