MULTIPLE RESPONSE 287 



as I am aware, no satisfactory explanation has hitherto been 

 offered. The phenomena which have been described in the 

 case of vegetable tissues show, however, that this is nothing 

 but an instance of multiple excitations in the retina caused 

 by strong stimulation. In the case of the plant these recurrent 

 excitations express themselves mechanically as twitches ; in 

 the case of the firefly as flashes of light ; and in the retina 

 as recurrent visual images. In these recurrent visual impulses 

 are found all the peculiarities which I have observed in 

 recurring excitatory impulses in Biophytuin. This is shown, 

 not only in simple cases, but also in the most complicated. 

 I shall presently give, in illustration of this, two parallel 

 instances of multiple excitation in Biopliytum and multiple 

 excitation in retina (see table, p. 288), in which the successive 

 intervals undergo similar cyclic changes. 



In order to measure accurately the intervals between 

 successive visual images, I use a special stereoscope, based on 

 my discovery of the phenomenon of binocular alternation of 

 vision, and by this means I have found that the after-effects 

 of light in the two eyes are not simultaneous but alternate — 

 that is to say. when the after-image in one eye is most vivid, 

 that in the other eye has just vanished, and vice versa. For 

 accurate measurement of this periodicity, it is necessary to 

 have the effect on one eye distinguished from that on the 

 other. For this purpose I have been accustomed to use a 

 stereoscope containing, instead of photographs, incised plates 

 with two inclined cuts, the right eye seeing the slit inclined 

 to the right, and the left eye that inclined to the left. When 

 the observer looks through this stereoscope turned towards 

 a bright sky, his two eyes are acted on by strong stimulus 

 of light. On closing the eyes, periodic visual impulses are 

 sent from the strongly stimulated retina just as the stimu- 

 lated area of BiopJiytuin was found to send out periodic 

 impulses.' 



' Bose, 'Binocular Altcination of W^xow,'' Respojise in the Liviu}; and Noii- 

 Lh'ing; p. 175. 



