258 VISION. 



black; a slight radial shadow will be seen within the white 

 (Charpentier). Under very favourable conditions a second and 

 oven a third may be observed. 



This effect is ascribed to the oscillatory response of the 

 retina to the stimulus. 



3. The spectrum top of Bentham. The white hemidisc is 

 divided into four equal sectors ; in each one of these a set 

 of concentric lines is described, in the first sector near the 

 periphery, in the second a quarter diameter nearer the centre, 

 and so on for the remaining sectors. 



Observe the colour effects produced on rotating the disc at 

 a medium speed ; the effect is reversed when the direction of 

 rotation is changed. 



The illumination should not be too brilliant. A slightly 

 yellow light is frequently more efficient than white light in 

 producing the effect. 



