SUNLIGHT AND THE EARTH'S ATMOSPHERE. 659 



ness from two to three, or half as much again, in going up to the top 

 it would gain half as much more, or become 4, while the ray near 40, 

 which has already increased to five times what it was, would increase 

 five times more, or to 25. Each separate ray increasing thus nearly 

 in some geocentric progression (though the heat, as a whole, does not), 

 you see how we are able, by repeating this process at every point, to 

 build up our outer or highest curve, which represents the light and 

 heat at the surface of the atmosphere. These have grown out of all 

 proportion at the blue end, as you see by the outer dotted curve, and 

 now we have attained, by actual measurement, that evidence which we 

 sought, and by thus reproducing the spectrum outside the atmosphere, 

 and then recombining the colors by like methods to those you have 

 seen on the screen, we finally get the true color of the sun, which tends, 

 broadly speaking, to blue. 



It is so seldom that the physical investigator meets any novel fact 

 quite unawares, or finds anything except that in the field where he is 

 seeking, that he must count it an unusual experience to come unex- 

 pectedly on even the smallest discovery. This experience I had on 

 one of the last days of work on the spectrum on the mountain. I was 

 engaged in exploring that great invisible heat-region, still but so par- 

 tially known, or, rather, I was mapping in that great " dark continent " 

 of the spectrum, and by the aid of the exquisite sky and the new in- 

 strument (the bolometer) found I could carry the survey further than 

 any had been before. I substituted the prism for the grating, and 

 measured on in that unknown region till I had passed the ultima Thule 

 of previous travelers, and finally came to what seemed the very end of 

 the invisible heat-spectrum beyond what had previously been known. 

 This was in itself a return for much trouble, and I was about rising 

 from my task, when it occurred to me to advance the bolometer still 

 farther, and I shall not forget the surprise and emotion with which I 

 found new and yet unrecognized regions below a new invisible spec- 

 trum beyond the farthest limits of the old one. 



I will anticipate here by saying that after we got down to lower 

 earth again the explorations and mapping of this new region were con- 

 tinued. The amount of solar energy included in this new extension of 

 the invisible region is much less than that of the visible spectrum, while 

 its length upon the wave-length scale is equal to all that previously 

 known, visible and invisible, as you will see better by this view, having 

 the same thing on the normal as well as the prismatic scale. If it be 

 asked which of these is correct, the answer is, " Both of them." Both 

 rightly interpreted mean just the same thing, but in the lower one we 

 can more conveniently compare the ground of the researches of others 

 with these. These great gaps I was at first in doubt about, but more 

 recent researches at Allegheny make it probable that they are caused 

 by absorption in our own atmosphere, and not in that of the sun. 



We would gladly have stayed longer, in spite of physical discom- 



