and the Intensities of Lights of different Colors. 



9 



ring. When viewed by the unassisted eye the differences in 

 hue are slight and difficult to define ; 

 but when viewed in rapid succession 

 through the adjoining tubes the dif- 

 ferences in hue are decided and may 

 be readily matched in color with 

 disks on the rotator. 



The duration of the time of 

 vision necessary to perceive contrast- 

 color. — A square screen was made, 

 as shown in fig. 10, with a square of 

 cardboard in its center, 8 cms on the 

 side, surrounded by a square of 

 translucent white paper, 35 cms on 

 the side. This screen was placed rig- 10. 



between the petroleum flame and 



the window so that its sides were equally illuminated. On 

 the side facing the window the central square appeared cyan- 

 blue. One-half of this square was covered by a strip of paper 

 so tinted with orange yellow that it appeared gray when in 

 juxtaposition with the other blue half of the square. 



On a rotator was placed two super- 

 posed black disks of the same diameter 

 (25 cms ). Near the periphery of each 

 disk was cut out an annular slot, 4 C ™ S 

 long and 6 mm wide, as shown in fig. 11. 

 By turning one of these disks on the 

 other the opening of this slot could be 

 varied from 4 cms to nothing. The rota- 

 tor is so made that the time of a rotation 

 of the disk can be held uniform and also 

 accurately measured. On gradually nar- 

 rowing the opening in the slit the dura- 

 tion of vision of the blue and gray square was reduced to 

 two" °f a second, when the blue half of the square could not 

 be distinguished from the gray half. 



When the screen was illuminated by a moi'e orange light, 

 obtained by placing a sheet of orange gelatine between the 

 lamp and the screen, the gray and blue were not distinguish- 

 able when the duration of vision was y-gVir of a second. 



In these experiments I only got 11/6 flashes of light from 

 the screen in a second, while the number of flashes required 

 to banish all flickering from the screen w r as accurately measured 

 as -^ of a second with the illumination of the lamp alone, and 

 -^ of a second when the lamp -light traversed the orange 

 gelatine film. These experiments show, what is well known, 

 viz : that as the intensity of the light diminishes the duration 



Fia.U. 



