The Electrical Response of the Eye to Stimulation by Light 413 



of the spectroscopic apparatus and the refracting media of the eye ; and 

 secondly, we must remember that these rays, although they reach the 

 retina, do not practically exert here a photo-electric action. 



We therefore may limit our calculation to rays of wave-lengths between 

 X = 0"460 and X = 0'670. The entire active radiating energy is then easily 

 calculated to be 5 times the area of diagram (fig. 25), making an amount 

 of h, = 00359 g. cal. per sec. 



The photographs in our plate where the weakest radiations are employed 

 are figs. 23 and 24. The strongest radiations have been employed in the 

 case of figs. 6 and 7. The ratio between the weakest and the strongest 

 -adiation is 10-» hg : h, ; that is, 1 : 1'18 X lO^o. 



aj,6« 



0.650 



PAio ojx 0450 qdoo 

 Fig. 25. — Diagram in which the wave-lengths of the normal spectrum 

 are plotted in a system of rectangular co-ordinates as abscissa, 

 and the values of Hj — the radiation entering the pupil — as ordinates. 

 The areas of the three parts of the diagram represent the radiation 

 of the three parts of the spectrum used for illuminating in g. cal. 

 per sec. 



Previous investigators have already repeatedly demonstrated that the 

 eye acts as a very sensitive reagent to light. 



Waller^ mentions that radiation by moonlight during not more than 

 001 sec. is able to evoke a strong action current, and that a distinct 

 galvanometric deflection is caused when the eye is illuminated by a white 

 card 10 feet distant from the eye which itself receives illumination from a 

 standard candle 10 feet distant. 



With regard to the -sensitiveness of light perception in the human eye, 

 we refer to the measurements of Langley, Grijns and Noyons and von 

 kries. 



Langley -^ gives as the least energy required to cause a light perception when 

 stimulating with green light 



6-7 X 10-" g. cal. = 2-8 x 10-» erg. 



» Loc. cit, p. 124. ' American Jour, of Science, p. 376, 1888. 



