We have now learnt something of the distance, the size, the
light, and the heat of the sun - the great source of the
sunbeams. But we are as yet no nearer the answer to the
question, What is a sunbeam? how does the sun touch our earth?
Now suppose I with to touch you from this platform where I stand,
I can do it in two ways. Firstly, I can throw something at you
and hit you - in this case a thing will have passed across the
space from me to you. Or, secondly, if I could make a violent
movement so as to shake the floor of the room, you would feel a
quivering motion; and so I should touch you across the whole
distance of the room. But in this case no thing would have
passed from me to you but a movement or wave, which passed along
the boards of the floor. Again, if I speak to you, how does the
sound reach you ear? Not by anything being thrown from my mouth
to your ear, but by the motion of the air. When I speak I
agitate the air near my mouth, and that makes a wave in the air
beyond, and that one, another, and another (as we shall see more
fully in Lecture VI) till the last wave hits the drum of your
ear.
Thus we see there are two ways of touching anything at a
distance; 1st, by throwing some thing at it and hitting it; 2nd,
by sending a movement of wave across to it, as in the case of the
quivering boards and the air.
Now the great natural philosopher Newton thought that the sun
touched us in the first of these ways, and that sunbeams were
made of very minute atoms of matter thrown out by the sun, and
making a perpetual cannonade on our eyes. It is easy to
understand that this would make us see light and feel heat, just
as a blow in the eye makes us see starts, or on the body makes it
feel hot: and for a long time this explanation was supposed to be
the true one. But we know now that there are many facts which
cannot be explained on this theory, though we cannot go into them
here. What we will do, is to try and understand what now seems
to be the true explanation of the sunbeam.
About the same time that Newton wrote, a Dutchman, named
Huyghens, suggested that light comes from the sun in tiny waves,
travelling across space much in the same way as ripples travel
across a pond. The only difficulty was to explain in what
substance these waves could be travelling: not through water, for
we know that there is no water in space - nor through air, for
the air stops at a comparatively short distance from our earth.
There must then be something filling all space between us and the
sun, finer than either water or air.
And now I must ask you to use all you imagination, for I want you
to picture to yourselves something quite as invisible as the
Emperor's new clothes in Andersen's fairy-tale, only with this
difference, that our invisible something is very active; and
though we can neither see it nor touch it we know it by its
effects. You must imagine a fine substance filling all space
between us and the sun and the starts. A substance so very
delicate and subtle, that not only is it invisible, but it can
pass through solid bodies such as glass, ice, or even wood or
brick walls. This substance we call "ether." I cannot give you
here the reasons why we must assume that it is throughout all
space; you must take this on the word of such men as Sir John
Herschel or Professor Clerk-Maxwell, until you can study the
question for yourselves.
Now if you can imagine this ether filling every corner of space,
so that it is everywhere and passes through everything, ask
yourselves, what must happen when a great commotion is going on
in one of the large bodies which float in it? When the atoms of
the gases round the sun are clashing violently together to make
all its light and heat, do you not think they must shake this
ether all around them? And then, since the ether stretches on
all sides from the sun to our earth and all other planets, must
not this quivering travel to us, just as the quivering of the
boards would from me to you? Take a basin of water to represent
the ether, and take a piece of potassium like that which we used
in our last lecture, and hold it with a pair of nippers in the
middle of the water. You will see that as the potassium hisses
and the flame burns round it, they will make waves which will
travel all over the water to the edge of the basin,, and you can
imagine how in the same way waves travel over the ether from the
sun to us.
Straight away from the sun on all sides, never stopping, never
resting, but chasing after each other with marvellous quickness,
these tiny waves travel out into space by night and by day. When
our spot of the earth where England lies is turned away from them
and they cannot touch us, then it is night for us, but directly
England is turned so as to face the sun, then they strike on the
land, and the water, and warm it; or upon our eyes, making the
nerves quiver so that we see light. Look up at the sun and
picture to yourself that instead of one great blow from a fist
causing you to see starts for a moment, millions of tiny blows
from these sun-waves are striking every instant on you eye; then
you will easily understand that his would cause you to see a
constant blaze of light.
But when the sun is away, if the night is clear we have light
from the starts. Do these then too make waves all across the
enormous distance between them and us? Certainly they do, for
they too are suns like our own, only they are so far off that the
waves they send are more feeble, and so we only notice them when
the sun's stronger waves are away.
But perhaps you will ask, if no one has ever seen these waves not
the ether in which they are made, what right have we to say they
are there? Strange as it may seem, though we cannot see them we
have measured them and know how large they are, and how many can
go into an inch of space. For as these tiny waves are running on
straight forward through the room, if we put something in their
way, they will have to run round it; and if you let in a very
narrow ray of light through a shutter and put an upright wire in
the sunbeam, you actually make the waves run round the wire just
as water runs round a post in a river; and they meet behind the
wire, just as the water meets in a V shape behind the post. Now
when they meet, they run up against each other, and here it is we
catch them. Fir if they meet comfortably, both rising up in a
good wave, they run on together and make a bright line of light;
but if they meet higgledy-piggledy, one up and the other down,
all in confusion, they stop each other, and then there is no
light but a line of darkness. And so behind your piece of wire
you can catch the waves on a piece of paper, and you will find
they make dark and light lines one side by side with the other,
and by means of these bands it is possible to find out how large
the waves must be. This question is too difficult for us to work
it out here, but you can see that large waves will make broader
light and dark bands than small ones will, and that in this way
the size of the waves may be measured.
And now how large do you think they turn out to be? so very,
very tiny that about fifty thousand waves are contained in a
single inch of space! I have drawn on the board the length of an
inch, and now I will measure the same space in the air between my
finger and thumb. Within this space at this moment there are
fifty thousand tiny waves moving up and down. I promised you we
would find in science things as wonderful as in fairy tales. Are
not these tiny invisible messengers coming incessantly from the
sun as wonderful as any fairies? and still more so when, as we
shall see presently, they are doing nearly all the work of our
world.
We must next try to realize how fast these waves travel. You
will remember that an express train would take 171 years to reach
us from the sun; and even a cannon-ball would take from ten to
thirteen years to come that distance. Well, these tiny waves
take only seven minutes and a half to come the whole 91 millions
of miles. The waves which are hitting your eye at this moment
are caused by a movement which began at the sun only 7 1/2
minutes ago. And remember, this movement is going on
incessantly, and these waves are always following one after the
other so rapidly that they keep up a perpetual cannonade upon the
pupil of your eye. So fast do they come that about 608 billion
waves enter your eye in one single second.* I do not ask you to
remember these figures; I only ask you to try and picture to
yourselves these infinitely tiny and active invisible messengers
from the sun, and to acknowledge that light is a fairy thing.
(*Light travels at the rate of 190,000 miles, or 12,165,120,000
inches in a second. Taking the average number of wave-lengths in
an inch at 50,000, then 12,165,120,000 X 50,000 =
608,256,000,000,000.)
But we do not yet know all about our sunbeams. See, I have here
a piece of glass with three sides, called a prism. If I put it
in the sunlight which is streaming through the window, what
happens? Look! on the table there is a line of beautiful
colours. I can make it long or short, as I turn the prism, but
the colours always remain arranged in the same way. Here at my
left hand is the red, beyond it orange, then yellow, green, blue,
indigo or deep blue, and violet, shading one into the other all
along the line. We have all seen these colours dancing on the
wall when the sun has been shining brightly on the cut-glass
pendants of the chandelier, and you may see them still more
distinctly if you let a ray of light into a darkened room, and
pass it through the prism as in the diagram (Fig. 7). What are
these colours? Do they come from the glass? No; for you will
remember to have seen them in the rainbow, and in the soap-
bubble, and even in a drop of dew or the scum on the top of a
pond. This beautiful coloured line is only our sunbeam again,
which has been split up into many colours by passing through the
glass, as it is in the rain-drops of the rainbow and the bubbles
of the scum of the pond.