40 THE FAIRY-LAND OF SCIENCE. 
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. 
Till now we have talked of the sunbeam as if it were 
made of only one set of waves, but in truth it is made 
of many sets of waves of different sizes, all travelling 
along together from the sun. These various waves 
have been measured, 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 instan- 
taneously that they are mingled together. In the same 
way light looks white to you, because all 'the different 
