The Electrical Response of the Eye to Stimulation by Light 389 



The microscopical objective, Zeiss' water-immersion D* moistened with 

 Ringer's solution, is placed almost in contact with the cornea of the frog's 

 eye. The latter is connected with a pair of du Bois-Reymond's non- 

 polarisable electrodes in such a way that one takes the potential of the 

 cornea and the other of the fundus oculi (fig. 2). 



The moist chamber is so arranged that the air within can be saturated 

 with water vapour, while the eye and the electrodes which are attached to 

 separate glass tubes projecting freely through openings in the floor of the 

 chamber, and which are nowhere in contact with its moist walls, remain 

 completely insulated. 



The electrodes are, by means of insulated wires, led to the galvanometer 

 in a manner similar to that which has been employed in this laboratory for 

 recording electrocardiograms.^ One has in this way an opportunity of 

 compensating the current of the resting retina, of measuring in a simple 

 and rapid manner the resistance of the preparation, and of regulating the 

 sensitiveness of the galvanometer as desired. 



Fig. 2. — Arrangement of the eye in relation to water-immersion 

 lens and non-polarisable electrodes. 



We need scarcely mention that we have repeatedly controlled the 

 exactitude of our adjustments. The insulation resistance lies as a rule 

 between 10^° and 10^^ ohm. The crater is so large that the cone of light 

 radiating through the slit extends all round beyond the margin of the 

 collimator lens. All lenses are accurately centred, and we experimentally 

 determined that all the light passing through the outer margins of L^ 

 contributes to the formation of S3. 



In conclusion, we did not omit to ascertain that all enclosing arrange- 

 ments were light tight. If Lj is covered, all diaphragms and the movable 

 screens may be withdrawn without the slightest photo-electric response 

 being obtained. 



III. Results 



1. The Form of the Photo-electric Reaction to Light of 

 Moderate Intensity. 



If the isolated eye, which has not shortly before been exposed to strong 

 light, be illuminated by rays of intermediate strength, a form of curve is 

 obtained similar to that recorded by previous observer.s. 

 1 Cf. "Weiteres iiber das Elektrokardiogranim," Pfliiper's Arcliiv, Bd. 122. p. 517, 1908. 



