;-}74 Einthoven and Jolly 



a frog that has been kept in the dark and again when the light is removed, 

 there is an increase, in the positive direction, of the current present during 

 darkness. A similar result is obtained when, during continued illumination, 

 the light varies rapidly in intensity. The strength of the current is, within 

 certain limits, proportional to the intensity of the light. The onset and 

 removal of light is attended with the same electrical changes when the 

 posterior half of the eyeball alone is employed. 



In the case of the viper, rabbit, dog, and cat, a negative variation 

 accompanies lighting, followed on continued illumination by a slow positive 

 variation, and darkening produces a positive variation. 



Dewar and M'Kendrick,^ who rediscovered the electrical changes 

 caused in the retina by light, found that " when diffuse light is allowed to 

 inpinge on the eye of the frog, after it has arrived at a tolerably stable 

 condition, the natural electromotive power is in the first place increased, 

 then diminished ; during the continuance of light it is still slowly dimin- 

 ished to a point where it remains tolerably constant ; and on the withdrawal 

 of light there is a sudden increase of the electromotive power nearly up to 

 its original position." 



The effect of moonlight upon the eye was found to be about equal to 

 that of a candle distant a few feet. The eye is more sensitive to variations 

 in light of weak intensity than to variations in light of great intensity. 



Certain of the colours of the spectrum were arranged by Dewar and 

 M'Kendrick, with reference to their power of altering the electromotive 

 force, in the following order — yellow, green, red, blue. 



It was found that the anterior segment of the eyeball, including cornea, 

 iris, and lens, yielded a current which was not affected by light. 



The interesting fact was discovered that in the crustacean eye the 

 retinal currents are reversed in direction, in accordance with the inverted 

 arrangement of the sensory epithelium. In investigating the eye of the 

 cat, the onset of light was found to be attended by a diminution of the 

 electromotive force ; during illumination the electromotive force gradually 

 rose to a point where it became steady, and on darkening a rise was well 

 marked. 



Kuhne and Steiner - occupied themselves chiefly with the investiga- 

 tion of the reactions of the isolated retina. They found that the electrical 

 change on lighting and darkening is a complex one, the variation being first 

 positive, then negative, and finally again positive. The reaction is divisible, 

 according to these observers, into two parts: the first, due to the onset and 

 continuance of illumination, consists of a negative variation preceded by a 

 positive ; and the second, caused by disappearance of the light, consists of a 

 simple positive variation. Ktihne and Steiner discuss the question 

 whether the former of these parts can be divided further into a positive 

 variation caused by the onset and a negative due to continuance of illumina- 



> Trans, of the Roy. Soc. Edin., vol. xxvii. p. 141, 1872-73. 

 •^ Phy.siol. Untersuch., Heidelberg, Bd. iii., S. 327, 1880. 



