Page:Structure and functions of the body; a hand-book of anatomy and physiology for nurses and others desiring a practical knowledge of the subject (IA structurefunctio00fiskrich).pdf/79

 the ball, instead of being round, is flattened from above down and so bulges in front. Consequently, owing to the greater distance from the lens to the retina, images are formed in front of the retina. Only nearby objects can be seen clearly, because the farther the object from the eye the farther in front of the retina the image is formed. Concave glasses are worn to enable near-sighted people to see at a distance. Hypermetropic or far-sighted eyes are flattened from before backward and can see only objects at a distance clearly, as those nearby form images behind the retina. For such eyes convex glasses are worn.

As the ordinary person approaches middle life, he becomes able to see better at a distance than near to. This presbyopia, as it is called, which is practically far-sightedness, is due to a partial loss of the power of accommodation in the lens, the result of a general loss of elasticity in the parts.

Another very common defect is astigmatism, a failure of the rays to focus upon a point, owing generally to a flattening in the surface of the cornea.

Color perception is also an important function of the eye. The waves of hyperluminous ether when of a certain rate of vibration give the sensation of heat and when their vibrations are more rapid they give the sensation of light. Each of the primary colors of the spectrum gives off a pretty definite number of light rays which travel through the air and enter the eye, the number of rays determining the color thrown upon the retina and the velocity determining the intensity of the color. Occasionally when light is passing through into the eye it is broken up as in a prism and the person gets a sensation as of all sorts of colors, chromatic aberration. Total or partial absence of sensitiveness to color is called color blindness. It is commonest in the form of inability to distinguish between red and green and is probably due to a defect in the retina.

Sometimes a hair follicle on the lid becomes infected