XV 
THE RELATION OF FACTS TO THEORIES 
3ii 
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 
