SUNBEAMS AND THEIR WORK. 39 
take only seven minutes and a half to come the whole 
92 millions of miles. The waves which are hitting 
your eye at this moment are caused by a movemenl 
which began at the sun only 7^ minutes ago. And re- 
member, 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 
single second.* I do not ask you to remember these 
figures ; I only ask you to try and picture to your- 
selves these infinitely tiny and active invisible mes- 
sengers from the sun, and to acknowledge that light is 
a fairy thing. 
But we do not yet know all about our sunbeam. 
See, I have here a piece of glass with three sides, called 
a prism. If I put it in the 
sunlight which is streaming /V /\ 
through the window, what ^ ^ 
happens? Look! on the FlG - 6 - 
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 
* Light travels at the rate of 192,000 miles, or 12,165,120,000 
inches, in a second. Taking the average number of wave- 
lengths in an inch at 50,000, then 12,165,120,000 X 50,000 = 608,- 
256,000,000,000. 
