168 On the Pliosplioragrapli of a Solar Spectrum. 



This brings us to Captain Abney's recent researches, which, 

 by the aid of the grating, carry the investigation referred to 

 the prismatic spectrum as far below the red as the red is 

 below the yellow. They are not to be regarded as an ex- 

 tension of exploration in the infra-red region (for they really 

 do not carry us beyond my own observations in 1843), but 

 as securing the resolution of these lines or bands into their 

 constituent elements. I had never regarded them as really 

 single lines ; the breadth or massiveness of their photographs 

 too plainly suggests that they are composed of many 

 associated ones. The principle of decreasing refrangibility 

 with increasing wave-length incapacitates the prism from 

 separating them ; but the grating (which spreads them out 

 according to their wave-length) reveals at once their composite 

 character. 



In Captain Abney's map, after leaving the red line A, we 

 find three groups : — (1) ranging from about 8150 to 8350 ; 

 (2) from 8930 to 9300 ; (3) from 9350 to 9800. These, ad- 

 mitting that the lines of the subsequent grating-spectra have 

 been excluded, are then the resolution of a, f3, j. 



I suppose that care has been taken to make sure of that, 

 either by absorbent media or by a subsidiary prism. If the 

 grating had been ruled in such a manner as to extinguish the 

 second spectrum, inconveniences would arise from the charac- 

 teristics thereby impressed on the first. 



In the phosphorographic spectrum on luminous paint, this 

 vast multitude of lines is blended into a mass which probably 

 can never be completely resolved into its elements, on account 

 of the propagation of phosphorescence from particle to particle. 

 I have resolved it into two or three constituent groups, and 

 frequently have seen indications of its capability of resolution 

 into lines, in the serrated aspect of its lateral edges. 



I believe that luminous paint enables us to approach very 

 nearly, if not completely, to the theoretical limit of the 

 prismatic spectrum. 



The history of these interesting infra-red lines is briefly 

 this. They were discovered by me in 1842, and an engraving 

 and description of them given in the ' Philosophical Maga- 

 zine.' They were next seen by Foucault and Fizeau in 

 1846, and a description of them presented to the French 

 Academy of Sciences. They were again detected by La- 

 manski with the thermopile in 1871. Their resolution into 

 a great number of finer lines was accomplished by Abney, 

 who gave a Bakerian lecture describing them before the Royal 

 Society in 1880. Finally they have been redetected by me 



