144 



INFRA-RED EMISSION SPECTRA. 



we have to consider the combined effect of atmospheric absorption, of 

 incomplete emission of the moon (emission minima from 8.5 to 9.8^), 

 and of selectively reflected energy from the sun. The latter will, no doubt, 

 vary the most in intensity. The computed emission curve is the most 

 intense at 10.2 /x, while the observed is the most intense at 8.3 \x (see fig. 103). 

 This is to be expected if the observed energy curve is the composite of the 

 selectively emitted energy of the moon, and the selectively reflected energy 

 of the sun, which is selectively transmitted by the earth's atmosphere. 

 The selectively reflected energy of the sun would to a certain extent fill up 

 the minima in the lunar emission curve, thus making it higher at 8.3 fx 

 than at 10.2 ft, and, as far as our present knowledge goes, would explain 

 the observed curves a, c, d, fig. 103, which lack a minimum at 8.5 fi. As 



5 6 7 8 9 10 ii 



Fig. 103. Emission of quartz (6); Emission from moon (Langley). 



a whole, from whatever standpoint we view this matter we come to the 

 same conclusion, viz, that in the region from 8 to 10 [x the energy emitted 

 from the moon consists of its own proper radiation and of reflected energy 

 from the sun. 



To sum up, we know that Langley observed radiation from the moon 

 in the region of 8 to 10 fi. He observed also direct radiation from the sun 

 in this same region; but we do not know how much of this is superposed 

 upon the direct radiation from the moon due to the latter being selectively 

 reflecting in this region of the spectrum. As stated in Carnegie Publica- 

 tion No. 65, page 115, computations which require all sorts of assump- 

 tions will not settle the question. Bolometric comparison of the spectrum 

 energy curves of the sun and of the moon made at high altitudes will be 

 of greater service in clearing up the matter. Very's 1 suggestion of search- 

 ing for a solar image in the lunar image, with a delicate heat-measuring 

 instrument covered by a screen with a pin-hole aperture, also deserves a 

 thorough trial. Exception has been taken to the writer's statement (loc. 

 cit.) that, of the radiation observed by Langley in the lunar spectrum at 



1 Very: Astrophys. Jour., 24, p. 353, 1906. 



