356 The Plwto-Electric Theory of Vision. 



about 4 x 10~ 7 erg per second, taking the energy emitted 

 by the unit candle as 5 X 10 5 ergs per second. Let us o also 

 assume that the average wave-length is about 5000 A.U. 

 (this is probably too short, but is sufficiently accurate for 

 this purpose), and we find that the energy of the corre- 

 sponding photo-electron on the quantum theory will be 

 equal to hv = H'V x 10~ 12 erg. 



If all the energy of the incident light is converted into 

 energy of the photo-electrons, this would lead to about 

 10 5 electrons per second being the minimum number visible, 

 and a current of 5 X 10~ 5 E.S.U. being the minimum 

 electronic current. 



In comparing this latter figure with the actual small 

 current obtained from the frog's retina, there are several 

 considerations to be taken into account : i. e., (a) a frog's 

 eye is very much smaller than a human eye; (6) the illu- 

 mination was not the minimum, but was intensely bright. 

 Also, in any case, we could not expect to get anything 

 approaching the theoretically possible current owing to 

 absorption of the electrons in the surface, loss of energy by 

 reflexion, and also the fact that even under ideal conditions 

 the total incident energy may not be entirely converted into 

 that of the emitted electrons. As regards (a) and (6), the 

 actual energy falling on the frog's retina per second was 

 approximately 10 4 ergs per second, or 2*5 x 10 10 times that 

 required to excite the human eye. This would give a 

 theoretically possible current of 1'25 x iO 6 E.S.U., or about 

 2xlO T1 times the actual current obtained. It is possible 

 that absorption in the surface might account for the 

 enormous discrepancy between these two figures, but 

 certainly no other consideration could do so. In this 

 connexion some results obtained by Elster and Geitel 

 (Phys. Zeits. xiii. p. 4:68, 1912) are' of interest, By the 

 use of a very sensitive potassium cell, they were able to 

 detect an amount of blue light imparting 3 x 10~ 7 erg 

 per second per sq. cm., and they found that under these 

 conditions the energy of the emitted electrons accounted 

 for 2 3 00 P ar ^ of the incident energy. As it has been 

 found that the photo- electric current for a substance is 

 usually a linear function of the light intensity, it would 

 from this appear probable that in all cases of photo-electric 

 action a sensible proportion of the incident energy would 

 be represented in the energy of the emitted electrons. Thus 

 we are driven to conclude that since in the case of the 

 retina only 5xl0 -12 part is so represented, it cannot be 

 considered to be photo-electric. The current obtained is 



