ELECTRO-PHYSIOLOGY 



735 



therefore negative as regards the cornea (Fig. 289). The current 

 has the same direction if the anterior electrode is placed on the 

 anterior surface of the retina itself, the front of the eyeball being 

 cut away, or if one electrode is in contact with the anterior and the 

 other with the posterior surface of the isolated retina. There is 

 nothing of special interest in this ; but the important point is that 

 if light be now allowed to fall upon 

 the eye, or upon the isolated retina, 

 characteristic electrical changes are 

 caused. These are spoken of as the 

 photo-electric reaction, and are best 

 studied by means of the string gal- 

 vanometer. The features of the curve 

 representing the photo-electric reaction 

 vary with the duration and intensity 

 of the illumination and with the pre- 

 vious condition of the eye as regards 

 illumination. A careful analysis of 

 the curves obtained under different 

 conditions supports the hypothesis that 

 there occur in the eye three separate 

 processes, which may for convenience 

 be considered to depend upon the existence in the retinafjjof 

 three separate photo-chemical substances (p. 934). When light 

 of moderate intensity is allowed to act upon an eye which has not 

 shortly before been exposed to strong light, a form of curve is 

 obtained which seems to represent the combined reaction of the 

 three substances (Einthoven and Jolly) (Fig. 290). After a latent 



FIG. 289. EYE-CURRENT. 





FIG. 290. PHOTO-ELECTRIC REACTION OF FROG'S EYE^(EINTHOVEN_AND 



JOLLY). 



The duration of the flash (of green light) was o'oi second. The eye had been 

 previously in the dark, i millimetre of the abscissa corresponds to 0-5 second, 

 i millimetre of the ordinate to 10 microvolts. Curve to be read from left to right. 



period a small preliminary negative deflection A is observed (down- 

 ward movement of the string). This is at once followed by a much 

 larger upward movement (positive variation) in the same direction 

 as the resting effect, the fundus becoming relatively more negative 

 to the cornea than before. After the peak B has been reached, the 

 curve sinks first rapidly, then more gradually, but soon mounts 

 again, and reaches a second maximum C, which is higher than B 



