The Electrical Response of the Eye to Stimulation by Light 415 



In conclusion, we may institute a comparison between the absolute 

 amount of the energy of the stimulus and that of the response. 



We choose as an example fig. 23, where we have already determined the 

 energy of stimulus as amounting to 147 x 10^^^ g. cal. The energy of 

 reaction must be calculated from the form of the curve. If the galvano- 

 meter is replaced by a wire of a small negligible resistance the current 



y 

 passing through it is A = :^ amp. Here V is the electromotive force in 



volts developed at each moment, while R is the resistance in ohms of the 

 preparation. For the present we assume that V remains unchanged, when 

 the resistance of the galvanometer is diminished. The energy of the 

 reaction during the time dt is expressed by YAdt, the total energy of the 



reaction by W = / YAdt or W = j dt Joule. 



In the figure 1 mm. abscissa = 0-2 sec, 1 mm. ordinate = 2 microvolts. 

 The resistance of the preparation is R = 9000 ohms. 



With the aid of the above data, the amount of W, as somewhat roughly 

 calculated from the form of the curve, is W = 2 x 10" '^ Joule = 48 x 1 0""" g.cal., 

 and it is thus evident that in the case of fig. 23 the energy of the reaction 

 is more than 30 times less than the energy of the stimulus. 



Following on this result, there are good grounds for stating as a general 

 rule that the absolute energy of the photo-electric reaction is always less 

 than that of the light stimulus. It is true that we have to consider the 

 possibility that in a curve which is recorded under other conditions, the 

 energy ratio might be altered in favour of the photo-electric reaction, but 

 in our collection of photographs, taken in very varying circumstances, we 

 have not found an example of this. 



The curve (fig. 23) has been chosen jilst because its energy ratio is 

 specially favourable to the photo-electric reaction. 



In judging of the energy of the photo-electric reaction, we have to take 

 into account that there exists short circuiting in the eye itself, and that the 

 current measured by the galvanometer is presumably only a small part of 

 the current passing through the eye. 



This last current is not easily determined, so we shall take, as is usually 

 done, the potential difference or the current, as these are measured by an 

 instrument outside the eye, to be the real photo-electric reaction. In our 

 above calculations we have assumed that the electromotive force developed 

 by the eye remains unchanged when the resistance of the galvanometer is 

 diminished, and we have assumed the amount of this lesistance to be equal 

 to zero. Both assumptions were made for the purpose of calciilating the 

 possible maximum of the reaction energy for this special case. If we take 

 the resistance of the galvanometer into account, we find for the reaction 

 energy an amount which is 175 times less, the resistance of the galvanometer 

 being 6800 ohms. 



The photo-electric reaction of the eye is, in regard to the energy ratios, 



