130 Mr. G. J. Burch. 



in the central or in the peripheral organ. Let A, fig. 3, represent a 

 peripheral organ, B a central organ, and C the nerve connecting them. 

 Let a stimulus, E, applied at A evoke a sensation, S, at B. Similarly, 

 let another stimulus, E-_>, applied at A evoke a corresponding sensation, 

 82, at B. Then the experimental facts may be stated thus : if there is 

 a certain time-interval of ^ of a second between EI and E 2 , and a 

 certain relation between them of intensity and duration, the sum of 

 the resulting sensations Si and So may be practically zero, or, at any 

 rate, very much smaller than either sensation by itself. 



FIG. 3. 



f 



i 



(1.) The first obvious deduction is that made by Exner, viz., that a 

 period of at least .y must elapse between the application of the 

 stimulus at A and the arousing of the sensation at B. 



(2.) It is also evident that this delay of T V is not simply the 

 transmission time of the nerve-response along C from A to B, for, if it 

 were, the second impulse would reach B too late to affect the first, 

 unless we suppose either a double track along C or the propagation 

 along a single track of a double response, with components travelling 

 at different rates a hypothesis not supported by any facts within my 

 knowledge. 



(3.) If the delay of T \/' occcurs at B we must also suppose either 

 it double track along C, or else that each nerve-fibre is capable of 

 transmitting two opposite kinds of response, namely, one with a 

 latency of 7 y arousing sensation, and the other deadening it, and 

 acting without delay. 



(4.) It seems simpler to suppose that this delay of r y occurs some- 

 where on the track between A and B, and probably close to A, e.g., at 

 D, and that it is quite distinct from any delay due to transmission. 



(5.) Not only do these experiments prove that there is a delay, but 

 that, under certain circumstances, a second stimulus may modify to 

 the point of annulling it the response to a previous stimulus. It has 

 been shown* that two stimuli in rapid succession may give rise in 

 nerve to what appears to be a single response, but in that case the 

 response is at least as strong as either stimulus would produce alone, 

 whereas here the response is less than it would have been without the 

 second stimulus. 



These facts point to the existence, probably in the retina, of some 

 mechanism sensitive to light, the function of which is to regulate for 



* G otcti and Burch, ' Journal of Physiology,' vol. 24, p. 410. 



