HEARING 345 



of physical rays which may occur, and consequently no theoretical limit 

 can be set to the color-tones perceptible by the human eye. (In practice 

 about 150 tones of color can be distinguished, and 700 degrees of bright- 

 ness; the total number of elementary chromatic sensations is in the 

 neighborhood of 30,000.) Whenever the combination of wave-lengths 

 in a beam of compound light is such that each sort of pigment is stimulated 

 equally at the same time, the sensation of whiteness is produced. 



COLOR-BLINDNESS is a technical defect of much practical and theo- 

 retical importance in which a person lacks the power to perceive certain 

 colors. About one in twenty of all males are more or less color-blind, 

 but not more than one in four hundred of females. It is often heredi- 

 tary. The most common form is that in which a confusion of red and 

 green occurs. One of the points in favor of Mrs. Franklin's theory is 

 that it explains the various sorts of color-blindness apparently better 

 than any other hypothesis, for it is necessary only to suppose a partial 

 or complete non-evolution of the color-pigments of the cones. 



SPACE-PERCEPTION. It is probable that the human eye does not at 

 first perceive the third dimension of space, that is depth away from the 

 eye. To the infant just born most likely the objects of the room appear 

 as if they were differently colored spaces on a screen hung before its 

 eyes. It is only when a child begins to use his muscles that the idea of 

 space in three dimensions begins to grow up in the mind. The sensations 

 which come from the muscles of the arms and of the legs in action are 

 probably assisted in this matter by the accommodative movements of the 

 eye-muscles. This in a word is the so-called empiric theory of space- 

 perception. The nativistic theory, on the contrary, supposes that the 

 perception of space is directly given visually from the moment the eyes 

 are first opened. 



HEARING. 



Only less informing than vision as to occurrences in the environment 

 is the experience called sound. Its nature we cannot discuss here any 

 more than we can that of light, for that is the province of philosophy. 

 It is enough to say that what comes to our afferent nervous end-organs 

 (ears) out of the environment is not sound but physical vibrations, usually 

 in the air. We and other animals make the sound and the light, in 

 some way, out of these. 



These sonorous vibrations of the environment are given out by all 

 rapidly vibrating elastic bodies: huge masses of gaseous matter in the 

 clouds disturbed by lightning, the air in a great organ-pipe, the head of 

 a drum, the string of a violin, or the cords in a singer's or speaker's 

 larynx. These and thousands of other natural and artificial objects 

 moving rhythmically, or otherwise, fast or slowly, make waves in the ear's 

 environment which the individual perceives as sound, whether noise or 

 music. These waves given out by moving objects, like hollow spheres, 

 radiate outward in all directions (in air at the rate of 332 meters (1093 



