The elasticity of the lens causes it at once to bulge
forwards, and it becomes more convex.

The ciliary muscle is thus known as the muscle of accommodation,
because it has the power to accommodate the eye to near and distant
objects. In this respect it corresponds in its use to the adjusting screw
in the opera-glass and the microscope.

330. The Eye Compared to the Photographic Camera. As an optical
instrument, the eye may be aptly compared, in many particulars, to the
photographic camera. The latter, of course, is much simpler in
structure. The eyelid forms the cap, which being removed, the light from
the object streams through the eye and passes across the dark chamber to
the retina behind, which corresponds to the sensitive plate of the camera.
The transparent structures through which the rays of light pass represent
the lenses. To prevent any reflected light from striking the plate and
interfering with the sharpness of the picture, the interior of the
photographic camera box is darkened. The pigmented layer of the choroid
coat represents this blackened lining.

In the camera, the artist uses a thumb-screw to bring to a focus on the
sensitive plate the rays of light coming from objects at different
distances. Thus the lens of the camera may be moved nearer to or farther
from the object. In order to obtain clear images, the same result must be
accomplished by the eye. When the eye is focused for near objects, those
at a distance are blurred, and when focused for distant objects, those
near at hand are indistinct. Now, in the eye there is no arrangement to
alter the position of the lenses, as in the camera, but the same result is
obtained by what is called "accommodation."

Again, every camera has an arrangement of diaphragms regulating the amount
of light. This is a rude contrivance compared with the iris, which by
means of its muscular fibers can in a moment alter the size of the pupil,
thus serving a similar purpose.

[Illustration: Fig. 130.--Illustrating the manner in which the Image of an
Object is brought to a Focus in a Photographer's Camera.]

331. The Refractive Media of the Eye. The eye is a closed chamber
into which no light can pass but through the cornea. All the rays that
enter the eye must also pass through the crystalline lens, which brings
them to a focus, as any ordinary lens would do.

Now, if the media through which the light from an object passes to reach
the retina were all of the same