IN FAVOR OF THE NEBULAR HYPOTHESIS. 31 
It consists of three tubes—one a small telescope, an¬ 
other a tube with a slit for the light to enter to be 
examined, and a third containing a scale by means of 
which all the small lines of the spectrum can be defi¬ 
nitely located and measured. These tubes are so 
arranged as to focus on a prism in the center. Here 
the light of any substance forming a spectrum can be 
examined. 
In Book III a spectrum of sunlight is illustrated. 
It is continuous, and shows what is often spoken of 
as the seven rainbow colors. Besides these colors 
Fig. 13.—Spectrum of Pollux, one of the bright stars in the con¬ 
stellation of the Twins. 
forming a continuous spectrum there are narrow, dark 
lines in it which can be seen only with a telescope. 
When we burn any substance in a fiame its light 
produces a change in the lines of the spectrum. As, 
for instance, if sodium is held on the point of a plat¬ 
inum wire in a gas-flame, a bright yellow band ap¬ 
pears in the spectrum. Hydrogen burned shows a 
broad, bright line in the orange and a narrow one in 
the blue. The mineral thallium (green twig) gives a 
single bright, green line, and so on. Each substance 
