212 Captain Abney on the Violet 



and 



E-R E 



wher 



h 



M M-i 



i i\ M x Mi-?-! 



If, as was done in the calculation of* Becquerel's experi- 

 ments, M— r and M x — ?\ were made to signify the resistance 

 of the gas at two different current-intensities, the resistance of 

 the gas would be found inversely proportional to the intensity 

 of the current, although in reality it is independent of that 

 intensity. 



XXIV. On the Violet Phosphorescence in Calcium Sulphide. 

 By Captain W. de W. Abney, F.R.S* 



IN some investigations in photography it became necessary 

 that I should study the phenomenon of phosphorescence 

 exhibited in calcium sulphide, such as is employed in Bal- 

 main's paint. And as one or two points of interest arose which 

 have not (as far as I am aware) been described before, I 

 have thought it might be of interest to lay the subject before 

 the Society. 



The phosphorescent light, which is a peculiar violet, can be 

 generated, if I may use the term, by day-light or candle-light 

 — by the former fairly brightly, and by the latter only feebly, 

 for reasons which will appear. In order to gain strong phos- 

 phorescence, the light from a magnesium-ribbon or the electric 

 light should be employed. Mr. "Warnerke has shown that £ 

 inch of ribbon of the former is sufficient to excite phospho- 

 rescence to the maximum point that can be obtained from 

 light of that brilliancy. Light of greater brightness, how- 

 ever, seems to excite it even more strongly. In a communi- 

 cation made to the Society by Lieut. Darwin, R.E., he gave 

 the formula for the decrease of phosphorescence after exci- 

 tation, from which it will be seen that it very rapidly dimi- 

 nishes in brilliancy. 



My first experiment was to determine the spectrum of the 

 emitted light ; and this I observed with a small spectroscope ; 

 and the emission-spectrum is shown in I, figure opposite. It 

 will be seen that to the eye the greatest luminosity is between 

 G and F, and a feebler one extending from between E and F 



* Communicated by the Physical Society, having been read at the 

 Meeting on January 28, 1882. 



