May 7, 1885) 
NATURE 
19 
we see its real colour most plainly, makes the sun look very 
plainly yellow or orange? 
We not only see here, in humid English skies, the ‘‘ orange 
sunset waning slow,” but most of us in these days of travel can 
perfectly testify that the clearest heavens the earth affords, the 
roxy tint on the snows of Mont Blanc, forerunninz the dawn, or 
the warm glow of the sun as he sets in Egyptian skies, show this 
most clearly—show that the atmosphere holds back the blue rays 
by preference, and lets the orange through. 
Tf, next, we ask, ‘‘ What has become of the blue that it has 
stopped?” does not that very blue of the midday sky relate the 
rest of the story—that blue which Prof, Tyndall has told us is 
due to the presence of innumerable fine particles in the air, 
which act selectively on the solar waves, diffusing the blue light 
towards us? I hope it will be understood that Prof. Tyndall is 
in no way responsible for my own inferences ; but I think it is 
safe at least to say that the sky is not self-luminous, and that, 
since it can only be shini g blue at the expense of the sun, all 
the light this sky sends us has been taken by our atmosphere 
away from the direct solar beam, which would grow both 
brighter and bluer if this were restored to it. 
Tf all that has been said so far renders it possible that the sun 
may be blue, you will still have a right to say that ‘‘ possi- 
bilities’ and ‘‘maybes” are not evidence, and that no chain of 
mere hypotheses will draw truth out of her well. We are all of 
one mind here, and I desire next to call your attention to what 
I think is evidence. 
Remembering that the case of our supposed dweller in the 
cave who could not get out-ide, or that of the inhabitants of the 
ocean-floor who cannot rise to the surface, is really like our own, 
over whose heads is a crystalline roof which no man from the 
beginning of time has ever got outside of, an upper sea to whose 
surface we have never risen ; we recognise that if we could rise 
to the surface, leaving the medium whose effect is in dispute 
wholly beneath us, we should see the sun as it is, and get proof 
of an incontrovertible kind ; and that, if we cannot entirely do 
this, we shall get nearest to proof under our real circumstances 
by going as high as we can in a balloon, or by ascending 
a very high mountain. The balloon will not do, because we 
have to use heavy apparatus requiring a solid foundation. The 
proof to which I ask your kind attention, then, is that derived 
from the actual ascent of a remarkable mountain by an expe- 
dition undertaken for that purpose, which carried a whole physi- 
cal laboratory up toa point where nearly one-half the whole atmo- 
sphere lay below us. I wish to describe the difference we found in 
the sun’s energy at the bottom of the mountain and at the top, 
and then the means we took to allow for the effect of that part of 
the earth’s atmosphere still over our heads even here, so that 
we may be said to have virtually got outside it altogether. 
Before we begin our ascent, let me explain more clearly what 
we are going to seek. We need not expect to find that the 
original sunlight is a pure monochromatic blue by any means, but 
that though its rays contain red, orange, blue, and all the other 
spectral colours, the blue, the violet and the allied tints were 
originally there in disproport’onate amounts, so that, though 
all which make white were present from the first, the 
refrangible end of the spectrum had such an excess of colour 
that the dominant effect was that of a blueish sun. In 
the same way, when I say briefly that our atmosphere has ab- 
sorbed this excess of blue and let the white reach us, I mean, 
more strictly speaking, that this atmosphere has absorbed ad/ 
the colours, but, selectively, taking out more orange than red, 
more green than orange, more blue than green ; so that its action 
is wh ly a taking ow/—an action like that which you now see 
going on with this sieve, sifting a mixture of blue and white 
beads, and holding back the blue while letting the white fall 
down. 
This experiment only rudely typifies the action of the atmo- 
sphere, which is di-criminating and selective in an amazing 
degree, and as there are really an infinite number of shades of 
colour in the spectrum, it would take for ever to describe the 
action in detail. It is merely for brevity, then, that we now 
unite the more refrangible colours under the general word 
“*blue,” and the others under the corresponding terms ‘‘ orange ” 
or ‘‘red.” 
All that I have the honour to lay before you, is less an an- 
nouncement of absol ite novelty than an appeal to your already 
acquired knowledge and to your reason as superior to the delu- 
sions of sense. I have, then, no novel experiment to offer, but 
to ask you to look at some familiar ones in a new light. 
We are most of us familiar, for instance, with that devised 
by Sir Isaac Newton to show that white light is compounded of 
blue, red, and other colours, where, by turning a coloured wheel 
rapidly, all blend into a grayish white. Here you see the ‘‘seven 
colours ” on the screen; but, though all are here, I have inten- 
tionally arranged them, so that there is too much blue, and the 
combined result is a very bluish white which may roughly stand 
for that of the original sun-ray. I now alter the proportion of 
the colours so as to virtually take out the excess of blue, and 
the result is colour'ess or white light. White, then, is not 
necessarily made by combining the ‘‘seven colours,” or any 
number of them, unless they are there in just proportion (which 
is in effect what Newton himself says) ; and white, then, may be 
made out of such a bluish light as we have described, not by 
putting anything to it, but by taking away the excess which is 
there already. 
Here, again, are two sectors—one blue, one orange-yellow 
with the blue in excess, making a bluish disk where they are 
revolved. I take out the excess of blue, and now what remains 
is white. 
Here is the spectrum itself on the screen, but a spectrum 
which has been artificially modified so that the blue end is rela- 
tively too strong. I recombine the colours (by Prof. Rood’s 
ingenious device of an elastic mirror), and they do not make a 
pure white, but one tinted with blue. I take out the original 
excess of blue, and what remains combines into a pure white. 
Please bear in mind.that when we “‘ put in” blue here, we have 
to do so by straining out other light through some obscuring 
medium, which makes the spectrum darker ; but that, in the case 
of the actual sunlight, introducing more blue, introduces more 
light, and makes the spectrum brighter. 
The spectrum on the screen ought to be male still brighter in 
the blue than it is—far, far brighter—and then it might repre- 
sent to us the original solar spectrum before it has suffered any 
absorption either in the sun’s atmosphere or our own. ‘The 
Frauenhofer lines do not appear in it, for these, when found in 
the solar spectrum, show that certain individual rays have been 
stopped, or selected for absorption by the intervening atmo- 
spheres ; and though even the few yards of atmosphere between 
the lamp and the screen absorb, it is not enough to show. 
Our spectrum, as it appears before absorption, might be com- 
pared to an army divided into numerous brigades, each wearing 
a distinct uniform, one red, one green, one blue, so that 
all the colours are represented each by its own body. If, to 
represent the light absorbed as it progresses, we supposed that 
the army advances under a fire which thins its numbers, we 
should have to consider that (to give the case of nature) this 
destructive fire was directed chiefly against those divisions 
which were dressed in blue, or allied colours, so that the army 
was thinned out unequally, many men in blue being killed off 
for one in red, and that by the time it has advanced a certain 
distance under fire the proportion of the men in each brigade 
has been altered, the red being comparatively unhurt. Almost 
all absorption is thus selective in its action, and often in an 
astoni-hing degree, killing off, so to speak, certain rays in 
preference to others, as though by an intelligent choice, and 
destroying most, not only of certain divisions (to continue our 
illustration), but even picking out certain files in each company. 
Every ray, then, has its own individuality, and on this I cannot 
too strongly insist ; for just as two men retain their personalities 
under the same red uniform, and one may fall and the other 
survive, though they touch shoulders in the ranks, so in the 
spectrum certain parts will be blotted out by absorption, while 
others next to them may escape. 
To illustrate this selective absorption, I put a piece of didy- 
mium glass in the path of the ray. It will, of course, absorb 
some of the light, but instead of dimming the whole spectrum, 
we might almost say it his arbitrarily chosen to select one 
narrow part for action, in this particular case choosing a narrow 
file near the orange, and letting all the rest go unharmed. In 
this arbitrary way our atmo phere operates, but in a far more 
complex manner, taking out a narrow file here and another 
there, in hundreds of places, all through the spectrum, but on 
the whole much the most in the blue, the Frauenhofer lines being 
merely part of the evidence of this wonderful quasi-intellizent 
action which bears the name of selective absorption. 
Before we leave this spectrum, let us recall one most important 
matter. We know that here beyond the red is solar energy in 
the form of heat which we cannot see, but not on that account 
°. 
any less important. More than half the whole power of the 
