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