224 Intelligence and Miscellaneous Articles. 



spectrum *. In repeating and varying these experiments several 

 years since, I was led to various interesting observations, and, in 

 particular, to indicate the position and wave-length of fine lines of 

 which some appear to me to have hitherto escaped the various 

 processes of investigation which have been applied to the study of 

 this region of the spectrum. 



It is known that thermoscopic methods reveal the existence in 

 the infra-red spectrum of sundry maxima and minima. In par- 

 ticular MM. Fizeau and Desains in "1 847 discovered the existence 

 of a cold band of which the wave-length is O001445 millim. ; and 

 M. Desains has quite recently f given a description of the calorific 

 spectrum of the sun which extends far beyond the limit observable 

 by means of the phenomena of phosphorescence, and shows, among 

 others, ten bands which appear to answer to those which will be 

 pointed out further on. Mr. LangleyJ, with the aid of his 

 bolometer, has likewise given an extensive drawing of the heat- 

 spectrum, which, however, comprises only seven or eight bands in 

 the region that forms the subject of the present memoir. 



The red and infra-red rays act upon phosphorescent substances 

 after the manner of heat, by at first accelerating the luminous 

 emissions and causing the substance to give out in a shorter time 

 the same sum of light which it would emit in a longer time and 

 with less intensity if it were sheltered from the radiation or the 

 calorific influence. The experiment above indicated presents two 

 phases : if the spectral influence has been of very short duration, 

 the impressed region appears at first brighter than the ground, and 

 gives a positive image of the spectrum with lines relatively dark ; 

 if the impression is prolonged, it exhausts the phosphorescence of 

 the corresponding region, which is extinguished, and the image 

 of the spectrum appears dark with bright lines. 



Generally only the second phase of the phenomenon is visible, 

 especially with hexagonal blende, which can give continuously a 

 negative image of the spectrum. Substances of which the phos- 

 phorescence lasts a long time, on the contrary, present very 

 clearly the first phase; and the positive image thus obtained 

 exhibits a remarkable fineness of details. I shall not here indicate 

 the simple experimental arrangements which permit these phe- 

 nomena to be studied with precision ; they will be explained in a 

 memoir to be published shortly. 



It was very important to vary the nature of the phosphorescent 

 screens. Besides hexagonal blende I used sulphides of strontium 

 and calcium, giving various shades by phosphorescence; and I was 

 led to observe that the phosphorographic images were the super- 

 position of the image of the solar spectrum and maxima and 

 minima of extinction peculiar to each substance, occupying regions 

 in the spectrum variable with each substance. It is moreover easy 

 to distinguish the fine lines of the spectrum from those maxima 

 which represent wide bands where the extinction is more rapid 

 than in the adjacent regions and which are reproduced with 

 various sources of light. 



* Comptes Rendus, t. Ixxvii. p. 302 (1873), & t. lxxxiii. p. 249 (1876). 



t Comptes Hendus, t. xcv. p. 434 (1882). J Ibid. p. 482 (1882). 



