360 Mr. Shelf ord Bidwell [March 5, 



images of the slit.* [The effect was demonstrated by means of a 

 rotating disc in the lantern, and is roughly indicated in Fig. 5.] 



The dark reaction known as the Charpentier effect, occurs at the 

 beginning of a period of illumination. There is also a dark reaction 

 of very short duration at the end of a period of illumination. I 

 should explain that owing to what is called the proper light of the 

 retina, ordinary darkness does not appear absolutely black : even in 

 a dark room on a dark night with the eyes carefully covered, there 

 is always some sensation of luminosity which would be sufficient to 

 show up a really black image if one could be produced. Now the 

 darkness which is experienced after the extinction of a light is for a 

 small fraction of a second more intense than common darkness. 



I believe that the first mention of this dark reaction occurs in the 

 article which I contributed to ' Nature ' in 1885, in which it was stated 

 that when the current was cut off from an illuminated vacuum 

 tube " the luminous image was almost instantly replaced by a corre- 

 sponding image which appeared to be intensely black upon a less dark 

 background," and which was estimated to last from J to J second. 

 " Abnormal darkness," it was added, " follows as a reaction after the 

 luminosity." 



In the Royal Society paper to which I have before referred the 

 point is further discussed, and a method is described by which the 



stage of reaction may be easily exhibited, 

 and its duration approximately measured. 

 If a translucent disc made of stout 

 drawing-paper and having an open 

 sector, is caused to rotate slowly in 

 front of a luminous background, a 

 narrow radial dark band like a streak 

 of black paint appears upon the paper 

 very near the edge which follows the 

 open sector. From the space covered 

 by this band when the disc was rotating 

 at a known speed, the duration of the 

 dark reaction was estimated to be about 

 ■5-^^ second. [The experiment was shown, 

 Fig. 6. and is illustrated in Fig. C] 



One more interesting point should 

 be noticed in the train of visual phenomena which attend a period of 

 illumination. The sensation of luminosity which is excited when light 

 first strikes the eye is for about ^L second much more intense than it 

 subsequently becomes. This is shown by the fact that the bright 

 band intervening between the leading edge of the white sector of a 

 Charpentier disc and the dark band, appears to be much more 

 strongly illuminated than any other portion of the sector. 



* Proc. Roy. Soc, vol. Ivi. p. 142 (1894). A similar observation was described 

 by Charpentier, Comptes Rendus, Jan. 1896. 



