92 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Stokes ^^ that the phenomenon varied with the areas involved, and 

 recently Dow ^^ has found that for small areas (i. e., nearly central and 

 hence mainly pure cone vision) Purkinje's phenomenon appears only 

 below about 0.2 meter-candle. This figure would quite certainly have 

 been somewhat higher had he used instead of red and signal-green 

 glass the primary red and green, but it is clear from his results that 

 the superposition of rod vision has a very considerable efiiect at moder- 

 ate illuminations. 



Finally, one must consider the luminosity curves at various intensities. 

 Figure 6 gives in curve a the relative luminosities of the spectrum 

 colors at fairly high intensity. The maximum is in the yellow, and the 

 falling off", especially on the red side, is very rapid. This seems to be 

 about the normal curve when the eye is fully in action. Curve b gives 

 the luminosity curve for an intensity of about 0.0007 meter-candle. 

 At this point color sensation is practically extinguished, and the maxi- 

 mum luminosity is perceptible, in what would seem the pure green were 

 the light brighter, very near the E line and at a point corresponding 

 to the inflection in the curves of Figure 5. This is practically the con- 

 dition of pure rod vision. Curve c. Figure 6, lends confirmatory evidence. 

 It is the luminosity curve obtained by Abney ^^ from a patient with pure 

 monochromatic vision. He had apparently an absolute central scotoma 

 (cones atrophied rather than replaced by rods 1 ), visual acuity greatly 

 subnormal (central vision absent), and nyctalopia. This is a typical 

 condition, nyctalopia being generally associated with central color sco- 

 toma, leaving peripheral vision but shghtly affected (Fick). The patient 

 apparently had no color perception, and his luminosity curve was prac- 

 tically identical with b, the normal curve for very weak light. 



It would be most interesting to get proper tests for luminosity in one 

 of the rare cases of congenital hemeralopia which would present the 

 reverse condition of rods inactive and cones nearly normal. A com- 

 parison of such a case with luminosity in the hemeralopia associated 

 with retinitis pigmentosa, in which peripheral vision is progressively 

 contracted, might give valuable evidence as to the existence of retinal 

 elements intermediate in function between rods and cones. 



To sum up this phase of the matter, rod vision seems to be predomi- 

 nant from the very threshold illumination up to several tenths of a 

 meter-candle, and to continue in force to all ordinary intensities, although 

 rather easily exhausted. It gives low visual acuity and shade-percep- 

 tion perhaps of the order of a tenth normal, but, such as it is, it is our 



" Nature, 32, 537. " Phil. Mag., Aug., 1906. 



13 Proc. Roy. Soc, 66, 179. 



