xii ELECTROMOTIVE ACTION IX THE EYK 473 



Kiihne and Steiner (/.c.) at first experimented in a dark room 

 divided into three parts. The two farthest contained the 

 galvanometer and telescope, while the eye was prepared in the 

 third room by a lamp. In the later experiments the galvano- 

 meter was placed in a light room, while the electrodes and 

 illuminating arrangements were in an adjacent and absolutely 

 dark chamber. The stimulus was made by an Argaud gas- 

 burner at a distance of 50-75 cm. from the preparation. The 

 lamp was turned up and down by an assistant at a signal, so 

 that the retina was suddenly illuminated or darkened. The 

 lead-off from the inner and outer surfaces of the retina was 

 effected by specially constructed clay electrodes, covered (Engel- 

 mann's method) with frog's lung. Each adequate and sudden 

 illumination with blue, green, yellow, red, or white light then 

 produced a considerable complex varia- 

 tion of the retinal current, with or 

 without the presence of visual purple. 

 The typical effect (Fig. 282) in a retina 

 containing the purple is a positive 

 variation at the moment of illumina- 

 tion (b c) rising rapidly to its maxi- 

 mum, and then passing quickly into the 

 negative variation. This phase reaches 

 its maximum (d e] during the impact 



FIG. 282. 



of light, is delayed some time at this 



point, and then declines very gradually to zero, even during con- 

 stant illumination. At the moment of darkness there is again a 

 sudden positive effect (e /), which must be regarded as the result 

 of a second stimulus due to the disappearance of light. The 

 mode of excitation is therefore in a measure comparable with 

 that from an electrical stimulus. As in the latter, the impact 

 and duration of the current on the one hand, and its disappear- 

 ance on the other, act as a stimulus, so with the impact of light 

 upon the retina, where the effects are visible on the galvanometer 

 as an initial diphasic (positive then negative) and a second 

 simple (positive) variation of the rest-current. The presence or 

 absence of visual purple appears from Kiihne and Steiner to 

 be of essential importance to the intensity of the retinal " current 

 of action." Not merely does the magnitude of the variations 

 differ in the two cases, being greater in the unbleached retina 



o 



