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 



