92 PROCEEDINGS OF THE AMERICAN ACADEMY 



Investigations on Light and Heat, made and published wholly or in part with 

 appropriation from the Rumeobd Fund. 



VI. 



ON CERTAIN REMARKABLE GROUPS IN THE 

 LOWER SPECTRUM. 



By Professor S. P. Langley. 



Presented Oct. 7, 1878. 



In first studying the diffraction spectrum for the purpose of learning 

 more of the laws governing the selective absorption of the sun's 

 radiant energy, both near its surface and in our own atmosphere, I 

 was much struck, as others have doubtless been, with the remarkable 

 band of lines on the less refrangible side of B. Seen under great 

 dispersion, they were quite unlike any thing I had before observed in 

 the spectrum, while the best maps, I found, gave no adequate idea 

 of their curious structure which possessed to me an unexanqjled formal 

 regularity. After more study of this portion, I commenced critically 

 to examine the A group, which is nearly the last extremity of the 

 visual spectrum toward the red, and which is so overwhelmed in the 

 diffuse light from brighter portions, that few have, I believe, ever 

 seen it in any detail. I now found another subject of surprise, in the 

 extraordinary resemblance which this group bore to the B group ; 

 a resemblance which could not be the result of accident, but which 

 has never been, so far as I can learn, publicly noticed, and of which 

 no published map, I have seen, gives any idea. 



It is, of course, known to professional students that in this region 

 such a structure and general resemblance exist ; but as I believe that, 

 without special precautions, its extraordinary completeness cannot be 

 made out, it is possible that a very careful drawing, founded wholly 

 on micrometric measurement, of what has never been fully delineated 

 will present some novelty even to the spectroscopist who may not 

 have made this region a special study. To others I may recall — 



