itation that keeps your feet on the
ground all the same. A clever man made up a story about some one who
invented a kind of stuff which stopped the force of gravitation going
through it, just as a solid body stops light; when this stuff was made,
of course, it went right away off into space, carrying with it anyone
who stood on it, as there was nothing to hold it to the earth! That was
only a story, and it is not likely anyone could invent such stuff, but
it serves to make clear the working of gravitation. These two tireless
forces, light and gravitation, run throughout the whole universe, and
carry messages of tremendous importance for those who have minds to
grasp them. Without light we could know nothing of these distant worlds,
and without understanding the laws of gravity we should not be able to
interpret much that light tells us.

To begin with light, what can we learn from it? We turn at once to our
own great light-giver, the sun, to whom we owe not only all life, but
also all the colour and beauty on earth. It is well known to men of
science that colour lies in the light itself, and not in any particular
object. That brilliant blue cloak of yours is not blue of itself, but
because of the light that falls on it. If you cannot believe this, go
into a room lighted only by gas, and hey, presto! the colour is changed
as if it were a conjuring trick. You cannot tell now by looking at the
cloak whether it is blue or green! Therefore you must admit that as the
colour changes with the change of light it must be due to light, and not
to any quality belonging to the material of the cloak. But, you may
protest, if the colour is solely due to light, and light falls on
everything alike, why are there so many colours? That is a very fair
question. If the light that comes from the sun were of only one
colour--say blue or red--then everything would be blue or red all the
world over. Some doors in houses are made with a strip of red or blue
glass running down the sides. If you have one in your house like that,
go and look through it, and you will see an astonishing world made up of
different tones of the same colour. Everything is red or blue, according
to the colour of the glass, and the only difference in the appearance of
objects lies in the different shades, whether things are light or dark.
This is a world as it might appear if the sun's rays were only blue or
only red. But the sun's light is not of one colour only, fortunat