xii ELECTROMOTIVE ACTION IN THE EYE 475 



since it is just this phase which alone appears in less excitable 

 preparations with instantaneous light stimuli. 



It is further remarkable that, as was found by Fuchs (4), 

 " the first (positive) phase of the variation produced by the 

 electric spark occurs incomparably more rapidly than in non- 

 instantaneous illumination." This is a strong argument for 

 considering the (photo-electric) variations as the expression of 

 the excitatory process in the sensory matter " (S. Fuchs). 



Every considerable variation- in intensity of illumination 

 produces, indifferently as to whether it occurs in the negative or 

 the positive direction, a positive variation of the rest-current ; as 

 expressed . on leading off from the isolated retina by twitching 

 movements, if the flame is made brighter or darker by turning 

 the gas up or down. " And since our eyes cannot perceive such 

 increments above a certain limen of intensity, the movements on 

 the galvanometer cease just before the highest luminosity is 

 attained " (Kiihne and Steiner). 



The retina is extraordinarily sensitive towards even the 

 merest trace of light (glimmer of a cigarette, flash of phos- 

 phorescent powder), so that it may be said, on the ground of 

 Kiilme and Steiner's experiments, that the galvanometer reacts 

 to the same intensities of light " that produce a clear sensation 

 in the eye." If the illumination is confined to the smallest 

 possible point of the retina, there is none the less a photo- 

 electric variation at every other point, however distant, due 

 either to currents deriving from the tract that is directly 

 illuminated, or to the effect of diffusion of light in the retina. 



The effect in the entire, uninjured eyeball differs essentially 

 from the course of the photo-electric variation in the isolated 

 retina as described above, since in preparations of maximal 

 excitability the second negative phase of the variation upon 

 the impact of light is wanting : so that even when the illumina- 

 tion is protracted for several minutes there is a uniform increase 

 between the first positive initial and second equally positive 

 terminal variation of the current. An effect analogous to that 

 in the isolated retina appears only with injured, fatigued, or 

 living bulbi (which may be compared with certain observations 

 of Dewar and M'Kendrick). 



The variations of the eyeball current are further much 

 smaller, in consequence of the less favourable conditions of 



