RELATIVE PHYSIOLOGICAL VALUE OF SPECTRAL LIGHTS 505 



following the use of such inadequate methods. In a recent paper 

 Hess (1) attempts to give an objective, quantitative comparison with 

 the human eye, by comparing the influence of different colored lights on 

 lower organisms with the influence of these same lights on the pupiQar 

 reflex of man. In this work, however, Hess makes use of a method, in 

 addition to using colored glasses, against which objections may be 

 raised. He varied the absolute intensity of his source, thus changing 

 the luminosity of the colored lights, until a just perceptible change in 

 the .size of the pupil was observed. The relative results which he 

 obtained with different colored lights he compared with those ob- 

 tained on lower organisms. But this is an analytic rather than a 

 synthetic method. Furthermore, it gives rise to physiological (sub- 

 jective) differences in the apparent relative brightness value of lights 

 of different color (Purkinje phenomenon). To have observed the 

 comparative amount of change in size of the pupil with colored lights 

 of the same intensity would have given more reliable results. 



To return to the objections against the descriptive qualitative com- 

 parisons of colored lights as judged by the human eye, it is well known 

 how variable such comparisons are likely to be on account of the vari- 

 able physiological factors involved. In the first place the state of 

 adaptation (light or dark) of the human retina is all important in the 

 judgment of the relative luminosity of lights of different color. In the 

 second place the absolute intensity of the colored lights must be con- 

 sidered, for the part of the spectrum of apparent maximum, brightness 

 shifts as the intensity of illummation is changed, being higher in a 

 strong than in a weak spectrum. Luminosity or apparent bright- 

 ness, as Trojand (2) points out, is a psychological variable which can- 

 not be expressed in physical units. It depends upon the absolute 

 value of retinal illumination and upon the general state of sensitivity 

 of the visual system. 



But the chief fault which we have to find is the failure on the part 

 of many investigators to take the unequal distribution of energy into 

 consideration. It seems evident, to obtain quantitative results on a 

 purely objective basis which will permit of comparisons being made 

 exclusive of subjective sensations and which will allow a conclusive cor- 

 relation of facts, that any comparative investigation of the influence 

 of spectral lights must take into consideration this unequal distribu- 

 tion of radiant energy in the spectrum. The quantity of this radia- 

 tion in the various colored lights should be ascertained, corrections 

 made for the unequal distribution or, what is better still, the lights of 



THE AMERICAN JOURNAL OF PHYSIOLOGY, VOL. 44, NO. 4 



