Fig. 22.—Anterior section of Eye, showing changed form of lens during the act of accommodation, a voluntary action in the eye. M, Ciliary muscle; I, Iris; L, Lens; V, Vitreous Humour; A, Aqueous Humour; C, Cornea and optic axis.
But as we are able to form a distinct image of near objects, and as we notice when we turn our gaze from far to near objects there is a distinct feeling of muscular effort in the eyes, there must be some means whereby the eye can readily adapt itself for focussing near and distant objects. In a photographic camera the focus can be readily altered, either by changing the lenses, employing a lens of greater or less curvature, or by altering the distance of the screen from the lens. The last method is obviously impossible in the rigid eye-ball, and therefore the act of focussing for near and distant objects is associated with a change in the curvature of the lens, a faculty of the eye termed accommodation ([Fig. 22]), a change chiefly accomplished by the ciliary (muscle) processes, which pull the lens forwards and inwards by virtual contracting power of the ciliary muscle, and by which its suspensory ligament is relaxed, and the front of the lens allowed to bulge forward. In every case, however, accommodation is associated with contraction of the iris, the special function of which is that of a limiting diaphragm (an iris-diaphragm), [Fig. 23].
In an ordinary spherical bi-convex lens, as already pointed out, the rays of light passing through the periphery of the lens come to a focus at a nearer point than the rays passing through the central portion. In this way a certain amount of blurring of the image takes place, and which, in optical language, is termed spherical aberration. This defect of the eye is capable of correction in three possible ways, and which it may be well to repeat: 1. By making the refractive index of the lens higher at its centre than at its circumference; (2) By making the curvature of the lens less near the circumference than at the centre; (3) By stopping out the peripheral rays of light by a diaphragm. The two latter methods are those resorted to in most optical instruments.
Fig. 23.—1. Equatorial section of Eyeball, showing Iris and Ciliary Processes, after washing away the pigment, × three diameters.
2. Nerves of the Cornea of Kitten’s Eye, stained with iodine.
3. Fibres or Tubules of Lens, × 250, seen to be made up of superimposed crenated layers, and is therefore not homogeneous in structure, but made up of a number of extremely fine tubules, whose curvatures are nearly spherical.
In the human eye an attempt is made to apply all these methods, but the most important is the third, that of applying the diaphragm formed by the iris, a circular semi-muscular curtain lying just in front of the anterior surface of the lens. The iris is also furnished with a layer of pigmental cells which effectually stop out all peripheral rays of light that otherwise would pass into the eye, creating circles of diffusion of a disturbing nature to perfect vision. This delicate membrane, then, is kept in constant action by a two-fold nerve supply, derived from five or six sources, which it is unnecessary to describe at length. But the eye, with all its marvellous adaptations, has an obvious defect, that of secondary or uncorrected chromatic aberration.
Chromatic Aberration of the Eye.—White light, as previously explained, is composed of different wave lengths; and accordingly as these undulations are either longer or shorter, so do they produce on the eye the impression of different colours. We have seen how a pencil of white light may, by means of a prism, be decomposed into a multi-coloured band. In an ordinary magnifying reading-glass these coloured fringes are always seen around the margins. In practical optics chromatic aberration is partially corrected by employing two different kinds of glass in the construction of certain combined lenses. In the human eye chromatism cannot be corrected in this way; hence a blue light and a red light placed at the same distance from the eye appears to be unequally distant: the red light requiring greater accommodation in the eye than the blue, and this accordingly appears to be the nearer of the two.