508 HENRY LAURENS AND HENRY D. HOOKER, JR. 



metric determinations of lights used in studying the color vision of 

 animals (23), (24), (25) and (26). 



Mast (27) in his recent paper clearly recognizes the necessity of 

 considering the amount of radiation in the various spectral lights, and 

 says (p. 489) that. 



To ascertain the relation between wave-length and stimulating efficiency it is 

 . . . . necessary to make correction for the unequal distribution of energy in 

 the spectrum. 



Such corrections were made. No objection can be raised against 

 the physical accuracy of this method, but certainly, as Ives (7) points 

 out, the measurement of the intensity of radiation directly at the eye- 

 slit is the ideal one since it eliminates the determination of correc- 

 tions. This is the actuating m.otive which prompted the construction 

 of the apparatus which will now be described. 



an apparatus delivering spectral lights of equal radiant energy 



content'-' 



The first problem was to decide upon an instrument with which to 

 measure the radiant energy in the spectral lights. It was our intention, 

 as above outlined, net to obtain data for the construction of the curve 

 representing the distribution of energy in the spectrum, but after hav- 

 ing ascertained the energy contained in different parts of the spectrum 

 to equate this so that it had a certain unit value throughout. 



For the measurem.ent of radiation or of the radiant energy content 

 of light, there are a number of physical instruments. For articles 

 dealing with the description and application of these the following, 

 among others, may be cited: Coblentz (28), (29), Schmidt (30), Pfund 

 (31), (32), (33), (34), Day (21) and Kunz and Stebbins (35). 



We finally decided to use a Hilger therm.opile and a Broca galva- 

 nometer. The thermopile which we have has a sensitive area of 20 



^ It gives us great pleasure to express our thanks to the following: Prof. H. 

 S. Uhler of the Physical Laboratory, Yale University, for advice and assistance 

 with the spectroscopic part of our apparatus; Prof. A. H. Pfund of the Physical 

 Laboratory, Johns Hopkins University, for advice concerning the radiant energy 

 measuring apparatus; Prof. H. A. Bumstead of the Physical Laboratory, Yale 

 University, for the loan of a radiomicrometer; Prof, J. B. Watson of the Psycho- 

 logical Laboratory, Johns Hopkins University, for advice on general matters of 

 apparatus — spectroscopic and energy measuring; and Prof. Philip E. Browning 

 of the Kent Chemical Laboratory, Yale University, for the generous way in 

 which he has placed a spectrometer at our disposal. 



