376 Einthoven and Jolly 



of the electrical response to light stimuli differing in intensity and duration, 

 and the relation, which the response to Jight at different intensities bears 

 to Fechner's law. He employed a slow-moving Deprez-d' Arson val 

 galvanometer for his research. This instrument, although fairly sensitive, 

 is not well adapted for following the rapid alternations of current present 

 in the response of the eye. A more suitable instrument for that purpose 

 is found in the capillary electrometer which was used by Gotch.^ The 

 electrical reaction of the eye to light was divided by this observer into 

 three portions: (1) the rise due to the sudden illumination, termed by him the 

 "on-effect"; (2) the continuous change occurring during the continuance of 

 illumination ; and (3) a second rise due to the sudden change from light to 

 darkness, termed by him the " off-effect." In addition to these changes, 

 careful examination of the records wdth a lens showed the presence in some 

 cases of a small negative deflection of short duration immediately preceding 

 the on-effect. The curve deduced from the electrometer records shows that 

 the on-effect having reached its maximum, subsides ; this subsidence is 

 checked, and a continuous effect is present during the illumination. The 

 continuous effect is not necessarily steady, but in some cases gradually 

 increases until it exceeds the value of the on-effect, and the off-effect 

 is superposed upon it. The off-effect depends for its production upon 

 previous illumination. When the illumination is of short duration — half a 

 second — it does not appear ; it is just visible with slightly longer illumina- 

 tion, and as the period increases the off-effect becomes more pronounced. 

 The latent period of the on-eftect varies with the temperature and with the 

 nature of the light. The delay was found to be shortest in the case of 

 white light, longest in the ease of red light, and intennediate in duration 

 with blue-violet light. It was not, however, possible under the conditions 

 of the research — the light being passed through filters of coloured fluid — 

 to determine the absolute intensities of the rays of diflerent wave-length 

 used as stimuli. The latent period of the off-effect is shorter than that of 

 the on-effect, nor does it vary with the nature of the illumination as does 

 the latter. 



Having regard to the facts that the on- and off-effects are both positive, 

 and that they differ in time relations, Gotch concluded that they cannot be 

 regarded as merely two different aspects of one chemical change, but that 

 there must be two distinct substances, one reacting: to light, the other to 

 darkness. 



The results obtained by Piper ^ from the eye of the frog agree generally 

 with those of Gotch. The latent periods at onset of light, according to this 

 ob.server, range from 0133 sec. to 0164 sec. The lighting effect is a sudden 

 increase of the electromotive force, which endures for 04 sec. to 05 sec. 

 and does not exceed 1 millivolt; upoi this follows typically a slight 

 diminution of the light effect. During illumination the curve remains 



' Jour, of Physiol., vol. xxix. p. 388, 1903, and vol. xxxi. p. 1, 1904. 

 ^ Arch. f. Anjit. u. Physiol., Suppl.-Bd., S. 133, 1905. 



