468 ALFRED O. GROSS 



1. INTRODUCTION 



Because of the abundance of arthropods, especially insects, 

 the ease with which they may be secured and their adaptability 

 to experimental work they have been used in many investiga- 

 tions of light reactions. A considerable number of these investi- 

 gations, made both in the field and in the laboratory, have 

 included tests upon the reactions of these animals to lights of 

 different colors. The light used in the field consisted of light 

 reflected from colored objects, such as flowers, colored glass, 

 paper, or cloth. For the experiments performed in the laboratory 

 the different colors were secured by filtering white light through 

 colored solutions or glass, or else from a prism spectrum. 



It is very apparent that hght reflected from objects in the 

 field, though having the advantage of a nearly natural condition 

 for insects, is very unsatisfactory since the different colors thus 

 produced are not optically pure or of the same intensity. One 

 could not be certain under these conditions whether the response 

 of the organism was due to the intensity of the light or to its 

 specific quality. Furthermore there are so many other uncon- 

 trollable factors which may be involved in experiments performed 

 in the field that the results of these investigations are of little 

 value in solving the problems of the reactions of animals to 

 lights of different colors. 



Colored lights produced by means of screens of glass or solu- 

 tions are also unsatisfactory since these lights with the possible 

 exception of the red are never pure but contain more or less of 

 a mixture of rays. Such screens have been found to transmit 

 the invisible heat rays also which when present undoubtedly 

 have their effect in stimulating organisms. 



For these reasons the use of colored light reflected from dif- 

 ferent surfaces or that produced by means of screens is to be 

 avoided in experiments involving a careful analysis of the reac- 

 tions of animals to this form of stimulus. At present the spectrum 

 is the best means of securing monochromatic lights because of 

 the purity of its colors, but it is little better than light produced 



