18 REPORT 1893. 



example, when the disc makes half a revolution in a second the dis- 

 tance is ten degrees, this obviously means that when light bursts into 

 the eye, the extinction happens one-eighteenth of a second after the 

 excitation.' 



The fact thus demonstrated, that the visual reaction consequent on an 

 instantaneous illumination exhibits the alternations I have described, has 

 enabled M. Charpentier to make out another fact in relation to the visual 

 reaction which is, I think, of equal importance. In all the instances, 

 excepting the retina, in which the physiological response to stimulus has 

 a definite time-limitation, and in so far resembles an explosion — in other 

 words, in all the higher forms of specific energy, it can be shown experi- 

 mentally that the process is propagated from the jiart first directly acted 

 on to other contiguous parts of similar endowment. Thus in the simplest 

 of all known phenomena of this kind, the electrical change, by which the 

 leaf of the Dion^a plant responds to the slightest touch of its sensitive 

 hairs, is propagated from one side of the leaf to the other, so that in the 

 opposite lobe the response occui'S after a delay which is proportional to 

 the distance between the spot excited and the spot observed. That in 

 the retina there is also such propagation has not only been surmised from 

 analogy, but inferred from certain observed facts. M. Charpentier has 

 row been able by a method which, although simple, I must not attempt 

 to describe, not only to prove its existence, but to measure its rate of 

 progress over the visual field. 



There is another aspect of the visual response to the stimulus of light 

 which, if I am not trespassing too long on your patience, may, I think, be 

 interesting to consider. As the relations between the sensations of colour 

 and the physical properties of the light which excites them, are among the 

 most certain and invariable in the whole range of vital reactions, it is 

 obvious that they afford as fruitful a field for physiological investigation 

 as those in which white light is concerned. We have on one side 

 physical facts, that is, wave-lengths or vibration-rates; on the other, 

 facts in consciousness — namely, sensations of colour — so simple that 

 notwithstanding their subjective character there is no difficulty in 

 measuring either their intensity or their duration. Between these there 

 are lines of influence, neither physical nor psychological, which pass from 

 the former to the latter through the visual apparatus (retina, nerve, 

 brain). It is these lines of influence which interest the physiologist. 

 The structure of the visual apparatus afiTords us no clues to trace them 

 by. The most important fact we know about them is that they must be 

 at least three in number. 



It has been lately assumed by some that vision, like every other 

 specific energy, having been developed progressively, objects were seen 



' Charpentier, ' Reaction oscillatoire de la Retine sous I'influence des excitations 

 lumineuses,' Arclivres de Pltysiol., vol. sxiv. p. 541, and Propagation de V action 

 oscillatoire, ice, p. 362. 



