SUNBEAMS AND THEIR WORK. 
coloured waves strike on your eye at once. You 
can easily make one of these cards for yourselves, 
only the white will always look dirty, because you 
Fig. 8. 
A, Cardboard painted with the seven 
colours in succession. 
B, Same cardboard spun quickly round. 
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 re 
main 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 should make us see one colour, and the 
slow waves another. This is a very difficult question, 
for we have a great deal still to learn about the effect 
of light on the eye. But you can easily imagine that 
colour is to our eye much the same as music is to our 
ear. You know we can distinguish different notes 
when the air-waves play slowly or quickly upon the 
drum of the ear (as we shall see in Lecture VI.), and 
somewhat in the same way the tiny waves of the ether 
play on the retina or curtain at the back of our eye, 
and make the nerves carry different messages to the 
