Page:Popular Science Monthly Volume 1.djvu/480

466 essential design of this tissue is to intercept the scattered rays. But, apart from its office of conducting nourishment to the eye, and secreting the humors by means of its numerous blood-vessels, the choroid has a second optical design to fulfil, which now brings us to the characteristic signs of the eye.

As you will perceive from Fig. 4, which gives a section of an eye from life, the choroid, after having accompanied the sclerotic to the edge of the cornea, goes on expanding anteriorly, and from henceforth bears another name, that of the iris, or, as we might say, the rainbow tissue. As this process, which likewise contains a quantity of pigment, lies behind the transparent cornea, it can be observed in all its minutiæ; and, on account of the rayed arrangement adopted by its fibres, it is frequently called the eyeball, or star. The iris is broken in the centre by an opening to which we apply the term pupil, being the visual aperture. It usually seems to be black.

The presence of the iris greatly diminishes the extent of surface designed for the reception of light; the whole pencil of rays that falls on the cornea, as supposed, in Fig. 3, not reaching the retina, but only as shown in Fig. 4, on that section of it which enters the pupil. Though much of the volume of light is thereby lost, the restriction is highly beneficial, by sharpening the image on the retina; for the refraction of the rays is much more equal in the centre than toward the margin.

The iris, however, has a still more important function to perform. It regulates the entrance of the light, being furnished with a muscular apparatus (ciliary muscle), which provides that in strong light the pupil contracts, and in duller light expands. Thus the iris plays the part of a so-called movable diaphragm, a common appliance in optical instruments, used to dull the light for the purpose of seeing better. You cannot but have observed this play of the pupil, and how it accommodates itself to the volume of light; nor can you be ignorant that the iris with its varied coloring from light blue to deepest brown is what we know as the color of the eye. It is perhaps less well known to you, however, that the peculiar pigment required for the darker colors of the iris comes only as we advance in life, and that, therefore, we all commence our earthly course with blue eyes; a fact already known to Aristotle.

The crystalline lens, which is held fast in its place by a very fine tissue, as shown in Fig. 4, from the curve of its surface, and its strong power of refraction, plays an important part by conducting the collected light to the picture on the retina. It has, however, another and extremely important design, which must here be carefully considered.

The requirements made on an optical instrument depending on lenticular effect, are different according as it is expected to project images of nearer or more distant objects. The light with very divergent rays, and proceeding from near objects, is collected to a picture