506 HENRY LAURENS AND HENRY D. HOOKER, JR. 



various wave-lengths be made quantitatively equal by equating the 

 radiant energy in them. 



It is true that psychologists and physicists are not unanimous re- 

 garding the respective merits of photometry (equation of brightness) 

 and of radiomicrometry (equation of radiant power) in the standardi- 

 zation of colored lights. This is due to the fact that they are primarily 

 interested in light as it influences the human eye. 



Photometric standardization (heterochromatic photometry) is a 

 method which gives us quantitatively exact information about different 

 colored lights, although as Parsons (3) says 



Photometry is one of the most faulty of physical measurements, chiefly, to 

 use a paradox, because it is in most cases, not really physical, but physiological. 



But a word should be said regarding the question of a photometric 

 standardization of colored lights for comparative work on "color 

 vision," and its seeming inadequacy. In the study of human vision 

 it is essential that the brightness value of colored light stimuli be 

 known and that work be done with visual stimuli of different wave- 

 length but of equal brightness (spectral energy at constant luminosity). 

 The use of such equally luminous stimuli for studying the stimulating 

 value of different wave-lengths on lower organisms would give us 

 quantitative results of interest but of questionable value except in so 

 far as we wished to note the effects on other sensitive protoplasm of 

 lights which are "equally bright" to the human eye. But the problem 

 of visibility (to the human eye) is one that requires a measurement 

 of the radiant energy content and a correction for the same. It deals 

 with luminosity at constant spectral energy. In the work of Nutting 

 (4), (5), Thiirmel (6), Ives (7) and Nutting (8) we have the results of 

 the study of this "visibility of radiation," which expresses the rela- 

 tive sensibility of the retina to light of different wave-lengths but of 

 equal energy (Nutting (9) ). 



Now in man, as Troland (2) says 



To determine the effect of stimulus intensity we may take any constant wave- 

 length and try our experiment with different values of the intensity, or to 

 find the influence of wave-length, we may select a definite intensity and vary the 

 wave-length. In this latter procedure we are obliged to equate the inten- 

 sities of the qualitatively different lights which we use. 



This latter problem, viz., the determination of the influence of 

 wave-length, is the one that we are primarily interested in when we 



