12 'Journal of Co7nparative Neurology and Psychology. 



Method of Controlling the Intensity of the Two Lights. 

 The diagrams (Figs. 1 and 2) show that the absohite intensity 

 of the entire spectrum may be changed by opening or closing the 

 slit /Sj. Since the present tests are not concerned with faint or 

 weak spectra (as such), the width of this slit, as has been stated, 

 was kept constant and as wide as was possible still to permit a 

 sharply defined spectrum at S^. The intensity of the two selected 

 bands can be altered separately in two ways: By attaching an iris 

 diaphragm (or better, possibly, an Aubert) to each of the two 

 projecting lenses, L^ and L^,, or by the use of an episcotister. 



In the experiments here reported, the intensity of the red and 

 of the green was controlled by the use of the iris diaphragm, while 

 that of the blue and of the yellow was controlled by the episcotister. 

 The chief objection to the diaphragm lies in the fact that the width 

 of the band changes slightl}^ when the diaphragm is opened or closed. 

 By an oversight, the spectroscopic reading was taken only when the 

 diaphragm was completely open. It was my intention to return 

 to the red-gr,een discrimination during the summer months when 

 it was possible to obtain sunlight, and to use both the diaphragm 

 and the episcotister, but I found that the time at my disposal did 

 not permit this. Accordingly, all the red-green tests reported below 

 were made with the arc as a source ; and all changes in intensity 

 of the two bands were made by means of the diaphragm (in table 

 of constants, e. g., "red maximum" was obtained by a wide open 

 diaphragm, "red minimum" by a fixed pinhole opening in the 

 diaphragm). On the other hand, all changes in intensity in the 

 blue-yellow tests were made by means of the episcotister. In the 

 present state of color photometry, it is desirable to have some check 

 upon photometric readings. The episcotister furnishes such a check 

 by allowing us to increase or decrease in a constant way the angular 

 opening through which the beam is allowed to pass. In the present 

 Avork, the episcotister proved eminently satisfactory. In the table 

 of constants given below, the maximum yellow, e. g., was the nonnal 

 intensity of the beam as it came from ^2 ; the minimum yellow was 

 the intensity of the beam after it had been interrupted by the 

 episcotister, set with a 30° opening (15° on each side). The angle 



