1897.] NEW YORK ACADEMY OF SCIENCES. 207 



flicker depends upon the difference in luminosity and is inde- 

 pendent of difference in color. The agreement among the re- 

 sults obtained by the three different methods employed would 

 seem indirectly to "warrant the assumption. The experiments 

 now to be described were undertaken for the purpose of obtain- 

 ing a direct proof. 



For each of four colored disks, yellow, green, blue and red, 

 respectively, a large and small gray disk was selected of the same 

 luminosity as the color. The selection was made by the method 

 of Least Flicker. In the experiments to be described both 

 large and small colored disks were used, but since the large and 

 the small disks of each color were cut from the same piece of 

 paper they are of the same luminosity. The numbers of the 

 large and small gray disks, and the colored disks of equal lumi- 

 nosities are given in the following table. Gray and colored 

 ■disks of the same luminosity are placed in the same horizontal 

 line. 



Colored disks of the same lumi- 

 nosities as the gray disks. 



Large and small red disk. 

 Large and small yellow disk. 

 Large and small green disk. 

 Large and small blue disk. 



Various pair-wise combinations were made among the large 

 and small disks respectivel3\ The disks were so chosen that 

 the difference in luminosity between two large disks combined 

 pair-wise should be the same as the diff'erence in luminosity be- 

 tween two small disks similarly combined. In the following 

 table the colors and numbers in the two columns on the left 

 refer to the large colored and gray disks of diff"ei'ent luminosities 

 which were combined pair-wise. The corresponding colors and 

 numbers in the two columns on the right refer to small colored 

 and gray disks presenting an equal difference in luminosity. 

 Thus : the luminosities of the disks referred to in column I. 

 are the same as the luminosities of the corresponding disks of 

 ■column III., the disks referred to in column II. are of the same 

 luminosity as the corresponding disks referred to in column IV. 



Large colored and gray disks which Small colored and gray disks which 

 were combined pair-wise. were combined pair-wise. 



(See Fig. Ill) 



