experimenters have fot he most part failed to take into 



i 



sideration "he fact that the quantity as well as the quality 



of the light stimulus varies with the different colors and 

 that the former variable must be eliminated before results 

 can be attributed to differences in wave length alonei Quite 



recently (/y/Y )such work has been aone end published with the 



die 



statement thst^ results! are Qualitative -eniy becau-e there is 

 no way of comparing lights of different colors as to amounts & 



,_ <tC*tx*-^e- /* 



of radiant energj . kowev-er there have been several methods de~ 



A 

 vised by means of which the relative intensity of monochro- 



matic lights van be measured. 



T he first exact work of this nature was done by itniep 

 and Minder C^f). They usea e blue and a red color screen and 

 a green solution with sunlight as the source of light. 



XX>t&4 



The wave lengths to which each/we^e transparent "/ere known;; 

 and the energy behind eacHi was determined ny means of a therme- 

 pile and a, d'Arsonval galvanometer. The interference of the 

 long heat rays w.-is prevented by inserting a water layer in 

 a parallel sided container between tne thermopile and the 

 source pf light. 



Day (/?/( }, obtained light of known wave length by mean 



of a spectfcum from ."ernt Glower^s formed by e carbon 

 bisulfide prisu and cut down by/a diaphragm witn narrow ver- 



tical slits whicn could be adjusted so as to permit any de- 



sired region of tne spectrum to be used. In t-iis adjustment p 

 spectroscope w;-s used to determine the exact range of wave- 

 lengths passing through the plit.in each of the four illumin- 

 attions used, --- red, yellow, gfeen, and blue. He measured 

 intensity of each with a Boys r: diomicromet, er, nd balanced" 

 them by varying the number of glowers employed in the lamps. 



"hus there was one glower for tne red light light, two for the 

 yellow, and three fot the green ajid biue. 



