2 SO 



SPECTROSCOPE AND VARIOUS SPECTRA [Ch. VIII 



§ 394. Line spectrum. — If a gas is made incandescent, the spec- 

 trum 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 metallic vapors heated to incandescence, e.g. 



*.4,i 



Y 



K7 U 



-r— 



— - 



B 



\.4fA 



K7u 



H 



C B 



HI 



Fig. 146-147. A Normal and a Prismatic Spectrum of Daylight. 



Fig. 146. Normal Spectrum of Daylight Showing the Segments of Color, 

 V B G Y R, and the Dark Lines, HGFEDCBA. 



In the normal spectrum produced by a grating the dispersion is directly- 

 proportional to the wave length of the light; hence the red is a broad band and 

 the violet-blue narrow. (Compare the prismatic spectrum where the red is 

 narrow and the blue broad.) 



0.4/i 0.7/x, the wave lengths between which the radiation is visible 

 (see fig. 144). 



Fig. 147. Prismatic Spectrum of Daylight. 



As glass does not disperse the different wave lengths in direct proportion 

 to their length, the width of the bands of color are strikingly unlike those in 

 the normal spectrum, the blue-violet being wide and the red very narrow. 



sodium. These spectra are usually obtained by heating some salt 

 of the substance (see § 405). 



§ 395. 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 Spec- 

 trum, or spectrum of sunlight. If this is examined by a good spectro- 

 scope 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 



