BIOLOGY IN RELATION TO OTHER NATURAL SCIENCES. 449 



eJectrieal discharge, which, as iu the case of vision, follows after a cer- 

 tain interval, lasts a certain time, first rapidly increasing- to a maxi- 

 mum of intensity, then more slowly diminishing. In like manner, as 

 regards the visual aij.paratus, we have, in the response to a sudden 

 invasion of the eye by light, 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 relations 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 struc- 

 ture is not only inactive, but its capacity for energizing is so com- 

 pletely lost that the same exciting cause which a moment before " let 

 oft'" the characteristic response is now without eftect. As regards 

 vision, it has long been believed that these general characteristics 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 disk* — the 

 eye seems to receive a double stroke, indicating that although the 

 stimulus is single and instantaneous, the response i« reduplicated. 

 The most x)recise of the methods we until lately possessed for investi- 

 gating the wax and wane of the visual reation, were not only diflBcult 

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

 particularly satisfactory when M. Oharpeutier, of Nancy, whose merits 

 as an investigator are i)erhaps 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 reac- 

 tion. It is difficult to explain even the simplest ai)i)aratus without 

 diagrams; you wi 11 however nnderstaiid the experiment if you will 

 imagine that you are contemplating a disk, like those ordinarily used 

 for color mixing; that it is divid ed by two radial lines which diverge 

 from each other at an aiigle of 60°; that the sector which these lines 

 inclose is white, the rest black ; that the disk 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 dis- 

 tance 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 ij^ in proportion to the frequency of the revolutions — 

 the faster the wider. If, for example, when the disk makes a half rev- 

 olution in a second the distance is ten degrees, this obviously means 

 that when light bursts into the eye, the extinction happens one-eight 

 eenth of a second after the excitation. t 



* The pheuomenon ia liest seen ■when, in a dark room, the light of a luminous 

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



ICharpentier "Reaction oscillatoire de la Retiue sous rinfluence des excitations 

 lumineuses," Arcliives de Physiol., vol. xxiv, p. 541, and Fropajjation de Vaction oncil- 

 latoire, etc., p. 362. 



SM 93 39 



