254 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
I. Introduction. 
Among the numerous papers concerning the reactions of animals 
and plants to light that have appeared in recent years, there are a 
considerable number of references to the reactions of organisms to 
different colored lights, though there are comparatively few papers 
which deal with these reactions alone. What work has been done on 
testing the reactions of organisms to different colored lights, however, 
cannot be considered as very fundamental. This is due to the fact 
that, in the majority of cases, the colored lights were obtained by 
filtering white light through colored media, either glass or colored 
solutions. This method of obtaining colored light has recently been 
shown to be practically worthless, for even the best of such screens 
also let through a large amount of dark heat rays. It is therefore 
impossible to state whether the reactions that occur under such 
conditions are to be attributed to the visible or to the invisible energy. 
Some little work has also been done with spectral light, but the 
results obtained by this method are hardly to be considered more 
valuable than those obtained by the use of screens, for the reason that 
no effort was made to render the lights used equal in the amount of 
radiant energy that they contained. If the curve representing the 
relative amount of radiant energy contained in the different regions 
of the prismatic spectrum of a common source of light be plotted, it 
will be seen to begin very low in the blue, and rise gradually until it 
reaches its maximum in the red. Some investigators have attempted 
to make what they considered the intensities of the different regions 
equal by making the brightness of the different lights, as judged by the 
human eye, the same. But this procedure is inaccurate, for the effect 
of the different lights on the human eye is not proportional to their 
energy content. Even the effect on the human eye varies with the 
amount of illumination, for in a brilliant spectrum the maximum 
brightness of the spectral colors is in the yellow, while in feeble illumi- 
nation this shifts to the green. The changing luminosity and color 
value of light with changes of light intensity — the Purkinje phenom- 
enon — is a clear indication that the relative brightness of the spectral 
colors to lower organisms is probably very different from the condi- 
tion found in the human eye, and that the human eye, therefore, 
cannot be employed as a means of measuring the relative intensities 
of light to be used in biological work. 
There is only one piece of investigation, so far as I know, in which 
