The Electrical Response of the Eye to Stimulation by Light 409 



The latent period of the wave C, the reaction of the third suKstance, is 

 difficult to measure, since the commencement of the wave is always masked 

 by the reaction of one or both of the other substances. 



Mention must be made of the measurements of Waller, who, in opposition 

 to Gotch and Brucke and Garten, observed latent periods which are as 

 great or even greater than ours. Speaking of latent periods of 3, 5, and 

 even 7 seconds, Waller says: ^ " Such an interval is altogether in excess of 

 any possible physiological lost time, and highly suggestive of a period of 

 hesitation during which two opposed currents were developed from thf' 

 retina at nearly equal rates." 



As Waller made use of a slowly acting Thomson galvanometer, he was 

 entirely right in supposing that opposite forces were in question which 

 compensated one another at first, while later one of them obtained the 

 mastery. The forces assumed by Waller are realised in our first and 

 second substances. 



Waller's explanation, however, excellent though it may be when a 

 slow galvanometer is used, does not hold good for the curves recorded by us, 

 for, where these show great latent periods, the actions of the various sub- 

 stances are practically completely isolated, while our measuring instrument 

 reacts almost instantaneously. 



From figs. 11, 12, 13, 14, 18, 23, and 24, we are, therefore, compelled to 

 conclude that, in fact, on weak stimulation a considerable latency occurs in 

 the action of those parts of the retina which give rise to the development 

 of electricity. 



The latency found in the photo-electric reaction of the frog's eye is in 

 complete agreement with the latency of light-perception in the human eye. 

 As evidence for this we may quote the work of two astronomers which has 

 not, so far as we know, been referred to up to the present in physiological 

 literature. 



Van de Sande Bakhuyzen^ uses as lightiiig point a small opening in 

 a copper plate, placed behind the flame of a petroleum lamp. Behind this 

 opening there is a metal screen which can be quickly withdrawn by means 

 of a strong spring. At the moment at which the margin of the screen 

 passes the light point, a metal projection from the screen dips in mercury. 

 In this way an electrical circuit is closed, by the aid of which a mark is 

 written upon the moving strip of paper of a recording apparatus. 



The observer at a distance of 25 metres from the point of light Wews the 

 appearing of the light through a telescope. At the moment at which he 

 perceives it he closes a second electrical circuit whereby another mark is 

 recorded on the same strip of paper. The distance between the two marks 

 measures the reaction time of the observer.^ 



In order to diminish the brightness of the light point, coloured glass is 



I Loc cit., p. 143. 



* Arch N^erlandai-ses dea pc. exactes et naturelles, ser. 2, L 6, p. 727. 

 For a critical review of the literature relating to measurements of reaction time see 

 Nyman, Skand. Arch. f. Phy.siol., Bd. 19, S. 365, 1907. 



