ELECTRICAL AND MECHANICAL STIMULATION. 1097 



The relation of after-image to the intrinsic light of the retina has 

 been already mentioned. According to Helmholtz, after-images are 

 modifications of intrinsic light ; according to others, they are independ- 

 ent phenomena, the former being of peripheral, the latter of central origin 

 (G. E. Mliller). 1 



Electrical stimulation of eye. Not only is the eye stimulated 

 by variations in the strength of a constant current applied to it, 

 but, during the passage of the current, sensations arise whose quality 

 depends upon the direction of the current. With an ascending 

 current 2 the visual field becomes brighter, with a descending current 

 the field becomes darker, even when its brightness is only due to the 

 intrinsic light of the retina. The field is also coloured, the exact nature 

 of the coloration having been described in different ways. With an 

 ascending current, violet has usually been seen ; with a descending 

 current, a yellow reddish according to Helmholtz ; 3 greenish according 

 to recent careful experiments by G-. E. Muller, 4 and a number of good 

 observers. The position of the optic disc is dark in the former case, 

 bright and bluish in the latter, effects due apparently to spatial induc- 

 tion. When the stimulation ceases, the bright violet field becomes dark 

 and yellowish; the dark yellow field becomes bright and bluish 

 (temporal induction). The sensation which arises on sudden closing of 

 the current has a higher intensity, but has the same character as that 

 which continues after closure (Purkinje, G. E. Muller); thus, with 

 closure of ascending and opening of descending current, there is a bright 

 violet flash ; with opening of ascending and closure of descending 

 current, there is a dark greenish-yellow sensation. 



An interesting point, determined by G. E. Muller, is that the con- 

 dition of retinal adaptation does not appreciably influence the effect of 

 electrical stimulation. 



Mechanical stimulation and pressure blindness. The phosphene 

 produced by localised pressure on the accessible parts of the eyeball is 

 blue and yellow, this being in agreement with the greater peripheral 

 extension of the fields for these colours. 



The results of mechanical stimulation of the optic nerve are very 

 important in relation to the doctrine of specific nervous energy. It 

 is generally supposed that when the optic nerve is cut during 

 extirpation of the eyeball, in a conscious individual, a flash of light is 

 experienced. This belief rests only on tradition. In 1882, Schmidt- 

 Kimpler 5 found that no flash occurred in several cases in which the 

 eyeball was removed without an anaesthetic, and there is no satisfactory 

 evidence that mechanical stimulation of the nerve produces a light 

 sensation. On the other hand, Schmidt-Eimpler found that electrical 

 stimulation of the stump of the optic nerve after extirpation, produced in 

 some cases a light sensation, which was described by one individual as of 

 the same nature as that which he had seen when treated by electricity 

 before removal of the eyeball. 



If mechanical pressure is exerted on the eyeball for some time, a 

 condition almost amounting to blindness is induced, in which objects can 



1 Ztschr.f. Psychol. u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1897, Bd. xiv. S. 40. 



2 I.e., with the anode over the eye. 

 "Handbuch d. physiol. Optik," 2te Aufl., S. 243. 



4 Ztschr.f. Psychol. u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1897, Bd. xiv. S. 329. 

 Refer to this paper for a full account of this subject. 



5 Centralbl.f. d. med. Wissensch., Berlin, 1882, Bd. xx. S. 1. 



