404: S. P. Langley — Invisible Solar and Lunar Spectrum. 



part of the band extends from 2 / "*45 to 3^*15 without a break. 



Minor cold bands at 3^7 and S^'69 are suspected. The evi- 

 dence for their existence may be seen by inspecting the high 



sun and low sun curves given in fig. 4. From-4^'0 to 4 // *5 we 

 have another region of almost complete absorption, followed 

 by a maximum at 4^*6, beyond which lies the longest break of 

 all, stretching from 5^ to llA The solar heat throughout the 



region from 5'" to 11" is probably far greater than it here 

 appears, and if the parts struck out by our atmosphere were 

 restored it would probably be found that a not wholly incon- 

 siderable portion of the sun's heat lies in this region, for it 

 must be borne in mind that even the maxima are doubtless in 

 some degree affected by a linear absorption, so that, because a 

 part of the spectrum appears to be almost entirely transmitted, 

 we cannot infer that it necessarily is so, or that it has not, 

 after passing through the upper strata of the air, already parted 

 with a considerable portion of its energy. While the position 

 of a line or band caused by light from the center of the sun 

 is, in the upper spectrum, unchangeable, in this extreme 

 lower spectrum (if we could photograph it, like the upper, on 

 cold and on hot days, with high sun and with low), the absorp- 

 tion would be seen to increase, not symmetrically with the cen- 

 ter of the band, but more on one side than another, so as to 

 considerably modify the position of greatest absorption. This 

 seems to be the explanation of a curious fact, which could not 

 have been anticipated in advance of observation ; that is, that 

 the centers of several of these bands and lines are under some 

 conditions found to be shifted to a recognizable extent, and 

 hence their wave-lengths are, within certain limits, variable. 



It seems as though the absorption, which we see exercised in 

 most of the visible spectrum by fine lines, which begin already 

 to show aggregation into a broader absorption band on the 

 borders of the infra-red (e. g., the well-known u A " of Fraun- 

 hofer), is, as we pass farther down into the invisible part, rep- 

 resented by wider and wider bands, like 0, W, Q (composed 

 themselves of lines probably). The heat between these local 

 regions of almost total absorption continues, as I have repeat- 

 edly before stated, to be (contrary to the old belief) apparently 

 transmitted with even greater facility than that in the visible 

 spectrum. These bands have grown larger, and closer and 

 closer together, as we have come down from the visible spec- 

 trum below the point where the old map ended, and not far 



below 5^ they seem to practically coalesce into one almost 

 unlimited cold band. We do not, let it be observed, assert 

 that the absorption is absolutely total even here, and, in fact, 



