﻿Phenomena in Photometry . 127 



appearance. After this point the blue also fades away until 

 nothing can be seen. 



There is, however, a distinct difference between the be- 

 haviour of the very big screen and the very small ones. In 

 the case of the latter the Purkinje effect is much weaker. 

 Both colours seem to fade away together, and, by the time 

 the Purkinje effect is really noticeable on the big screen, the 

 very small screen can scarcely be seen at all. 



In fact, to see the Purkinje effect really well, it is necessary 

 to stand quite close up even to the big field. 



A very interesting physiological explanation of these effects 

 has been given by M. Sartori in a recent paper *. 



Dotted about over the retina are two varieties of light- 

 perceiving organs, known from their appearance as the 

 ki rods " and the " cones ,s respectively. The rods, it is 

 thought, are sensitive to light but cannot perceive colour. 

 Light of any colour appears to them white, but they are 

 most sensitive to blue light. They are, moreover, sensitive 

 to very weak light, but as the illumination is increased they 

 become, as it were, saturated, and do not respond any further. 

 The cones, on the other hand, perceive colour, but are most 

 sensitive to yellow-green light, and while they do not respond 

 at the low illuminations- at which the rods can act, they con- 

 tinue to respond further to increased stimulus, once they have 

 started, long after the rods have ceased to do so. 



At normal illuminations, therefore, it is the cones which 

 chiefly act, and we see colour. At very low illuminations 

 the action of the rods is predominant, and we cannot see 

 colour, while light of a bluish colour shines out with a whitish 

 appearance. As the illumination is increased the cones 

 suddenly begin to act and the colours appear. Then takes 

 place what has been called "The Battle of the Bods and 

 Cones." It is while this battle is in progress that the Purkinje 

 effect is noticeable. 



But the Purkinje effect is complicated by the fact that the 

 rods and cones are unequally distributed over the retina. At 

 the yellow spot the cones are predominant. Consequently, 

 the Purkinje effect is much weaker when the field of view 

 subtends a small angle at the eye. 



This uneven distribution of the rods and cones will also 

 explain the fact, referred to above, that at low illuminations 

 the size of the field of vievv can produce much greater 

 differences in the results. 



In order to gain an idea at what illumination the effect 



* JElectrotechnik und Maschinenbau, March IS, 1906. 



