The Electrical Response of the Eye to Stimulation by Light 403 



third substance itself, consisting in a slow descent of the string, appears 

 much later and fairly isolated. 



The general result is that we can observe in a series of curves obtained 

 from a dark eye, whore the light has gi-adually been lengthened in duration, 

 that the darkening effect, in the first curves a negative deflection, becomes 

 in the later ones a positive deflection. The latter, on further lengthening 

 the duration of lighting, gradually increases in size. In the conflict 

 between negative and positive deflections there is sometimes seen an 

 upward movement, which is immediately preceded by a small downward 

 one. 



To illustrate reactions of this description we reproduce in the first 

 place fig. 18, where 1 mm. abscissa = 02 sec, and 1 mm. ordinate = 2 

 microvolts. The intensity of the stimulation cannot be given exactly, as 

 the weakening of the light has been brought about by the aid of a 



Fig. 18. — Conflict between the reactions of the three substances. Dark eye. Absc. 1 mm. = 0'2 

 sec. Ordin. r mm, =2 microvolts. Green light. Intensity of illumination = 10-* Ig. Z, light ; 

 d, darkness. Instead of the usual upward deflection on darkening, due to the first substance, 

 there is seen a downward deflection Bj caused by the action of the second substance. 



coloured light screen in addition to the usual diaphragms. The light 

 intensity may be estimated at 10"^ or 10"^ Ig. 



One observes that in these circumstances the negative deflection A is 

 not present, while the sunnnit B is developed in the ordinary way. As a 

 result of the darkening there is seen in place of the usual upward directed 

 summit A^ a descent of the curve which is denoted by Bj. So deep a 

 depres.sion at Bj as occurs here is not found again in our whole collection 

 of photogi'aphs. 



We desire, in the second place, to draw attention to figs. 19, 20, and 21, 

 which were obtained successively from the same eye. In all three figures 

 1 mm. abscissa equals 02 sec, 1 mm. ordinate equals 26 microvolts. 

 In each figure there is reproduced in front of the photo-electric response a 

 control curve E which is obtained by introducing a potential difterence of 

 200 microvolts, which remains constant for a short time. The summits A 

 and B arc present in the three figures. In the first of the series, 

 fig. 19, the darkening reaction at B^ is seen as a sudden descent of the 

 VOL. I., NO. 4. — 1908. 27 



