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 Zoélogical 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 
