and we know that the waves which make up red light are larger and
more lazy than those which make violet light, so that there are
only thirty-nine thousand red waves in an inch, while there are
fifty-seven thousand violet waves in the same space.

How is it then, that if all these different waves making
different colours, hit on our eye, they do not always make us see
coloured light? Because, unless they are interfered with, they
all travel along together, and you know that all colours, mixed
together in proper proportion, make white.

I have here a round piece of cardboard, painted with the seven
colours in succession several times over. When it is still you
can distinguish them all apart, but when I whirl it quickly round
- see! - the cardboard looks quite white, because we see them all
so instantaneously that they are mingled together. In the same
way light looks white to you, because all the different coloured
waves strike on your eye at once. You can easily make on of
these card for yourselves only the white will always look dirty,
because you cannot get the colours pure.

Now, when the light passes through the three-sided glass or
prism, the waves are spread out, and the slow, heavy, red waves
lag behind and remain at the lower end R of the coloured line on
the wall (Fig. 7), while the rapid little violet waves are bent
more out of their road and run to V at the farther end of the
line; and the orange, yellow, green, blue, and indigo arrange
themselves between, according to the size of their waves.

And now you are very likely eager to ask why the quick waves