S. P. Langley — Invisible Solar and Lunar Spectrum, 409 



we have measured indications of lunar heat (and possibly of 

 solar) greater than are shown on this drawing, and whose wave- 

 lengths exceed one-fiftieth of a millimeter. It is not likely 

 that the more refrangible of that extremely feeble heat, which 



we here particularly describe, is much less than ll ,a . 



I think we may now feel justified in saying that we probably 

 know some of the main facts about the solar spectrum, so far 

 as terrestrial absorption is concerned. Broadly speaking, they 

 are these : 



Hardly one-fourth of the solar energy, as we receive it, is visi- 

 ble, at least without special precautions. Of the remaining 

 three-fourths, far the larger portion of the heat actually re- 

 ceived lies in the region above 2^*8, which has already been 

 delineated, but if it were not for terrestrial absorption, the heat 

 in the region below it would be relatively so much greater, 

 that it is probable that of the original solar energy, before ab- 

 sorption, more than three-fourths is invisible. 



The effects of terrestrial absorption appear in the visible 

 spectrum chiefly by means of the telluric Fraunhofer lines, so 

 that our first impression on looking at it is that these lines only 

 occasionally interrupt the play of light and color by which the 

 solar energy makes itself known through the sense of vision. 

 As we go down into these lower parts of the infra-red region, 

 we find (directly contrary to the old belief) that, broadly speak- 

 ing, the radiation apparently grows more and more transmis- 

 sible by our atmosphere, and this because the heat rays between 

 the lines grow more and more transmissible, while the lines 

 themselves, though growing into broader bands of almost total 

 absorption, have not yet extinguished the hot regions between 

 them, so that, even taking regions of transmission and absorp- 

 tion together, on the whole here, i.e. above 3^, more of dark 

 heat in proportion seems transmitted than of light heat. We 

 may roughly illustrate* this portion of the sun's spectrum after 

 absorption by saying that if it were visible, it would present 

 almost the appearance of diffuse luminous bands on a dark 

 field, somewhat like those seen in the spectra of stars of the 

 fourth type. As we have observed that absorption seems to 

 increase in the same direction in the sun's own atmosphere as 

 in ours, it would be interesting to know if anything analogous 

 exists in the absolute solar spectrum ; i. e., before absorption 

 in the sun's atmosphere, but this we are not as yet able to 

 determine. 



Since, in the part of the spectrum described here for the first 

 time, the same process of aggregation into ever wider and 



* Figure 3. 



