378 Einthoven and Jolly 



to the original level during lighting lasts two or three times as long as the 

 ascent. The positive darkening reaction does not alter the course of the 

 continuous effect. When the eye is submitted to short repeated illumina- 

 tions the successive on-efFects are superposed upon the continuous effect. 

 The state of adaptation of the eye has an important influence upon the 

 continuous effect. When the eye is light-adapted no second rise is 

 exhibited on the curve, but after the on-effect the current sinks gradually 

 during illumination without, however, reaching the zero line. During a 

 series of experiments on the dark-adapted eye the continuous effect is 

 evoked more strongly at first and becomes progressively weaker as 

 illumination is repeated. The continuous effect is exhibited not only by 

 the whole eye but also by the posterior half. This is inconsistent with 

 Piper's view that it is derived from the iris muscles. 



The continuous effect differs frOm that just described when the experi- 

 ments are performed not upon the isolated eye but upon the eye, remaining 

 in situ, of a curarised frog. Here the continuous effect does not exhibit 

 subsidence during illumination but remains constant at a high level, the 

 illumination being continued for half an hour. 



The latent period of the on-effect is on an average 02 sec. The very 

 short latent periods obtained by Fuehs cannot be attributed to the short 

 duration of the light stimuli used by him, as similar short stimuli were 

 made use of by Briicke and Garten and yielded latencies of 0108 sec. to 

 01 11 sec. Fatigue may be observed in the on-effect which diminishes in 

 strength after repeated stimulation of the eye. The o^-effect differs from 

 the continuous effect in that it is but little affected by the state of 

 adaptation of the eye. The off-effect is found, as a rule, with dark- 

 adapted eyes, especially after long illumination, to be stronger than the 

 on-effect, while in light-adapted eyes the latter is usually as strong as or 

 occasionally stronger than the off-effect. The off-effect is, as a rule, some- 

 what steeper than the on-effect. 



Briicke and Garten have extended their researches over a large 

 number of vertebrates and find a marked similarity of reaction in the 

 different classes when the eyes are quite fresh and investigated under the 

 most favourable conditions. From their results it appears probable that 

 the electromotive changes caused in the eye by the stimulation of light 

 are essentially of the same nature throughout the vertebrate series, and 

 when allowance is made for the reversed direction of the currents, also in 

 crustsuieans. 



Although the electrical response of the eye to stimulation by light has 

 been studied by numerous observers, there has not, so far, been undertaken 

 a systematic investigation of the electromotive changes which are caused 

 by stimuli of very varying strength. Such an investigation, however, 

 can, as we hope to show, contribute to our comprehension of the retinal 

 processes. 



