RELATIVE PHYSIOLOGICAL VALUE OF SPECTRAL LIGHTS 507 



make a comparative study of the stimulating value of colored lights. 

 We are therefore obliged to equate (either directly, or later by correc- 

 tions) the radiant energy, in order to have a quantitative unit for the 

 comparison of quaHtatively different hght, or for the sake of working 

 with objective, not subjective, units of Ught. 



We beUeve then, to repeat, that in the study of the evolution of 

 color vision, or of the comparative investigation of the stimulating 

 efficiency of spectral lights, it is absolutely essential that the distribu- 

 tion of the energy in the spectrum be considered, whether it is be- 

 Ueved that it will affect the results obtained or not. This is just as 

 important as is the necessity of having as pure and saturated fights as 

 possible. Although it has long been recognized that the sensation of 

 color brightness (the indication of the stimulating value to the human 

 eye) must be due to a combination of wave-length and energ}% and the 

 curve for the distribution of this energy' known (10), even the recent 

 work on the investigation of the relative efficiency of different wave- 

 lengths has, with few exceptions, failed to take this matter into con- 

 sideration. Loeb and Wasteneys (11), (12), for instance, do not even 

 refer to the possible influence of the unequal distribution of energy in 

 the spectrum on the results which they obtained. This objection, 

 among others, may also be made to the work of Hess (13). 



The disregard of this matter is, however, not universal. Hertel in 

 various investigations (14), (15), (16), determined, with a thermopile 

 and galvanom.eter, the energy content of the fights of different wave- 

 length which he used. Knicp and Minder (17), also using a thermo- 

 pile and galvanometer, measured the absolute energy in their colored 

 fights, and equated it (see p. 629 ff.). These investigators used filters 

 for obtaining their colored fights, although they attempted to throw 

 out the infra red and ultra violet rays. (See p. 634.) Blaauw (18, 

 see pp. 271 ff.), corrected for the unequal distribution of radiant energy 

 by means of calculations of its amount in the spectrum which he used, 

 and Mast (19) referred to the importance of considering the amount of 

 radiant energy. 



In the work of Lam-ens (20), Day (21) and Gross (22) a real begin- 

 ning was made in the study of the relative stimulating value of fights 

 of different wave-length but of equal radiant energy content.^ Com- 

 parative psychologists also have recognized the importance of radio- 



1 This work was done at the Harvard Zoological Laboratory and the credit 

 for its inception should be given to Prof. G. H. Parker. 



