280 Professor S. P. Langley [April 17, 



grown from i° to 1°, or increased five times. These mountain 

 measurements give another spectrum, the energies in each part of 

 which are defined by the middle dotted line, which we see indicates 

 very much greater energy whether heat or light in the blue end than 

 below. Next, the light or heat which would be observed at the surface 

 of the atmosphere is found in this way. If the mountain top rises 

 through one-half the absorbing mass of this terrestrial atmosphere (it 

 does not quite do so, in fact), and by getting rid of that lower half, 

 the ray 60 has grown in brightness from two to three, or half as much 

 again ; in going up to the top it would gain half as much more, or 

 become 4J, 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 re- 

 producing the spectrum outside the atmosj^here, and then recombining 

 the colours by like methods to those you have seen on the screen, we 

 finally get the true colour 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 un- 

 expectedly 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 

 partially 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 instrument (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 travellers, 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 unrecognised regions below 

 — a new invisible spectrum 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 was 

 continued. 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 pre- 

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

 view, having the same thing on the normal as well as the prismatic 



