On the reappearance of the A it will be noted that it comes into view from the inner side, the x being seen before it.  If now we move the book towards its original place, the A will again disappear, coming again into view from the outer side when the o is seen before it.

328.  Inner Structure of the Eye.  Let us imagine an eyeball divided through the middle from above downwards.  Let us now start in front and observe its parts (Fig. 127).  We come first to the cornea, which has just been described.  The iris forms a sort of vertical partition, dividing the cavity of the eyeball into two chambers.

[Illustration:  Fig. 128.—­Diagram illustrating the Manner in which the Image of an Object is brought to a Focus on the Retina.]

The anterior chamber occupies the space between the cornea and the iris, and is filled with a thin, watery fluid called the aqueous humor.

The portion behind the iris forms the posterior chamber, and contains the crystalline lens and a transparent, jelly-like fluid, the vitreous humor.  This fluid is never renewed, and its loss is popularly described by the phrase, “when the eye runs out.”

Experiment 149.  The retina is not sensitive where the optic nerve enters the eyeball.  This is called the “blind spot.”  Put two ink-bottles about two feet apart, on a table covered with white paper.  Close the left eye, and fix the right steadily on the left-hand inkstand, gradually varying the distance from the eye to the ink-bottle.  At a certain distance the right-hand bottle will disappear; but nearer or farther than that, it will be plainly seen.

The vitreous humor fills about four-fifths of the eyeball and prevents it from falling into a shapeless mass.  It also serves to hold the choroid and the retina in position, and to maintain the proper relations of the inner structures of the eye.

The iris consists of a framework of connective tissue, the surface of which is lined by cells containing pigment, which gives color to the eye.

Bundles of involuntary muscular fibers are found in the substance of the iris.  Some are arranged in a ring round the margin of the pupil; others radiate from it like the spokes of a wheel.  When the circular fibers contract, the pupil is made smaller, but if these fibers relax, the radiating fibers cause the pupil to dilate more or less widely.

329.  The Crystalline Lens.  Just behind the pupil and close to the iris is a semi-solid, double-convex body, called the crystalline lens.  It is shaped like a magnifying glass, convex on each side, but with the posterior surface more convex than the anterior.  In health it is perfectly clear and transparent, and highly elastic.  When the lens becomes opaque, from change in old age, or from ulcers or wounds, we have the disease known as cataract.

[Illustration:  Fig. 129.—­Diagram showing the Change in the Lens during Accommodation.