EFFECT OF LIGHT ON THE RETINA 181 



pigment and a maximal stretching (dark position) of the cones. 

 It would seem as if a tonus, not only of the cones, but of the 

 pigment epithelial cells, was removed or destroyed and that they 

 both relaxed. Dittler ('07) and Garten ('07) were not able to 

 corroborate Herzog's results. Ciarten, on the other hand, found 

 that when the optic nerve of a frog is cut a marked stretching 

 of the cones may take place, but does not mention any effect 

 on the pigment, while Dittler (p. 306) says that destruction of 

 the central nervous system, cutting of the optic nerve and enu- 

 cleation of the bulbus has not the slightest effect upon the length 

 of the cones. 



Since the migration of the pigment and the contraction of the 

 cones is relatively so small in amount, it was considered inter- 

 esting to find out whether a greater amount of migration and of 

 contraction could not be induced by means of other stimuli. 

 The most natural stimulus to try was an electric current and 

 therefore this was done, both induced and constant currents 

 being used. 



The experiments and results of stimulating the retina with an 

 induced current will first be considered. The eyes, both dark 

 and light, were removed as quickly as possible and placed in a 

 watch glass containing a little 0.7 per cent salt solution. In 

 one series of experiments both electrodes were placed on the 

 optic nerve, in another, one was placed on the nerve and the other 

 on the cornea. Stimuli of moderate strength were used and for 

 a period of from 10 to 15 minutes. Upon examination of sec- 

 tions of such eyes it is seen that the pigment undergoes migra- 

 tion, but also a pronounced bunching and massing up in various 

 places. The cones are also affected by the induced current 

 being broader and shorter than normally the myoid being con- 

 siderably contracted (figs. 6 and 10 and table 4). All of the cones 

 did not show this effect, some of them appearing like normal 

 dark cones and similar in measurements. 



Engelmann ('85, p. 508) found that stimulation of the intact 

 or enucleated dark eye of the frog with an induced current of 

 moderate strength, produced the light condition of both cones 

 and pigment. 



