306 bulletin: museum of comparative zoology. 



neglected or else unsuccessfully coped with. This error consists in 

 ascribing the effect of colored light solely to its wave-length and of 

 ignoring the factor of intensity. 



In order to separate the effects of color from those of intensity in 

 monochromatic light it was necessary to have recourse to some special 

 device for measuring the intensities of lights of known but different 

 wave-length. The instrument with which this was finally accom- 

 plished was the thermo-electric apparatus of Boys, known as a radio- 

 micrometer, to be briefly described later. With the aid of this 

 instrument a fact was discovered which prohibited the use of 

 mono-chromatic light obtained through colored solutions. Since the 

 infra-red waves, so potent in thermal energy, penetrated all solutions 

 tested, it was impossible to get a measure of the intensity of the colors 

 uncombined with that of the infra-red, and it was therefore necessary 

 to resort to spectral light. 



When the physical obstacle of intensity had been overcome, atten- 

 tion was turned to biological considerations. The subject of the 

 migration of pigment in the retina was suggested to me for study, and 

 since it was still an unsettled question whether the quality of light 

 did in reality have any influence upon this phenomenon, the field 

 seemed to be one full of promise. The problem was accordingly 

 formulated as follows : — to determine whether different regions of 

 the spectrum, when reduced to equal intensity, were equipollent in 

 eliciting the migration of pigment; and if they were not, to seek, as 

 a corollary to the problem, some quantitative expression for the 

 difference in efficiency between the red and blue ends of the spectrum. 

 The research was carried on in the Zoological Laboratory of Harvard 

 University under the supervision of Prof. G. H. Parker, to whom I am 

 indebted for much in the way of valuable suggestion, kindly criticism, 

 and encouragement throughout the work. 



II. Historical Review. 



The vertebrates which have served in the study of pigment migra- 

 tion in the eye are fishes, doves, and especially frogs; while among 

 the invertebrates the crayfishes, moths, and butterflies have been 

 experimented upon. Between the years 1877, when Boll ('77) dis- 

 covered the pigment-migration in the vertebrates and 1894, when 

 Kiesel ('94) worked upon a moth, Plusia gamma, the frog chiefly has 

 served for the study of the efficiency of colored lights in evoking this 



