INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 30I 



produced. These pulsations are not usually noticed in visual 

 sensation, owing firstly to the absence of a standard of com- 

 parison, and secondly to the fact that though the impressions 

 on the individual retinae undergo variation, the sum total of the 

 two remains constant. I have been able to provide the neces- 

 sary comparison-standards by having two distinguishable 

 images produced in the two eyes, the fluctuation in the visual 

 excitation in one eye being thus capable of detection by com- 

 parison with that in the other. It would have been impossible 

 to detect this fluctuation had the excitatory variation taken 

 place in the two eyes simultaneously, i.e. if the maximum 

 excitation in the one had occurred at the same moment as the 

 maximum excitation in the other. But I have found that, as 

 regards excitation, there is a relative difference of phase, of 

 half a period, between the two eyes, so that the maximum effect 

 at the given moment in one eye corresponds to the minimum 

 in the other. It is owing to this fact that the periodic excita- 

 tions in each retina are brought out in an unmistakable 

 manner by the following experiment, which consists in look- 

 ing at two slits through the modified stereoscope described in 

 the previous chapter. One of the two slits inclines to the 

 right and the other to the left, and on looking through these 

 at a bright sky, the right eye perceives a bar of light turned, 

 say to the right, and the left eye a bar turned to the left, 

 the resultant impression being that of an inclined cross. 



When the stereoscope is turned to a bright sky, and the 

 cross looked at steadily for some time, it will be found, owing 

 to pulsatory excitation in each eye, that when one arm of 

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

 vice versa. These periodic fluctuations are- perceived con- 

 tinuously under the constant action of light. 1 



I shall now proceed to the demonstration of the very 

 interesting rhythmic movements caused in plants under 

 constant light-stimulus. As we wish to prove that the 

 cause of automatic movements lies in the action of some 



1 The experiment will be found described in detail in my book, Response in 

 the Living and Non-Living, p. 175. 



