SPECTRAL COLOR AND STIMULATION 483 



I have for some time held the opinion that the study of reac- 

 tions to colors, aside from its importance in comparative psy- 

 chology and physiology, ought to yield results which will throw 

 light on the nature of those chemical changes in the organisms, 

 which are associated with the reactions to light (Mast '11, pp. 

 320, 363). But for this purpose it is necessary to ascertain 

 more in detail the relation between the wave-length and stimu- 

 lation than it has been ascertained in previous work. It is highly 

 essential to use monochromatic hght of such a nature that it 

 can be measured directly or indirectly in terms suitable for 

 comparative work, preferably in terms of energy, so as to be able 

 to give the relative stimulating efficiency of the different wave- 

 lengths. It is also highly essential to ascertain the stimulat- 

 ing efficiency for all regions in the spectrum that are at all 

 effective, not merely for those that are most effective. These 

 ideas have been the guiding principles in the following experi- 

 ments. 



METHODS AND MATERIAL 



The methods used in the following experiments are based upon 

 those used in observations on orientation of organisms in a field 

 of light consisting of two beams crossing at righj:; angles ('07, pp. 

 132-134; '11, pp. 86-89). In these observations it was found 

 that among the organisms which orient all of those without image 

 forming eyes proceed toward or from a point situated between 

 the two beams and that the location of this point depends upon 

 the relative effectiveness of these two beams. If the illumina- 

 tion in the two beams is the same in quality and quantity, so 

 that the stimulating effect is the same, then the point lies half 

 way between them, provided the organisms in a single beam 

 travel parallel with the rays and do not deflect to the right or 

 the left. 



This being true, it is obvious that whenever such organisms, 

 exposed to light from two sources, proceed toward or from a point 

 midway between them it may be concluded that the light re- 

 ceived from the two sources is equal in stimulating effect, no 

 matter how much it may differ either in quantity or in quality. 



