May 29, 1885. 



SCIENCE, 



449 



of the mountain we measure all these over once more, 

 we shall find all are hotter; so that we must up there 

 make all our lines higher, but in very different 'propor- 

 tions. At 60, for instance, the heat (and light) may 

 have grown from 2° to 3°, or increased one-half, 

 while above 40 the heat (and light) may have grown 

 from j° to 1°, or increased five times. These moun- 

 tain measurements give another spectrum, the ener- 

 gies 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 ob- 

 served 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 2 

 to 3, 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 repeat- 

 ing 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 any thing 

 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 expe- 

 rience I had on one of the last days of work on the 

 spectrum on the mountain. I was engaged in ex- 

 ploring 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 farther 

 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 Thuie 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 unrecog- 

 nized regions below, — a new invisible spectrum be- 

 yond 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 map- 

 ping 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 previously known, visible and in- 

 visible, 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 re- 

 searches at Alleghany make it probable that they are 

 caused by absorption in our own atmosphere, and not 

 in that of the sun. 



We would gladly have staid longer, in spite of 

 physical discomfort; but the formidable descent and 

 the ensuing desert journey were before us, and cer- 

 tainly the reign of perpetual winter around us grew 

 as hard to bear as the heats of the desert summer had 

 been. On Sept. 10 we sent our instruments and the 

 escort back by the former route, and, ourselves unen- 

 cumbered, started on the adventurous descent of the 

 eastern precipices by a downward climb, which, if 

 successful, would carry us to the plains in a single 

 day. I at least shall never forget that day, nor the 

 scenery of more than Alpine grandeur which we 

 passed in our descent, after first climbing by frozen 

 lakes in the northern shadow of the great peak, till 

 we crossed the eastern ridges, through a door so nar- 

 row that only one could pass it at a time, by clinging 

 with hands and feet as he swung round the shoulder 

 of the rocks — to find that he had passed in a sin- 

 gle minute from the view of winter to summer, the 

 prospect of the snowy peaks behind shut out, and 

 instantly exchanged for that below of the glowing val- 

 ley and the little oasis, where the tents of the lower 

 camp were still pitched, the tents themselves invisi- 

 ble, but the oasis looking like a green scarf dropped 

 on the broad floor of the desert. We climbed still 

 downward by scenery unique in my recollection. 

 This view of the ravine on the screen is little more 

 than a memorandum made by one of the party in 

 a few minutes' halt part way down, as we followed 

 the ice-stream between the tremendous walls of the 

 defile which rose two thousand feet, and between 

 which we still descended, till, toward night, the ice- 

 brook had grown into a mountain torrent, and, look- 

 ing up the long vista of our day's descent, we saw it 

 terminated by the peak of Whitney, once more lonely 

 in the fading light of the upper sky. 



This site, in some respects unequalled for a physi- 

 cal observatory, is likely, I am glad to say, to be 

 utilized ; the president of the United States having, 

 on the proper representation of its value to science, 

 ordered the reservation, for such purposes, of an area 

 of a hundred square miles about and inclusive of 

 Mount Whitney. 



There is little more to add about the journey back 

 to civilization, where we began to gather the results 

 of our observation, and to reduce them; to smelt, 

 so to speak, the metal from the ore we had brought 

 home, — a slow but necessary process, which has oc- 

 cupied a large part of two years. The results, stated 



