Cosine Flicker Photometer. 



649 



method," now almost invariably used when careful photo- 

 metric tests are attempted, eliminates all want of photometric 

 symmetry in a stationary photometer and would completely 

 eliminate the above source of error, if the same portion of 

 the scale was used in both experiments. 



This instrument can be utilized either on the Equality of 

 Brightness or Flicker principle. 



This is accomplished as follows. 



Fig. 4. 



The image of the wedge, as formed by the mirror M, is 

 outside the focal length of the convex lens L 1? and this 

 together with the lens L 2 forms a real image of the illuminated 

 surfaces in the plane of the aperture at A. The eye of the 

 observer applied to the third lens L 3 sees a magnified image 

 of the aperture and the illuminated surfaces — both simul- 

 taneously in focus. 



If, now, an observer is comparing, say, a red source with 

 a green one, the field of view appears to him as shown in 

 fig. 5 a, when the lens L x is stationary. He then adopts as 

 photometric balance the position of the photometer-wedge in 

 which the red and green fields appear equally bright. 



The element of flicker may, however, be introduced by 

 modification of the method due to Rood *. The lens, L x , is 

 mounted on springs and attached to a cord passing over a pin 

 mounted eccentrically on the pulley of a small electric motor. 

 When the motor is caused to rotate the lens oscillates to and 

 fro and the boundary between the photometrical surfaces 

 * Amer. Journal of Science, 1899, p. 194. 



