18 
NATURE 
[May 7, 1885 
in all our lives seen the true colour of the sun, which is in reality 
blue rather than white, as it looks, so that this air all about and 
aboye us is acting like a coloured glass roof over our heads, or a 
sort of optical sieve, holding back the excess of blue in the 
original sunlight, and letting only the white sift down to us. 
I will first ask you, then, to consider that this seeming colour- 
lessness of the air may be a delusion of our senses, due to 
habit, which has never given us anything else to compare it 
with. 
If that cave had been lit by sunshine coming through a reddish 
glass in its roof, would the perpetual dweller in it ever have had 
an idea but that the sun was red? How is he to know that the 
glass is ‘‘coloured”’ if he has never in his life anything to com- 
pare it with? How can he have any idea but that this is the 
sum of all the sun’s radiations (corresponding to our idea of white 
or colourless light) ; will not the habit of his life confirm him 
in the idea that the sun is red; and will he 1ot think that 
there is no colour in the glass so long as he cannot go outside to 
see? Has this any suggestion for us, who have none of us ever 
been outside our crystal roof to see? 
We must all acknowledge in the abstract, that habit is equally 
st.ong in us whether we dwell in a cave or under the sky, that 
what we have thought from infancy will probably appear the 
sole possible explanation, and that, if we want to break its 
chain, we should put ourselves, at least in imagination, under 
conditions where it no longer binds us. 
The Challenger has dredged from the bottom of the ocean 
fishes which live habitually at great depths, and whose enormous 
eyes tell of the correspondingly faint light which must have 
descended to them through the seemingly transparent water. It 
will not be as futile a speculation as it may at first seem, to put 
ourselves in imagination in the condition of creatures under the 
sea, and ask what the sun may appear to be to them; for if 
the fish who had never risen above the ocean floor were an 
intelligent being, might /e not plausibly reason that the dim 
greenish light of his heaven—which is all he has ever known— 
was the full splendour of the sun, shining through a medium 
which all his experience shows is transparent ? 
We ourselves are, in very fact, living at the floor of a great 
aérial sea, whose billows xoll hundreds of miles above our heads. 
Is it not at any rate conceivable that we may have been led into 
a like fallacy from judging only by what we see at the bottom ? 
May we not, that is, have been led into the fallacy of assuming 
that the intervening medium above us is colourless because the 
light which comes through it is so ? 
I freely admit that all men, educated or ignorant, appear to 
have the evidence of their senses that the air is colourless, and 
that pure sunlight is white, so that if I venture to ask you to 
listen to considerations which have Jately been brought forward 
to show that it is the sun which is blue, and the air really acts 
like an orange veil or like a sieve which picks out the blue and 
leaves the white, I do so in the confidence that I may appeal to 
you on other grounds than those I could submit to the primitive 
man who has his senses alone to trust to; for the educated intel- 
ligence possesses those senses equally, and in addition the | 
ability to interpret them by the light of reason, and before this 
audience it is to that interpretation that I address myself. 
Permit me a material illustration. You see through this 
glass, which may typify the intervening medium of air or water, 
a circle cf white light, which may represent the enfeebled disk 
of the sun when so viewed. Is this intervening glass coloured 
or not? It seems nearly colourless ; but have we any right to 
conclude that it is so because it seems so? Are we not taking 
tt for granted that the original light which we see through it is 
white, and that the glass is colourless, because the light seems 
unaltered, and is not an appeal to be made here from sense to 
reason, which, in the educated observer, recalls that white light 
is made of various colours, and that whether the original light 
is really white and the glass transparent, or the glass really 
coloured and so making the white, is to be decided only by ; 
experiment, by taking away the possibly deceptive medium? I 
can take away this glass, which was not colourless, but of a 
deep orange, and you see that the original light was not 
white, but intensely blue. If we could take the atmosphere 
away between us aud the sun, how can we say that the same 
result might not follow? To make the meaning of our illustration 
clearer, observe that this blueness is not a pure spectral blue. 
It has in it red, yellow, blue, and all the colours which make 
up white, but blue in superabundance ; so that, though the white 
is, so to say, Jatent there, the dominant effect is blite. The 
glass coloured veil does not put anything 7, but acts I repeat 
like a sieve straining oz/ the blue, and letting through to us the 
white light which was there in the bluishness, and so may not 
our air do so too? 
I think we already begin to see that it is at any rate conceiv- 
able that we »ay have been hitherto under a delusion about the 
true colour of the sun, though of course this is not proving that 
we have been so, and it will at any rate, I hope, be evident that 
here is a question raised which ought to be settled, for the blue- 
ness of the sun, if proven, evidently affects our present know- 
ledge in many ways, and will modify our present views in optics, 
in meteorology, and in numerous other things. In_ optics, 
because we should find that white light is »o¢ the sum of the sun’s 
radiations, but only of those dregs of them which have filtered 
down to us ; in meteorology because it is suggested that the tem- 
perature of the globe and the condition of man on it, depend in 
part on a curious selective action of our air, which picks out parts 
of the solar heat (for instance, that connected with its blue li,ht), 
and holds them back, letting other selected portions come to us, 
and so altering the conditions on which this heat by which we 
live, depends; in other ways, innumerable, because, as we 
know, the sun’s heat and light are facts of such central import- 
ance, that they affect almost every part of scientific knowledge. 
It may be ashed what suggested the idea that the sun may be 
blue rather than any other colour. 
My own attention was first directed this way many years ago 
when measuring the heat and light from different parts of the 
sun’s disk. It is known that the sun has an atmosphere of its 
own which tempers its heat, and, by cutting off certain radiations 
and not others, produces the spectral lines we are all familiar 
with. These lines we customarily study in connection with the 
absorbing vapours of sodium, iron, and so forth, which produce 
them ; but my own att-ntion was particularly given to the regions 
of absorption, or to the colour it caused, and I found that the 
sun’s body must be deeply bluish, and that it would shed blue 
light except for this apparently colowless solar atmosphere, 
which really plays the part of a reddish veil, letting a 
little of the blue appear on the centre of the sun’s disk where it is 
thinnest, and staining the edge red, so that to delicate tests the 
centre of the sun is a pale aqua-marine, and its edge a garnet. 
The effect I found to be so important, that if this all but invisible 
solar atmosphere were diminished by but a third part, the tem- 
perature of the British islands would rise above that of the torrid 
zone, and this directed my attention to the great practical im- 
portance of studying the action of our own terrestrial atmosphere 
on the sun, and the antecedent probability that our own air was 
also and independently making the really blue sun into an ap- 
parently white one. We actually know then, beyond conjecture, 
by a comparison of the sun’s atmosphere, where it is thickest, 
and where it is thinnest, that an apparently colourless atmosphere 
can have such an effect, and analogous observations which I 
have carried on for many years, but do not now detail, show 
that the atmosphere of our own planet, this seemingly clear air 
in which we exist like creatures at the bottom of the sea, does 
do so. 
We look up through our own air as through something so 
limpid in its purity that it appears scarcely matter at all, and we 
are apt to forget the enormous mass of what seems of such light- 
ness, but which really presses with nearly a ton to each square 
foot, so that the weight of all the buildings in this great city, for 
instance, is less than that of the air above them. 
I hope to shortly describe the method of proof that it too has 
been acting like an optical sieve, holding back the blue ; but it 
may naturally be asked, ‘‘Can our senses have so entirely de- 
ceived us that ihey give no hint of this truth, if it be one ? is the 
appeal wholly to recondite scientific methods, and are there no 
indications, at least, which we may gather for ourselves?” I 
think there are, even to our unaided eyes, ind cations that the 
seemingly transparent air really acts as an orange medium, and 
keeps the blue light Lack in the upper sky. 
If I hold this piece of glass before my eyes, it seems colour- 
less and transparent, but it is proved not to be so by looking 
through it edgewise, when the light, by traversing a greater 
extent, brings out its true colour, which is yellow. Every one 
knows this in every-day experience. We shall not get the 
colour of the ocean by looking at it ina wine-glass, but by gazing 
through a great depth of it ; andso it is with the air. If we look 
directly up, we look through where it is thinnest ; but if we look 
horizontally through it towards the horizon, through great thick- 
nesses, as at sunrise or sunset, is it not true that this air, where 
