202 ELECTRO-PHYSIOLOGY CHAP. 



the ci/c than in the taste-sense. The following facts are taken 

 from Helmholtz's latest communication (49). 



If the eye is excited by variations of current of adequate 

 intensity, one electrode being applied to the forehead or closed 

 eyelids and the other to the neck, more or less pronounced 

 flashes of light appear across the entire field of vision, while 

 if galvanic currents are employed they occur both on closing and 

 on opening the current. 



Stronger currents are usually required to produce a persistent 

 effect from a steady constant current, than for make and break 

 flashes. In order to avoid these, as also twitches of the muscles 

 on making and breaking the current, Helmholtz found it advisable 

 to place two metal cylinders, surrounded with paste saturated 

 with salt solution, and connected with the two poles of a Daniell 

 battery of 12-24 cells, at the edge of the table where the subject 

 was sitting. The forehead is firmly applied to one cylinder, 

 while the hand touches the other, and if this is clone gradually 

 the effects of alternating the currents are quite inconsiderable. 

 The direction of current varies according as the forehead touches 

 now one and now the other of the cylinders. " If a weak 

 ascending current is led through the optic nerve, the dark field of 

 vision of the closed eye becomes brighter than before, and takes 

 on a greyish-violet hue. The point at which the nerve enters 

 looks at first like a dark circular disc in the bright field. The 

 illumination soon diminishes in intensity, and disappears altogether 

 on breaking the current. As the field of vision becomes 

 obscured, the previous blue tinge is replaced by a contrasting 

 reddish yellow from the subjective light of the retina." " On 

 closure of the opposite, descending direction of current there is a 

 marked effect, i.e. the field of vision (which is illuminated only 

 from the intrinsic light of the retina) usually becomes darker 

 than before, and appears to be reddish yellow ; the entrance point 

 of the optic nerve alone stands out as a bright blue disc upon the 

 dark fundus. On breaking this direction of current the field of 

 vision becomes bright again, and bluish white in colour, while 

 the optic disc is obscure." Other observers have described the 

 phenomena somewhat differently ; the views of Bitter, Purkinje, 

 Helmholtz, and Brenner are summarised (after Bossbach) in the 

 following table. 



