S. P. Langley — Invisible Solar and Lunar Spectrum. 403 



the appearance which this heat region might be expected to 

 show to an eye which could see it. 



Inordinately long as our new chart fig. 2b may seem, we see 

 that the scale is, nevertheless, contracted to the last degree, so 

 that the entire visible spectrum is compressed into less than 0.2 of 

 an inch, seen on the left or violet end. Next we have the solar 



infra-red, already described and shown to exist to 2^*8, which 

 includes its great absorption bands, 0, W, Q, previously investi- 

 gated with glass prisms. 



The principal lunar heat lies chiefly beyond the great wave- 

 length of l) mm 01 (10^), and ere we reach it we pass over a 



region between 5^ and 11/* (many times the length of the 

 entire visible spectrum), where the solar heat seems to have 

 been, to our present means of recognition, entirely absorbed, 

 probably chiefly by our own atmosphere. 



It will be convenient, however, after noting the extent of 

 the whole region shown in ^. 2a, to commence' our descrip- 

 tion with the detailed portion shown in figures 3a and 3b, to 

 which the reader is referred in illustration of what immediately 

 follows. 



The lowest bands already described, as seen in the spectrum 



of a flint glass prism, are the small ones co^ and o» 2 , near 2 1 ". Be- 

 low this point all is believed to be here given for the first 

 time. The bands in this new region are undoubtedly due, 

 chiefly, if not wholly, to telluric absorption, and they are 

 notably variable, depending on the season of the year and 

 still more on the hour of the day. As the sun sinks, its rays, 

 passing through increasing air masses, suffer absorptions which 

 singularly change the appearance of the bands ; as is shown in 

 figures 4 and 5 which, however, are drawn upon the pris- 

 matic, not the normal, scale. Figure 4 is identifiable with the 

 portion extending from X to Y on the latter. 



Observations made during the winter indicate that the band 



at 2 / " , 64, figures 3a, 3b, is, with a high sun, largely filled up, 

 especially on the less refrangible side. At noon a subordinate 

 maximum has been found within the low sun limits of this 



band at 2 //, 94, and a second one at 2^*80 frequently accom- 

 panies it, producing subordinate minima at 2^*89 and 3^*02. As 

 the absorption increases, with a sinking sun, these subordinate 



maxima disappear to a very great extent, that at 2^'SO being 

 the first to vanish as well as the quickest to grow, so that 

 at noon, on a cold day, it not only surpasses the maximum 



at 2^*94, but even begins to approach that at 3^*20, while, 

 when the sun's altitude is less than 10° the nearly uniform 



