544 THE POPULAR SCIENCE MONTHLY. 



in the heavens or on the earth, will give a spectrum. If it be 

 a solid or liquid body, we shall have what is called a contin- 

 uous spectrum — that is, a colored band bright from end to end, 

 with no sign of any dark or bright lines about it. By continuing 

 to heat this body until it becomes a mass of incandescent gas, the 

 spectrum will become entirely changed, and will consist of a series 

 of bright lines on a dark background, the number and position of 

 the lines depending on the substance heated. But suppose, now, 

 that the light from an incandescent solid or liquid body passes 

 through a gas, what kind of a spectrum should we have ? Ex- 

 periment shows that in this case we get a continuous spectrum 

 crossed by dark lines, these dark lines being produced by the 

 peculiar power that a gas possesses of absorbing those particular 

 rays of light which it emits. Thus we see that if we are dealing 

 with incandescent solid or liquid bodies, we obtain continuous 

 spectra ; if with incandescent gases, bright-line spectra ; and if 

 with absorption, dark-line spectra ; the position of the lines in all 

 cases revealing the chemical nature of the substances. 



So much, then, for the general idea of the nature of a spec- 

 trum. There are some additional points to be considered when 

 we are dealing with stars. If we observe the spectrum of a star 

 at rest, we shall obtain lines, whether bright or dark, in their 

 normal place in the spectrum. These lines will be peculiar to 

 certain substances, and, in fact, their presence in the star is 

 determined simply by them. If we deal with the light from a 

 body which is not an apparent point, the lines will still keep the 

 same positions, for the same reason, but each one of them will be 

 broadened equally. 



Let us now suppose the star no longer stationary, but moving 

 with a considerable velocity. In this case the wave-length of 

 each line will be no longer the same; but the line will have 

 altered its position in the spectrum to an extent depending on 

 the movement of the star toward or from the earth. The result 

 produced in the spectrum will be the same with regard to the 

 number of lines as was the case when the star was assumed to 

 be motionless, but the lines will all have received a slight shift, 

 either to one side or the other of their initial positions, according 

 as the star is approaching or receding. If instead of one we now 

 deal with two stars of the same chemical and physical structure, 

 traveling with different velocities, either toward or away from 

 us, the spectrum would show each line doubled, and the more 

 rapid the relative motion the coarser will be the doubling. If 

 the stars were so physically constituted that the same chemical 

 substances were present in both, but giving bright lines in one 

 and dark lines in the other, the spectrum would present a series 

 of bright lines, each accompanied by a dark one, on one side or 



