EYES OF THE CHAMELEON.

wishes to have its own way, and there is no accordance of action. But when the creature is undisturbed, the

eye which receives the strongest impression propagates it to the common centre, and prevails on the other eye

EYES OF INSECTS.

to follow that impression, and direct itself to the same object. The chameleon, moreover, may be asleep on one side, and on the other awake. Thus says Dr. Weissenborn, who came to these conclusions:-" When cautiously approaching my specimen at night with a candle, so as not to awaken the whole animal by the shaking of the room, the eye turned towards the flame would open and begin to move, and the corresponding side to change colour; whereas, the other side would remain for several seconds longer in its torpid and unchangeable state, with its eye shut."

The simple eyes of insects resemble those of higher animals, but the compound eyes may well fill us with amazement. These organs are two in number. When examined with a microscope, their surface appears divided into a multitude of sixsided facets, between which minute hairs are generally seen. The number of facets is various. Thus, in the ant, there are 50; in the common housefly 4,000; in some dragon-flies, upwards of 12,000; and more than double this number has been counted in other insects.

EYES OF CRUSTACEOUS ANIMALS.

As the eyes of insects cannot be moved, the number of facets appears to compensate for this want. Here, however, it may be asked, do they see distinctly with every one of these, or with only one? To this it may be replied, that the probability is, that only one acts perfectly at a time, the insect seeing imperfectly with the rest. Thus, we perceive in all its distinctness a part of a landscape, while we see confusedly the whole. The eyes of crustaceous animals, the crab and lobster, for example, are compound, like those of insects. In the two compound organs of the latter there are about 5000 eyes.

In all vertebrate animals the eye occupies a cavity, more or less surrounded with bone, its walls being formed by the frontal bone above, and by those of the face below. This cavity is lined with a layer, or cushion of fat, on which the eyeball reposes. It contains also the muscles which move the eye, the lachrymal gland, and the orifices into the duct, conveying the fluid secreted from the eye into the nostrils.

In man, the eyes, protected by the edge of the overarching forehead, from which arise the eyebrows, look forward, and have a comparatively bounded sphere of vision. In the lower mammalia, especially the timid and gentle, as the deer or hare, each eye is situated laterally, looking from opposite sides of the

EYES OF THE GIRAFFE.

head, and is large and projecting. The sphere of vision is thus extended, so that the approach of enemies, numerous as they are, may be more readily discerned. In the giraffe, this projection of the eye is so great,

Eyes of the Giraffe.

that, in connexion with its lateral position, the animal is really as capable of seeing behind it as before, as the engraving will clearly show. Hence arises one of the great difficulties which the hunter experiences of getting within gun-shot.

CHAPTER III.

FUNCTION OF VISION-REFRACTION OF LIGHT-COLOUR, MAGNITUDE, DISTANCE, AND MOTION.

Ir is now desirable to consider the human eye as an optical instrument. In doing so, we must look again at the part already described as the retina-the delicate, transparent, filmy web, which is an expansion of the optic nerve. This nervous tissue, be it observed, is alone sensible to the rays of light.

Here, however, another fact requires to be noticed. When rays of light pass from a rare into a denser medium, they are refracted or bent out of the straight line in which they naturally proceed. The rays of light, therefore, emanating in a straight line from a given object, on touching the cornea pass through the aqueous humour, which being denser than air causes them to be thus refracted. After passing through this humour they proceed through the lens, suffering still further refraction, next through the vitreous humour;