136 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Intensity versus Color. — As there are two kinds of 

 perception of sound — viz., sound as simple sound or 

 noise, loud or faint, and sound as tone or pitch, high or 

 low, acute or grave — so there are two kinds of percep- 

 tion of light, viz., light as simple light, bright or faint, 

 and light as color. In both sound and light the one is a 

 question of quantity, the other of quality. In both cases 

 the one is a question of strength of vibration or wave- 

 height, the other of rate of vibration or wave length. 

 In both, too, there is a limit to the range of perception. 

 In the case of sound the range is great — viz., from the 

 lowest, sixteen per second, to the highest, some thirty to 

 forty thousand per second, or more than eleven octaves. 

 In the case of light it is very restricted, four hundred 

 million-million to nearly eight hundred million-million, 

 or about one octave. 



Primary versus Mixed Colors. — Primary or pure 

 colors are such as are simple sensations. Mixed or sec- 

 ondary colors are such as may be made by mixtures of 

 the primaries in various proportions. The former are 

 few, the latter almost infinite in number. Both primary 

 and secondary colors may be again mixed with black or 

 white, and give rise to an infinite number of shades of 

 each. 



Primary Colors. — There is much difference of 

 view as to which and how many colors should be called 

 primary. Brewster (and Newton before him) made 

 three— viz., red, yellow, and blue, rejecting green because 

 it can be made by mixing blue and yellow pigments. 

 Young, and after him Helmholtz and nearly all physicists, 

 make also three, but they are red, green, and violet or 

 blue approaching violet, rejecting yellow because a 

 mixture of spectral red and spectral green makes a kind 

 of yellow. From the purely physical point of view un- 

 doubtly Helmholtz and the physicists are right, and 



