This darkness of the pupillary aperture is attributable partly to obvious causes, such as the natural contraction of the pupil or _iris_ which occurs under light--this contraction limiting the number of rays which can enter the eye. Then that black pigment which lines the iris absorbs a great deal of light; and thus, as in the case of albinos, whose eyes are deficient in pigment, or where the pupil is dilated, either through disease or by artificial agents, these obstacles for seeing into the living eye are removed. But still the main difficulties are not cleared away; and if you take for example an albino animal, such as one of those beautiful little white-furred rabbits, whose rosy eyes look like fiery opals edged with swan's down, and dilate the pupils with atropine, it is still not possible to see clearly the details of the structure within and at the back of the eye. This is by reason of the structure of the eye as an optical instrument, and because the rays of light in entering and in emerging from it undergo refraction, according to definite laws. The light which penetrates the eye traverses the transparent retina, producing the impression necessary for sight, and is partly absorbed by the black pigment of the choroid; but a great number of the rays are reflected; for here there is no exception to the general rule that some of the rays of light falling upon any substance are always reflected. These rays, in returning, are refracted through the vitreous body and lens, just as they were in entering the eye, with the object then of causing them so to converge as to produce upon the retina a clear and definite image of whatever external object they started from. Similarly, then, on their emergence they are refracted chiefly by the lens and cornea, so as to form an image in the outer air, the emergent rays coinciding in their path with that which they took when entering, and the image formed in the air being conjugated with the retinal image; being formed, therefore, on the same side, varying with the position of the lens and object, and the accommodation of the eye. Thus, then, to perceive this aerial image, derived from the retinal reflection, the eye of the observer needs to be placed in the axis of the converging rays; but since this is also the axis of the entering rays, he will of necessity in that position cut off those rays altogether of the light proceeding, say, from a lamp, or the source of light opposite to the eye to be illuminated.