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The rays of light proceeding from any point, whether luminous of itself, or merely reflecting light, fall on the cornea of the eye, and are by that slightly refracted: they then enter the aqueous humour, which, being terminated in front by a convex surface, further diminishes their divergence: the uvea stops all the extreme rays, and suffers only those near the axis of each pencil to pass through the pupil:[1] these enter the chrystalline, which, with the vitreous humour which succeeds it, completes the refraction, and collects all the different cones of light to points on the retina, which, it is supposed, is affected by them, and transmits the sensations excited in it by means of the optic nerve to the brain, where they become the means of conveying intelligence to the mind. It is admirably placed, so as to admit the greatest possible extent of view, as may be seen by comparing Figs. 150, 151, the latter of which shows what would be the effect of a diaphragm placed in front of the eye of such a magnitude, as to admit rays only to the central part of the chrystalline. It must necessarily be much smaller than the pupil, and would therefore afford very little light, or else allow no vision in any direction at all oblique to the axis of the eye.
Dr. Wollaston has ingeniously imitated this part of the construction of the eye in his periscopic lenses, composed of two plano-convex ones, joined at their plane sides, with an intervening diaphragm pierced at the centre (Fig. 152.). They concentrate rays of very great obliquity with wonderful accuracy.
The rays collected at the back of the eye form what is called in Optics, an image of an external object, which is of course inverted like that formed by a common convex lens. This image may be easily seen in the eye of a dead animal, when the outer coats are cut away from the back of it.(See Fig. 153.)[errata 1]
138.It will naturally occur to the reader, that an eye of an invariable form can produce the image above described, only of an object at one particular distance from it, as rays proceeding from a point beyond that distance would converge within the eye, before they met the retina, and the image of a nearer object would be
- ↑ The magnitude of the pupil is regulated so as not to let in so much light as would hurt the eye, or confuse the refraction: for this reason it is more contracted, cæteris paribus, in looking at a near object than a distant one.
Errata
