S. P. Langley — Invisible Solar and Lunar Spectrum. 399 



yellow and orange light exists in relatively enormous quantity 

 in the neighboring parts of the visible spectrum, and irreg- 

 ularly diffused and reflected portions of this light re-appear 

 where they do not belong and overpower the radiation legiti- 

 mately there. Still, we can put a colored glass before the slit 

 and cut off the intruding light in a great measure, and we can 

 see the extraneous light which comes in, and allow in some de- 



free for its effects ; but here, in the actual case of the unseen 

 eat in the far more remote spectral region we are about to de- 

 scribe, all radiations, both the feeble ones we would study and 

 the intruders on them which we would avoid, are alike invisible, 

 and we are, of course, unable in any case to use glass, since this 

 is opaque to all the rays now in question. If any one familiar 

 with the visible spectrum will imagine himself as trying to dis- 

 criminate with his eyes shut between these different compo- 

 nents of the apparent radiation just. below Fraunhofer's A, and 

 endeavoring while blindfold to say how much of it legitimately 

 belongs there and how much does not, he will have a better 

 conception of the difficulties peculiar to our actual field of re- 

 search, though still an inadequate one, since the total heat 

 radiation here is at best less than the hundredth part of that in 

 the vicinity of the A line, which we have used in illustration. 

 For the clearer understanding of this, I must, in anticipation 

 of what follows, remark that while in the solar spectrum the 

 maximum heat, as we all know, appears not very far from the 

 red, so that the heat corresponding in a general sense to the short 

 waves is great, and to the still longer ones small, in the lun ar 

 invisible spectrum the reverse is the case; for here, speaking 

 generally, the solar reflected heat found in the upper part of 

 the lunar spectrum is less than the heat apparently radiated 

 from the moon's own soil, which is of great wave-length, and 

 which we have found in the extreme region of the spectrum 

 we are now studying. In other words, the typical solar spec- 

 trum heat is greatest in the relatively short wave-length ; the 

 typical lunar spectrum heat is greatest in the long wave-lengths. 

 The explanation of the curious fact that this particular quality 

 of heat may be more easily recognized where it exists in a less 

 degree in a lunar spectrum than where it is found in a relatively 

 great degree as in the solar, will be still clearer if we consent 

 (in continuance of the illustration) to further compare this 

 lunar invisible radiation of great wave length to the deep- 

 red light from a piece of scarcely luminous hot iron. This 

 peculiarly deep red is seen with little difficulty in the iron 

 in a dark room, but never in daylight ; yet it is of a quality 

 which we know from theory must exist in far greater 

 degree in the daylight itself ; nor do we, even when we would 

 isolate it in a certain part of the solar spectrum, see it there, 



