478 ELECTRO-PHYSIOLOGY 



from the middle of the cornea and a point posterior to it or near 

 the equator, in the upper half of the eyeball of a " dark " frog, 

 when (with complete obscurity of this eye) the other eye was 

 illuminated. The same occurred after removal of the skin as 

 well as of the mucosa of the palate, although in a lesser degree. 

 In opposition to the photo -electrical variations with direct 

 illumination, absence of the second positive phase occurred 

 only with sudden obscurity. After dividing the optic nerve, 

 there was no galvanic effect from illumination of the opposite 

 side. Chemical stimulation (application of a crystal of salt to 

 the retina of the opened eyeball) produced considerable variations 

 of the rest-current in the other eye, first in the positive and 

 then in the negative direction. 



Starting from the experiments of Kiihne and Steiner upon 

 instantaneous illumination of the retina, Sigm. Fuchs (4) has 

 recently been attempting to determine more exactly the time- 

 relations of the photo-electrical variations in the frog's eye, 

 and to discover whether (as might be anticipated) the excitation 

 of the apparatus of the optic nerve, and therewith of the ac- 

 companying light sensation, occurs subsequently to the stimulus 

 which discharges them. A series of opening sparks were 

 discharged by means of Bernstein's rheotome as adequate 

 stimuli for the retina, the galvanometer being closed, on the 

 other hand, at a variable interval after each impact of light. 



The form and time-relations of the curve of variation could 

 thus be investigated by the same method as the negative 

 variation of the muscle or nerve current. The experiment was 

 of course performed after compensating the current of rest. 

 Kiihne and Steiner had previously made observations upon 

 the E.M.F. of the latter, which are approximately confirmed 

 by the valuable data obtained by S. Fuchs with the Poggendorff- 

 du Bois-Eeymond method of compensation. 



The E.M.F. of the current of rest during a single experiment 

 was sufficiently constant to give assurance that the conditions 

 underwent no substantial alteration during the period of investi- 

 gation. Each rheotome experiment started " from the appearance 

 of the spark at the moment of opening the retina circuit in the 

 rheotome. The characteristic position of the slider must be 

 regarded as the zero-point, from which the experiment starts 

 each time, or to which it returns." Under these conditions 



