i.] RETINAL CURRENTS 11 



"positive" the opposite direction from fundus to cornea. And 

 I recognise in the deflection to the left that we have just 

 witnessed what I am accustomed to regard as a 

 reaction of the third stage, viz., a negative current, 

 directed from before backwards in the eyeball. And 

 I may just mention that a few hours ago, when I 

 put up this eyeball for ex- 

 periment (under difficulties 

 inseparable from the pres- 

 ence of belated workmen in 



a new laboratory), it gave a - -- - - 



_ . T NofmcLi response. 



positive deflection, such as FlG 



have learned to be typical of 



a fresh and normal eyeball, and have chosen to characterise 



as a reaction of the first stage. 



These have been details of the laboratory, details that you 

 have hardly followed as I had to do, to make plain to myself 

 (and state to you) what was the direction of current in the 

 eyeball indicated by the direction of movement of the spot of 

 light. Those of you who are actual workers will appreciate 

 the importance of attention to such details, and the hopeless 

 confusion arising; from doubtful determinations of direction of 



O 



observed currents. Those of you who content themselves with 

 the literature of the subject, will find it almost impossible to 

 realise what writers mean by "positive" and "negative" effects. 

 But the response we have just seen is not of the type that 

 I wish to show in a first lecture ; it is of what will be 

 described later as a response of the third type, negative instead 

 of positive ; the eyeball may have been kept waiting too long, 

 or have been accidentally compressed in the course of pre- 

 paration. So I shall repeat the experiment on another eye- 

 ball that has just been carefully prepared and set up for me 

 by the assistant. 



And now, as you see, the response to light is a normal 

 response of the first type, viz., positive, for the spot has moved 

 to your right during the illumination, indicating current through 



