ADDRESS. 17 



Probably everyone is acquainted with some of tlie familiar proofs that 

 an object is seen for a much longer period than it is actually exposed to 

 view ; that the visual reaction lasts much longer than its cause. More 

 precise observations teach us that this response is regulated accordino- to 

 laws which it has in common with all the higher functions of an organism. 



If, for example, the cells in the brain of the torpedo are 'let off' 



that is, awakened by an external stimulus — the electrical discharo-e 

 which, as in the case of vision, follows after a certain interval, lasts a 

 certain time, first rapidly increasing to a maximum of intensity, then 

 more slowly diminishing. In like manner, as regards the visual appa- 

 ratus, we have, in the response to a sudden invasion of the eye by lio-ht, a 

 rise and fall of a similar character. In the case of the electrical organ 

 and in many analogous instances, it is easy to investigate the time rela- 

 tions of the successive phenomena, so as to represent them graphically. 

 Again, it is found that in many physiological reactions, the period of 

 rising ' energy ' (as Helmholtz called it) is followed by a period during 

 which the responding structure is not only inactive, but its capacity for 

 energising is so completely lost that the same exciting cause which a 

 moment before 'let off' the characteristic response is now without effect. 

 As regards vision, it has long been believed that these general charac- 

 teristics of physiological reaction have their counterpart in the visual 

 process, the most striking evidence being that in the contemplation of a 

 lightning flash — or, better, of an instantaneously illuminated white disc ' — 

 the eye seems to receive a double stroke, indicating that, althouo-h the 

 stimulus is single and instantaneous, the response is reduplicated. The 

 most precise of the methods we until lately possessed for investigating 

 the wax and wane of the visual reaction, were not only difficult to carry 

 out but left a large margin of uncertainty. It was therefore particularly 

 satisfactory when M. Charpentier, of Nancy, whose merits as an in- 

 vestigator are perhaps less known than they deserve to be, devised an 

 experiment of extreme simplicity which enables us, not only to observe, 

 but to measure with great facility both phases of the reaction. It is 

 difficult to explain even the simplest apparatus without diagrams ; you 

 will, however, understand the experiment if you will imagine that you 

 are contemplating a disc, like those ordinarily used for colour mixino- ; 

 that it is divided by two radial lines which diverge from each other at 

 an angle of 60° ; that the sector which these lines enclose is white, the 

 rest black ; that the disc revolves slowly, about once in two seconds. 

 You then see, close to the front edge of the advancing sector, a black 

 bar, followed by a second at the same distance from itself but much 

 fainter. Now the scientific value of the experiment consists in this, that 

 the angular distance of the bar from the black border is in proportion 

 to the frequency of the revolutions — the faster the wider. If, for 



' The phenomenon is best seen when, in a dark room, the light of a luminous 

 spark is thrown on to a white screen with the aid of a suitable lens. 



1893. n 



