RESPONSE OF RETINA TO STIMULUS OF LIGHT 433 



of dark vision, as RO ME RO ME, and so on. In this 

 curious instance one sees better with eyes closed than open ! 



The next problem that presents itself is that of demon- 

 strating the pulsatory nature of the visual sensation in each 

 eye, under the constant action of light. The stereoscope, 

 with its two inclined slits, is now taken and turned towards 

 the bright sky, and when the design is looked at steadily it 

 will be found that, owing to pulsatory excitation in each eye, 

 and the binocular alternation of vision, when one bar of the 

 cross begins to be dim the other becomes bright, and vice 

 versa. The success of this experiment is determined by the 

 fact that the image in each eye forms a comparison-standard 

 for the other. And as the changes in the two eyes proceed 

 in opposite directions, the visual fluctuations during the con- 

 tinuous action of light, are brought out with the greatest 

 distinctness. It may be stated here that the period of this 

 visual oscillation has an average value of about four seconds. 

 It is, generally speaking, shorter with young people, and 

 longer with old. Even in the same individual, again, it is 

 modified, according as the previous condition has been one of 

 rest or activity. I give here a set of readings given by an 

 observer : 



In this connection it may be interesting to mention that 

 the period of a single oscillation in a multiple response, 

 measured in frog's retina, under experimental conditions, was 

 ten seconds. In the case of the retina of Wallago, the aver- 

 age period was twenty seconds. 



F F 



