38 THE FAIRY-LAND OF SCIENCE. 
it out here, but you can see that large waves will make 
broader light and dark bands than small ones will, 
and that in this way the size of the waves may be 
measured. 
And now how large do you think they turn out to 
be ? So very, very tiny that about fifty thousand 
waves are contained in a single inch of space ! I 
have drawn on the board the length of an inch,* and 
now I will measure the same space in the air between 
my finger and thumb. Within this space at this 
moment there are fifty thousand tiny waves moving 
up and down ! I promised you we would find in 
science things as wonderful as in fairy tales. Are not 
these tiny invisible messengers coming incessantly 
from the sun as wonderful as any fairies ? and still 
more so when, as we shall see presently, they are doing 
nearly all the work of our world. 
We must next try to realize how fast these waves 
travel. You will remember that an express train 
would take 171 years to reach us from the sun; and 
even a cannon-ball would take from ten to thirteen 
years to come that distance. Well, these tiny waves 
take only seven minutes and a half to come the whole 
9 1 millions of miles. The waves which are hitting your 
eye at this moment are caused by a movement which 
began at the sun only 7^ minutes ago. And remember, 
this movement is going on incessantly, and these 
waves are always following one after the other so 
rapidly that they keep up a perpetual cannonade 
upon the pupil of your eye. So fast do they come 
that about 608 billion waves enter your eye in one 
* The width of an inch may be seen in Fig. 12, p. 60. 
