CH, VI,-] MICRO-SPECTROSCOPE AND POLARISCOPE. 123 



\ i8r. Continuous Spedtrum. — In case a good artificial light or 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. 



\ 182. Line Spectrum. — If a gas is made incandescent, the spectrum it produces 

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

 dark lines or bauds in the spectrum. The most striking and interesting of the ab- 

 sorption 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 (§ 182). 

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

 come 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. 114). 

 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. 



\ 184. Absorption Spectra from Colored Substances. — While the solar spectrum 

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

 tained with light which has passed through or has been reflected from colored ob- 

 jects 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, as 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 

 spectrum dark lines or bands, but the bands are usually much wider and less 

 sharply defined. Their number and position depend on the substance or its con- 

 stitution (Fig. 116), and their width, in part, upon the thickness of the body. 

 With some colored bodies, no definite bands are present. The spectrum is simply 

 restricted at one or both ends and various of the other colors are considerably 

 lessened in intensity. This is true of many colored fruits. 



\ 185. Angstrom and Stokes' Law of Absorption Spectra. — The wave lengths of 

 light absorbed by a body when light is transmitted through some of its substance 



