144 AKNTJAL REPORT SMITHSONIAN INSTITUTION, 1941 



spectrum is simply a map of the colors spread out into a band begin- 

 ning with violet at one end and passing through blue, green, and 

 yellow into red at the other. It can be produced in several dif- 

 ferent ways, the simplest of which is by a triangular piece of glass 

 called a prism. Wlien white light passes through a prism the violet 

 part of the light is bent a certain amount when it comes out, the 

 green a little less, and the red still less. The final result is a con- 

 tinuous band of color extending from violet to red. You have all 

 seen flashes of such a spectrum when sunlight falls upon a cut-glass 

 bowl or the edge of a beveled mirror. 



Now the important fact is that any chemical element, when heated 

 to the point where it vaporizes and gives out light, gives it in a 

 pattern of colored bright lines which is unique for each element 

 and defines it absolutely. Some patterns are comparatively simple, 

 while others are exceedingly complex. For example, sodium has 

 relatively few lines in its spectrum and nearly all the light which 

 sodium vapor emits is concentrated in two strong lines of orange 

 color. These are so dominant that they define the color of a sodium 

 lamp completely, as you all know who have ridden through the 

 yellow glare of the street lights now in such common use. Similarly 

 neon gas has its strongest lines in the red portion of the spectrum 

 and hence neon signs are red to the eye, while mercury light with 

 an exceedingly strong green line in its spectrum is predominantly 

 green in color. On the other hand the vapor of iron produced in an 

 electric arc has an extraordinarily rich spectrum consisting of some 

 2,000 lines distributed throughout all the colors of the spectrum. In 

 the absence of predominant lines of one color, luminous iron gas 

 appears nearly white to the eye. 



One other fact should be remembered before we pass to our 

 immediate astronomical applications of the spectrum. A hot, solid 

 body or one consisting of dense gases gives out a spectrum which is a 

 continuous band of color, not one of bright lines. When the light from 

 such a body, a star for example, passes through a gas of somewhat 

 lower temperature the gas will absorb light of just the color of its 

 characteristic lines, and we shall have a pattern of absorption or 

 dark lines. For example, when the light from the filament of an 

 ordinary incandescent lamp is passed through a slightly cooler tube 

 of sodium vapor we see the two strong yellow lines of sodium as dark 

 lines on the yellow background of light given by the filament. 



In astronomy we have almost a precise analogy to the filament and 

 tube of sodium vapor. The body of the sun or of a star corresponds 

 to the filament and gives out a continuous band of color, while the 

 gaseous atmosphere corresponds to the sodium tube and produces the 

 absorption lines. The principal difference is that the atmosphere of 



