On the right the lens is arranged for distant vision, the ciliary muscle is relaxed and the ligament D is tense, so flattening by its compression the front of the lens C; on the left the muscle A is acting, and this relaxes the ligament and allows the lens B to become more convex, and so fitted for the vision of near objects.]

The lens is not placed loosely in the eyeball, but is enclosed in a transparent and elastic capsule suspended throughout its circumference by a ligament called the suspensory ligament.  This ligament not only retains the lens in place, but is capable of altering its shape.  In ordinary conditions of the eye, this ligament is kept tense so that the front part of the lens is flattened somewhat by the pressure on it.

All around the edge, where the cornea, sclerotic, and choroid meet, is a ring of involuntary muscular fibers, forming the ciliary muscle.  When these fibers contract, they draw forwards the attachment of the suspensory ligament of the lens, the pressure of which on the lens is consequently diminished.  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.