The Electrical Response of the Eye to Stimulation by Light 379 



II. Method of Investigation. 

 ]. General Remarks. 



We have in our work employed exclusively isolated frogs' eyes. We 

 have been enabled on the one hand, by means of the string galvanometer ^ 

 which, for the retinal currents, may be regarded as the most sensitive 

 instrument available, tx) record and measure very weak electromotive 

 forces, such as are evoked by light of extremely low intensity; on the 

 other hand, we have tried by a suitable System of lenses to concentrate 

 light of as great intensity as possible upon the retina of the eve under 

 observation. 



2. The Arc Light. 



The weakening of any light may be continued indefinitely. On the 

 other hand, the increase of the intensity of light radiating upon a given 

 area is restricted within limits, theoretically as well as practically. 



We have chosen as the- source of illumination the crater of the arc light, 

 which of all terrestrial sources of light possesses the greatest intrinsic 

 intensity. In making this choice it has been necessary to consider, in the 

 first place, whether the radiation from the crater is sufficiently constant 

 for use in a series of observations an essential part of which consists in 

 the use of measurable quantities of light. 



With regard to this we may observe that we have never employed the 

 entire crater. If we desired to avail ourselves of the maximum of white 

 light we made use of a relatively large part of the crater, while in all 

 other cases only a small area of its central depression was employed. 

 Thus in all our experiments the question as to tlie constancy of the crater 

 light reduces itself to the question of the constancy of the light which is 

 radiated per square millimetre from the crater. 



We may assume that this constancy is sufficiently guaranteed. Thus 

 Violle 2 demonstrated that through variations in current intensity of 10 

 to 400 amp., and of energy from 500 to 34,000 watt, the brightness of 

 the crater does not change. It has even been suggested from several sides 

 to take the light derived from 1 ram."^ of the crater as a light unit and 

 thus raise the crater to the position of a light standard. In all probability 

 the temperature of the crater is that of vaporising carbon. 



On the other hand, some difficulties arise; thus, for example, Petavel' 

 says : " Even when the most favourable conditions are selected and the 

 intensity of current and the length of the arc are maintained constant, 



> Cf. " Eiri neues Galvanometer," Annalen der Physik, 4 Folge, Bd. 12, S. 1069, 1903. 

 " Ueber einige Anweiiduiigen dcs SaitengalvanonieterR," ibid., Bd. 1 4, S. 182, 1904. " Weitere 

 Mitteilungen tiber das Saitengalvanometer," ibid., Bd. 21, S. 483, 1906. 



2 Cf. Waidncr and Burgess, Bulletin of the Bureau of Standards, Washington, 

 vol. i. p. 109, 1904. 



3 Proc. Roy. Soc. London, vol. Ixv. p. 469, 1900. 



