The Electrical Response of the Eye to Stimulation by Light 405 



Finally, we draw attention to the differenct- which exists between figs. 

 6 and 7 on the one hand, and the series of figs. 19, 20, and 21 on the other. 

 In both cases very strong light was used, but in the first-mentioned figures 

 we had the organ as completely as possible a light eye. In the last- 

 mentioned figures we endeavoured to keep it as completely as possible 

 a dark eye. 



3. Light of Difterent Colours. 



We shall now mention some further results of our investigatrons, and 

 in the first place, those obtained by illuminating with rays of different 

 wave-lengths. Our expectation that we should find marked differences in 

 the form of the photo-electric reaction when light of different wave-lengths 

 is employed for stimulation has not been realised. When experiments are 

 performed with light of a single colour there appear very different forms 

 of photo-electric reactions according as we have to do with a light eye or 

 with a dark eye, and according as we have illuminated the eye with 

 weaker or stronger light, and during shorter or longer periods. This is 

 sufficiently proved by the figures of our plates. To study the influence 

 which the variation of colour exerts upon the development of these 

 numerous forms would require a long and detailed investigation which we 

 have not had an opportunity of carrying out. We have only been able in 

 our investigations to confirm what is already known, viz. that for the 

 same energy of stimulation the reaction to green rays is stronger than that 

 to red and blue.^ 



4. Rhythmical Reaction on Continuous Stimulation. 



We take the opportunity of referring in a word to the possibility of a 

 rhythmical reaction to constant illumination. As a rule the eye reacts to 

 constant illumination with an electrical current which increases and 

 decreases very gradually, but in some cases it is open to question if this 

 rule holds good. In fig. 22 an example of this is reproduced. We have 

 here to do with a light eye which is illuminated by strong white light 

 02 I„. Here 1 mm. abscissa = 0"2 sec, 1 mm. ordinate=lo microvolts. The 

 string shows, during each period of illumination l^, l^, and /g, rhythmical 

 Oiscillations which fail (luring the periods of darkness d■^^ and fZg. 



After making the record shown in fig. 22, the eye was maintained for a 

 (juarter of an hour in the dark and thereaftci- exposed again to light of the 

 same strength, with the result given in fig. 15. The value of 1 mm. 

 abscissa has reniainetl unchanged and is equal to 02 sec, while the 

 value of 1 mm. ordinate is increased to 18 microvolts. At E a control 

 curve is recorded, while the reaction evoked by a short exposure to strong 

 light shows the usual summits A, B, A^, and C. We draw attention to the 



' Himstedt and Nagol, Bericbte der Naturforsch. Ges. zu Freiburg, Bd. xi., p. 153, 

 1901. 



