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.