SUNBEAMS AND THEIR WORK. 39 
single second.* I do not ask you to remember 
these figures ; I only ask you to try and picture to 
yourselves these infinitely tiny and active invisible 
messengers 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 
through the window, what ^A ./\ 
happens ? Look ! on the ^ ^ 
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 jx^ Fi s- 7 
dancing brf the 
wall when the sun 
has been shining 
brightly on the 
cut - glass pen- 
dants of the chan- 
delier, and you Coloured spectrum thrown by a prism on the 
may see them Still DE, Window-shutter. F, Round hole in it 
more distinctly if A B C, Glass prism. MN.WalL 
you let a ray of light into a darkened room, and 
pass it through the prism as in the diagram (Fig. 7). 
* 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. 
