S. P. Langley on the Selective Absorption of Solar Energy, 169 



on our charts is marked l^'dl, and in doing so had reached 

 the furthest band then certainly observed. 



Captain Abney*, in 1880, mapped by photography the 

 infra-red prismatic spectrum as far as wave-length 1^*075 

 with a precision and completeness till then wholly unknown, 

 besides giving the wave-lengths of lines for which he derives 

 by an extrapolation curve a position at X= F'240, and indi- 

 cating a band still beyond. 



Captain Abney had previously published a map of the 

 diffraction-spectrum extending to \= 0^*9682. Dr. J. W. 

 Draper f , by the aid of Captain Abney 's map, believes he has 

 identified the lines a, /3, y he saw in 1842 with groups repre- 

 sented by Abney at X= 0^8150 to 0^-8350, (T-8930 to 0^9300, 

 and (T-9350 to (T'9800. 



On our chart we have given Draper's a, j3, y according to 

 his own locations of them. He believes these to be the same 

 lines seen by himself, Foucault and Fizeau, and Lamansky. 

 According to Draper, then, the lowest limit of his own or any 

 other researches known to him in 1881 did not extend much 

 beyond wave-length 1^0000. It appears to us probable, how- 

 ever, that Lamansky 's lowest point was below this: and we 

 give a copy of Lamansky's curve (fig. 3), which the reader 

 can compare with the positions on our present charts. 



Fig. 3. 



•^ TitiHe -RiiU 



These brief references concern only what belongs to our 

 immediate purpose, and are not offered as a history of the 

 subject. 



Recent Observations on the Invisible Prismatic Spectrum, 

 After the return from Mount Whitney, observations were 

 taken at Allegheny with the train of apparatus used on the 



* Phil. Trans. 1880. 



t Proceedings of the American Academy, 1881. 



Phil. Mag. S. 5. Vol. 15. No. 93. March 1883. N 



