42 THE FAIRY-LAND OF SCIENCE. 
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 
brain : and the colour we see depends upon the num- 
ber of waves which play upon the retina in a second. 
Do you think we have now rightly answered the 
question What is a sunbeam? We have seen that it 
is really a succession of tiny rapid waves, travelling 
from the sun to us across the invisible substance we 
call " ether," and keeping up a constant cannonade 
upon everything which comes in their way. We have 
also seen that, tiny as these waves are, they can still 
vary in size, so that one single sunbeam is made up 
of myriads of different-sized waves, which travel all 
together and make us see white light ; unless for some 
reason they are scattered apart, so that we see them 
separately as red, green, blue, or yellow. How they 
are scattered, and many other secrets of the sun-waves, 
we cannot stop to consider now, but must pass on to 
ask 
What work do the sunbeams do for us f 
They do two things they give us light and heat. 
It is by means of them alone that we see anything. 
