ELECTROMOTIVE PHENOMENA OF THE EYE 



811 



been observed in the frog's tongue on excitation of the glosso-pharyngeal 

 nerve. 



Eye- Currents. If two unpolarizable electrodes connected with a 

 galvanometer are placed on the excised eye of a frog or rabbit, one on 

 the cornea and the other on the cut optic nerve, or on the posterior 

 surface of the eyeball, it is found that a current passes in the eye from 

 optic nerve to cornea, the fundus of the eye being therefore negative 

 as regards the cornea (Fig. 307). The current has the same direction 

 if the anterior electrode is placed on the an- 

 terior 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 ether 

 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 galvanometer. The features of 

 the curve representing the photo-electric reaction vary with the duration, 

 and intensity of the illumination and with the previous 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 con- 

 sidered to depend upon the existence in the retina of three separate 

 photo-chernical substances. When light of moderate intensity is 

 allowed to act upon an eye which has not shortly before been exposed 



Fig. 307. Eye-Current. 







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Fig. 308. Photo-Electric Reaction of Frog's Eye (Einthoven and Jolly). The 

 duration of the flash (of green light) waso-oi second. The eye had been pre- 

 viously 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. 



to strong light, a form of curve is obtained which seems to represent 

 the combined reaction of the three substances (Einthoven and Jolly) 

 (Fig. 308). After a latent period a small preliminary negative deflec- 

 tion A is observed (downward movement of the string). This is at once 

 followe I 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 



