42 
IA T OES 
[Aay 14, 1885 
mountain camp, and found that it had suffered less than might | tions, On the screen is the spectrum as seen in the desert, 
be expected, considering the pathless character of the wilderness. 
We went to work to build piers and mount telescopes and 
siderostats, in the scene shown by the next illustration on the 
screen, taken from a sketch of my own, where these rocks in 
the immediate foreground rise to thrice the height of St. Paul’s. 
We suffered from cold (the ice forming 3 inches deep in the 
tents at night) and from mountain sickness, but we were too busy 
to pay much attention to bodily comfort, and worked with 
desperate energy to utilise the remaining autumn days, which 
were all too short. 
Here, as below, the sunlight entered a darkened tent, and 
was spread into a spectrum, which was explored throughout by 
the bolometer, measuring, on the same separate rays which we 
had studied below in the desert, all of which were different up 
here, all having grown stronger, but in very different propor- 
) 
: 
drawn on a conventional scale, neither prismatic nor normal, 
but such that the intensity of the energy shall be the same in 
each part, as it is represented here by these equal perpendiculars 
in every colour. Fix your attention on these three as types, and 
| you will see better what we found on the mountain, and what 
| We inferred as to the state of things still higher up, at the surface 
of the aéreal sea. 
You will obtain, perhaps, a clearer idea, however, from the 
following statement, where I use, not the exact figures used in 
calculation, but round numbers, to illustrate the process em- 
ployed. I may premise that the visible spectrum extends from 
H (in the extreme blue) to A (in the deepest red), or from near 
40 (the ray of forty one hundredth thousandths of a millimetre in 
wave-length) to near 80. All below 80, to the right, is the 
| invisible infra-red spectrum. 
Now, the shaded curve above the spectrum represents the 
amount of energy in the sun’s rays at the foot of the mountain, 
and was obtained in this way :—Fix your attention for a moment 
on any single part of the spectrum, for instance, that whose 
wave-length is 60, If the heat in this ray, as represented by the 
bolometer at the foot of the mountain, was (let us suppose) 2°, 
on any arbitrary scale we draw a vertical line, 2 inches, or 
2 feet high over that part of the spectrum, If the heat at 
another point, such as 40, were but a 4°, a line would be drawn 
there a quarter of an inch high, and so on, till these vertical 
lines mark out the shaded parts of the drawing, the gaps and 
depressions in whose outline correspond to the ‘‘cold bands” 
already spoken of. Again, if on top 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 z very different propor- 
vions. At 60, for instance, the heat (and light) may have grown 
from 2° to 3°, or increased one-half, while above 4o the heat 
(and light) may have grown from 2° to 1°, orjincreased 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 orlight inthe 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 tothe top it would gain half as much 
more, or become 44, 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 geo- 
centric progression (though the heat, as a whole, does not), 
you see how we are able, by repeating this process at every 
Distribution of Solar Energy at Sea-level and at various Altitudes. 
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 oniside the atmosphere, 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 unexpectedly on even the smallest discovery. This 
experience I had on one of the last days of work on the spec- 
trum 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_mea- 
sured 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 toadvance the bolometer still farther, and I shall not forget 
the surprise and emotion with which I found new and yet un- 
recognised 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 
