XV THE RELATION OF FACTS TO THEORIES 311 



stituted, that it responds differently to the varying 

 stimuli of the separate rays. Thus it responds to 

 the rays of long wave-length by the formation of 

 simple pigments, and to those of short wave-length 

 by the formation of more complex pigments, so that 

 there is a relation between the molecular weight of 

 the pigments produced and the wave-lengths of the 

 rays producing them. If we combine this statement 

 with the previous one as to the relation existing 

 between the colour of a pigment and its molecular 

 weight, it would seem that red light produces red 

 pigments of simple composition ; violet light, violet 

 pigments of complex composition, and so on. 

 Further, the previous suggestion as to the gradual 

 appearance of the rays, accounts for the order in 

 which the pigments appear. 



Before proceeding further with Dr. Simroth's 

 theory, we may note that so far it is in its details 

 largely an adding together of the suggestions of 

 others. Thus, the suggestion that the action of red 

 light on organisms is to cause them to produce red 

 pigment, is merely the suggestion as to the photo- 

 graphic sensibility of living beings which has already 

 been made in various quarters. As every one 

 knows, the essence of the process of photography 

 lies in the fact that certain chemical substances are 

 extremely sensitive to the action of light. When 

 the photographer exposes a plate to light in his 

 camera, the sensitive substance with which it is 

 covered is rapidly decomposed by the action of the 

 light, and dark-coloured substances are produced. 

 So great is this sensitiveness that the brightest rays 

 of the incident light correspond to the darkest parts 



