If by means of the eye we merely had a sensation varying in intensity according to the amount, and in character according to the colours of the light before us, vision would be a sense completely comparable with taste or smell. But to produce the effect of the landscape, there is required, in addition, an exceedingly fine power of localization of the impressions made by different rays, not indeed a power which enables us to perceive, as in touch, the spot where the stimulus is applied, but one by which the relative positions of all the rays entering the eye at one time may be recognised.

It is further necessary that every nerve-termination or sensitive point in the eye shall receive only one ray at a time, and that it shall be the proper ray. In the case of insects, this is managed by every nerve-termination being placed at the bottom of a long dark-walled tube, so that it is affected by none but the ray which falls vertically on it. But in all vertebrata, as well as in the cephalopodous molluscs, of which the cuttle fishes are a familiar example, the object is achieved in much greater perfection by an optical apparatus which throws the inverted image of the landscape on a sensitive surface at the bottom of a dark chamber.

The whole optical apparatus, as well as the sensitive surface, is contained within the eyeball; the range of vision is increased, and the two eyes are enabled to act in concert, by means of muscles which turn the eyeballs; and inasmuch as the fore part of the eye must be preserved from opacity, whether from dryness, scratching, or the nutritive changes consequent on irritation, it is protected by the eyebrows, eyelids, eyelashes, and a lachrymal apparatus.