The _Sclerotic_, or outer covering, is the white, firm membrane, which forms the larger visible portion of the eyeball. It is covered in front by a colorless, transparent segment, termed the _cornea_, which gives the eye its lustrous appearance. Within the sclerotic, and lining it throughout, is a thin, dark membrane termed the _Choroid_. Behind the cornea it forms a curtain, called the _iris_, which gives to the eye its color. The muscles of the iris contract or relax according to the amount of light received, thus enlarging or diminishing the size of the circular opening called the _pupil_. The _Retina_ is formed by the optic nerve, which penetrates the sclerotic and choroid and spreads out into a delicate, grayish, semi-transparent membrane. The retina is one of the most _essential_ organs of vision, and consists of two layers. A spheroidal, transparent body, termed the _crystalline lens_, is situated directly behind the pupil. It varies in density, increasing from without inward, and forms a perfect refractor of the light received. The space in front of the crystalline lens is separated by the iris into two compartments called respectively the _anterior_ and _posterior chambers_. The fluid contained within them, termed the _aqueous humor_, is secreted by the cornea, iris, and ciliary processes. The space behind the crystalline lens is occupied by a fluid, called the _vitreous humor_. This humor is denser than the other fluids and has the consistency of jelly, being perfectly transparent. "The function of the crystalline lens is to produce distinct perception of form and outline."[3] The transparent humors of the eye also contribute to the same effect, but only act as auxiliaries to the lens. [Illustration: Fig. 62.] The figure on the next page represents the course of the rays of light proceeding from an object _a b_, refracted by the lens, and forming the inverted image _x y_ on the screen. All rays of light proceeding from _b_ are concentrated at _y_, and those proceeding from _a_ converge at _x_. Rays of light emanating from the center of the object _a b_ pursue a parallel course, and form the center of the image. Rays of light passing through a double convex lens converge at a point called the _focus_. In the organ of vision, if perfect, the focus is on the retina, which serves as a screen to receive the image or impression. We have a distinct perception of the outline of a distant hill, and also of a book lying before us. The rays of light we receive from these objects cannot have the same focus. How, then, can we account for the evident accommodation of the eye to the varying distances? Various theories have been advanced to explain this adjustment; such as changes in the curvature of the cornea and lens; a movement of the lens, or a general change in the form of the eyeball, by which the axis may be lengthened or shortened. [Illustration: Fig. 63.]