146 ANIMAL SENSE ORGANS 



to look at a book in his hand. The change of focus is 

 brought about by a thickening of the crystalline lens, and 

 this, in turn, results from the contraction of tiny muscles 

 within the eye-ball. Second, is the adjustment of the 

 diaphragm, known as the iris, to regulate the amount of 

 light entering the eye. That the pupil is small in bright 

 light and large in dim has been noticed by nearly every- 

 one. The change in size of the pupil is the result of 

 adjustments in the iris, whereby light is either largely 

 excluded or allowed to enter freely. 



The Perception of Color. — A feature of sight not 

 yet touched upon is the recognition of color. The ability 

 to perceive color contributes greatly to the utility of sight 

 in man, as well as to the enjoyment he derives from it. 

 The presence of bright colors in birds, fish, insects, and 

 other forms which have eyes, has led to the general belief 

 that they share with man the ability to perceive color. 

 Whether these lower forms actually do possess color vi- 

 sion or not is not easy to prove, and at the present time 

 we must admit that we know very little as to how wide- 

 spread among animals color vision is. In ourselves, color 

 vision is evidently due to special properties possessed by 

 the sensitive retinal cells. These properties are not pres- 

 ent in equal degree in the eyes of all people. Color blind- 

 ness is a condition in which some colors, usually red and 

 green, appear as neutral tints instead of as definite colors. 

 Color blindness is fairly common, particularly among 

 males, of whom four out of every hundred, on the average, 

 are color blind. 



The Perception of Distance is accurate only within 

 a comparatively limited range. This is because it de- 

 ponrls upon the combined use of the two eyes. Whenever 

 one looks at a near object his eyes are turned in to some 

 extent in order to bring both of them to bear directly. 

 The " turning in " is accomplished by the muscles which 

 move the eye-balls, and the judgment of how far the eyes 

 have been converged is made through the muscle sense 



