Principles underlying the Flicker Photometer. 59 



of photometers of the ordinary variety. In this communica- 

 tion the author wishes to summarise the results of some 

 further experiments on this point which seem to throw some 

 light on the theory of the flicker photometer : this, however, 

 as Mr. A. P. Trotter has justly remarked, is essentially 

 physiological, and is therefore correspondingly difficult to 

 investigate. 



Before proceeding further, however, it may be well to 

 recapitulate briefly the main points in this theory of the action 

 of rods and cones on the retina, as it will be frequently 

 referred to in what follows. 



According to this theory, there exist on the retina two 

 distinct varieties of minute light-perceiving organs, known, 

 from their appearance, as the " rods " and the " cones " re- 

 spectively. The rods, it is thought, are sensitive to light 

 but unable to perceive colour, as such ; they are, however, 

 most sensitive to bluish-green light, probably in the neigh- 

 bourhood of 0*51 to 0*52 /x.. Light of all wave-lengths seems 

 to be usually perceived by these organs as white *. These 

 organs are also sensitive to very weak light but, as the 

 illumination is increased, they become as it were saturated 

 and do not respond further to increased stimulus. 



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

 sensitive to yellow-green light, probably usually near 0*58 jx, 

 and, while they are insensitive to the \erj weak order of 

 illumination to which the rods respond, they continue to 

 become more active, under the influence of increased stimulus, 

 once they have started, long alter the rods have ceased to 

 do so. 



As explained in the paper referred to above, this theory 

 seems to account very satisfactorily for the Purkinje effect. 

 Moreover, an experiment described therein also suggested 

 that, as the Purkinje effect only became very noticeable to 

 the author, for small retinal areas, at illuminations below 

 about 02 lux, this might be the order of illumination at 

 which the cones finally go out of action, and cone- vision 

 is more or less completely replaced by rod-vision. In view 

 of what follows this figure is rather important, and we may 

 next briefly refer in passing to a confirmatory experiment 

 which acts as a useful check upon it. 



According to the rod and cone theory, the yellow-spot 

 effect is to be ascribed to the peculiar distribution of rods 



* According to a recent paper by Professor G. J. Burcli (Proc. Roy. 

 Soc. 1905) the retina, which, as a rule, seems to lack the power of colour- 

 perception at very low illuminations, regains it if kept in the dark for 

 some hours. 



