CHROMATICS SPECTRA. 411 



angular distance from the slit itself. A pure spectrum is also 

 obtained by passing the beam through several prisms in succession, 

 thus increasing the dispersion. 



638. Fraunhofer's Lines. A pure solar spectrum 

 is not continuous, but is crossed by numerous dark lines, 

 many hundreds of which have been counted and accu- 

 rately mapped. The more conspicuous of these dark lines 

 are distinguished by letters of the alphabet, as shown in 

 Fig. 315. Each of these dark lines indicates that a par- 

 ticular kind of ray is wanting in solar light. 



AaBCD E6P G HH' 



ROY G B I V 



FIG. 315. 



(a.) The spectra of incandescent solids are continuous, from the 

 extreme red to a limit depending upon the temperature. The 

 spectra of incandescent gases (not containing solid particles in 

 suspension) are non-continuous, consisting of a number of definite 

 bright lines. A candle or gas flame gives a continuous spectrum 

 because it is chiefly due to the incandescence of solid carbon 

 particles. 



(&.) The spectroscope is an instrument for producing and observing 

 pure spectra. It has proved to be one of the most powerful aids 

 to modern science. It affords the most delicate means of chemical 

 analysis ; by its aid several elements have been discovered ; the 

 presence of 7TTF ^. Tnnr of a grain of sodium has been detected by 

 "spectrum analysis." It is of incalculable importance to the 

 astronomer. For definite information, the pupil is necessarily 

 referred to some of the excellent manuals upon the subject recently 

 published. 



639. Synthesis of White Light. By analysis, 

 we have shown that white light is composed of seven 

 primary colors. The same fact may be shown synthetically, 

 for by recombining these spectrum colors white light will 

 be produced. 



