CH. VI] MICRO-SPECTROSCOPE AND POLARISCOPE 137 



VARIOUS KINDS OF SPECTRA 



By a spectrum is meant the colored bands appearing when the light traverses 

 a dispersing prism or a diffraction grating, or is affected in any way to separate 

 the different wave lengths of light into groups. When daylight or some good 

 artificial light is thus dispersed one gets the appearance so familiar in the 

 rainbow. 



\ 190. Continuous Spectrum. — In case a good artificial light as the electric 

 light is used the various rainbow or spectral colors merge gradually into one 

 another in passing from end to end of the spectrum. There are no breaks or gaps. 



$ 191. Line Spectrum. — If a gas is made incandescent, the spectrum it pro- 

 duces consists, not of the various rainbow colors, but of sharp, narrow, bright lines, 

 the color depending on the substance. All the rest of the spectrum is dark. 

 These line spectra are very strikingly shown by various metals heated till they are 

 in the form of incandescent vapor. 



I 192. Absorption Spectrum. — By this is meant a spectrum in which there are 

 dark lines or bands in the spectrum. The most striking and interesting of the 

 absorption spectra is the Solar Spectrum, or spectrum of sunlight. If this is exam- 

 ined carefully it will be found to be crossed by dark lines, the appearance being as 

 if one were to draw pen marks across a continuous spectrum at various levels, 

 sometimes apparently between the colors and sometimes in the midst of a color. 

 These dark lines are the so-called Fraunhofer Lines. Some of the principal ones 

 have been lettered with Roman capitals, A, B, C, D, E, F, G, H, commencing at 

 the red end. The meaning of these lines was for a long time enigmatical, but it 

 is now known that they correspond with the bright lines of a line spectrum (§ 191). 

 For example, if sodium is put in the flame of a spirit lamp it will vaporize and 

 become luminous. If this light is examined there will be seen one or two bright 

 yellow bands corresponding in position with D of the solar spectrum (Fig. 121). 

 If now the spirit-lamp flame, colored by the incandescent sodium, is placed in the 

 path of the electric light, and it is examined as before, there will be a continuous 

 spectrum, except for dark lines in place of the bright sodium lines. That is, the 

 comparatively cool yellow light of the spirit lamp cuts off or absorbs the intensely 

 hot yellow light of the electric light ; and although the spirit flame sends a yellow 

 light to the spectroscope it is so faint in comparison with the electric light that the 

 sodium lines appear dark. It is believed that in the sun's atmosphere there are 

 incandescent metal vapors (sodium, iron, etc. ), but that they are so cool in com- 

 parison with the rays of their wave length in the sun that the cooler light of the 

 incandescent metallic vapors absorb the light of corresponding wave length, and 

 are, like the spirit lamp-flame, unable to make up the loss, and therefore the pres- 

 ence of the dark lines. 



\ 193. Absorption Spectra from Colored Substances. — While the solar spec- 

 trum is an absorption spectrum, the term is more commonly applied to the spectra 

 obtained with light which has passed through or has been reflected from colored 

 objects which are not self-luminous. 



It is the special purpose of the micro-spectroscope to investigate the spectra of 

 colored objects which are not self-luminous, i. e., blood and other liquids, various 

 minerals, as monazite, etc. The spectra obtained by examining the light reflected 

 from these colored bodies or transmitted through them, possess, like the solar 



