Principles underlying the Flicker Photometer. 73 



speed. One of these disks is provided with red and black 

 alternate sectors, the other with the same number of green 

 and black sectors. 



The two disks are illuminated with a very weak intensity and 

 are driven at a slow speed. If the illumination is very weak 

 indeed the red sectors appear to the eye black and the green 

 sectors white, in accordance with the Purkinje effect. Under 

 these conditions we naturally cannot perceive any flicker at 

 all in the disk with red and black sectors. 



At a somewhat brighter but still weak illumination, how- 

 ever, the red sectors will be visible, and we shall then see a 

 flicker with both disks. But there is one striking difference 

 in the conditions under which the flicker is best seen in the 

 two cases. 



In the case of the green and black disk we find that when 

 we look straight at the disk we see no flicker ; but by looking 

 at it obliquely we immediately see the curious " wobbly " 

 violent flicker which, it has been suggested, is mainly con- 

 nected with the part of the retina where the rods predominate. 

 In the case of the red and black disk the exact converse is the 

 case. So long as we can see a flicker at all we can perceive 

 it best by looking straight at the disk, i. e. by using the 

 central region of the retina. But when we look at the disk 

 obliquely the flicker becomes less apparent and, at a certain 

 order of illumination, vanishes entirely. In addition the 

 quality of the flicker is of a distinctly less violent kind and 

 rather resembles the fine variety which is utilized in the 

 flicker photometer. 



It may be added that the order of illumination most 

 favourable for the exhibition of the effect with the green 

 and black disk is somewhat lower than that necessary for 

 the red and black disk. A certain amount of care is thus 

 necessary in order to exhibit both effects side by side 

 successfully. 



This experiment serves to show that the central part of 

 the retina, where cones predominate, and the peripheral 

 portion where there are chiefly rods, differ noticeably in the 

 conditions under which flicker is best seen, and encourage 

 the suggestion that there is something distinct about the 

 nature of the flicker in the two cases.] 



We may therefore imagine that, as the speed of a flicker is 

 gradually increased from zero, the following changes occur. 

 First the flicker is mainly of the violent kind associated 

 with the rods, which tends to mask the perception of the 

 fine ^ariety; this is best seen by averted gaze. Next with 



