Principles underlying the Flicker Photometer. 63 



was utilized in observing both brightness and flicker, and 

 the illumination o£ the surfaces viewed was the same in each 

 case. Under these circumstances, the two sensations would 

 be compared under exactly similar conditions, and if the 

 physiological principle of both instruments is the same, 

 should lead to similar results. 



Comparison of the Yellow-spot Effect for Equality 

 of Brightness and Flicker Photometers. 



The Rood type of photometer is particularly convenient 

 for the investigation of this point for, by keeping the lens 

 still, one may use it as an ordinary instrument, and then, by 

 oscillating the lens, compare the brightness of the two photo- 

 metrical surfaces, still at the same distance from the eye, by 

 a flicker method. 



For this purpose the writer made use of the arrangement 

 shown in fig. 2. The surfaces of a Ritchie Wedge, W, con- 

 sisting of fine unglazed cardboard, were illuminated by the 



Fig. 2. 

 Arrangement of Rood Type of Photometer. 



+ \ / + 



A \ / B 



two heterochromatic sources of light to be studied, A and B. 

 In front of the wedge a convex lens, L, is mounted on springs 

 and attached to a cord which passes round the pin P mounted 

 eccentrically on the pulley of the electric motor M. In front 

 of the lens was placed a small black screen S pierced with 

 an aperture 5 cms. high and 2 cms. wide. 



If, now, the motor is stationary, the observer balances the 

 brightness of the red and green by the equality of brightness 

 method. But when the motor is set in rotation a band of 

 flicker is produced between the coloured surfaces, since the 



