THE ORGANS OF SPECIAL SENSE. 73 



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 en- 

 able near-sighted people to see at a distance. Hyper- 

 metropic or far-sighted eyes are flattened from be- 

 fore 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 be- 

 comes 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 fail- 

 ure 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 cer- 

 tain rate of vibration give the sensation of heat and 

 when their vibrations are more rapid they give the sen- 

 sation of light. Each of the primary colors of the spec- 

 trum gives off a pretty definite number of light rays 

 which travel through the air and enter the eye, the num- 

 ber of rays determining the color thrown upon the ret- 

 ina 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 aberra- 

 tion. 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 



