18S1.] 071 Selenium and its applications to the Photophoney dc. 629 



which I used iu tlic first iiistauco was an oxy-hydrogcu lime-light, and 

 the spectrum was formed with a bisulithido of carbon prism. Tho 

 experiment was repeated six times, and three dillerent selenium cells 

 were used. Tho results were precisely the same in every case, and 

 proved in the most marked manner that the greatest influenco 

 occurred in the boundary-line between the red and the orange ; thus 

 differing completely from the results obtained both by Captain Sale 

 and by Professor Adams. Moreover, the resistance when the selenium 

 cells were placed in the ultra-red, two inches beyond the limits of the 

 visible siicctrum, was in every case lower than when it was in the 

 blue, indigo, and violet But even in the ultra-violet the resistance 

 of the cells was lower than when they were quite removed from the 

 spectrum. 



My friends Mr. Preecc and Mr. W. H. Coffin, who were present 

 during these experiments, suggested that it would be desirable to vary 

 them by making use of different sources of light, and different methods 

 of dispersion ; and a few days afterwards, by the great kindness of 

 Mr. Norman Lockyer, they were repeated in Mr. Lockyer's laboratory 

 by Mr. Preece and myself with the electric light and a magnificent 

 diffraction grating. Nine experiments were made with three cells, 

 and the results were as absolutely concordant as those which we had 

 previously obtained ; but they all concurred in placing the maximum 

 at the extreme edge of the red, tlius agreeing with Captain Sale's 

 observations. One other remarkable effect must be noticed. In the 

 case of a single cell — that which I distinguish as No. 6 — with which 

 three experiments were made, a second maximum was observed in 

 every case in the greenish yellow, though the effect was about 20 per 

 cent, smaller than at the extreme red. The electric light, however, 

 is from its great unsteadiness most unsuitable for experiments of this 

 nature, and since no such exceptional phenomenon was ever observed 

 before or since, I am inclined to believe that, by a coincidence which 

 however remarkable is by no means impossible, the light happened to 

 be unusually intense just on the three occasions when this particular 

 cell was in the greenish yellow. A third series of ex2)eriments made 

 with a gas flame and a bisul2)hide of carbon prism, agreed with the 

 first in placing the maximum at the orange end of the red. Many 

 more combinations of sources of light and dispersion remain to be 

 tried, but time for these and for innumerable other experiments which 

 have suggested themselves has hitherto been wanting : for an opera- 

 tion which may be described in a dozen words not unfrequently 

 requires as many hours for its performance. 



By the help of a reflecting galvanometer I now propose to show 

 you the various effects produced by different parts of the spectrum of 

 the electric light formed by a bisulphide prism upon the resistance of 

 a selenium cell. The maximum deflectitm is seen to occur when the 

 selenium is at the extreme outer edge of the red. 



The effect of interposing various coloured glasses between a gas 

 flame and the selenium cell was also tried. The greatest effect was 



