2-S4 PIIOCEEDINGS OF THE AMERICAN ACADEMY 



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 sjjectrum, incon- 

 veniences would arise from the characteristics thereby impressed on 

 the first. 



In the phosphorographic spectrum on luminous paint, this vast mul- 

 titude 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 its 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 sj^ectrum. 



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 Magazine." 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 

 Lamanski with the thermopile in 1871. Their resolution into a great 

 number of finer lines was accomplished by Abney, who gave a 

 Bukerian lecture describing them before the Royal Society in 1880. 

 Finally, they have been redetected by me in the shining rectangle, just 

 above the theoretical limit of the prismatic spectrum, given by many 

 phosphorescent substances. 



University of New York 

 Dec. 1, 1880. 



