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.



Week 5

Till now we have talked of the sunbeam as if it were made of only
one set of waves of different sizes, all travelling along
together from the sun. These various waves have been measured,