CH. XVI ABSORPTION SPECTRA 637 



is a common practice in experiments with spectra to put the slit 

 close to the condenser as for a lantern slide, but this lessens the 

 brilliancy of the spectrum. 



ABSORPTION SPECTRA 



901. The apparatus being arranged as above indicated to 

 project a continuous spectrum, all that remains is to insert the 

 absorbing medium between the light source and the screen, it 

 makes little difference where. The appearance of the spectrum 

 would be the same even if the absorbing medium were held between 

 the eye of the observer and the screen. As a practical matter it is 

 best to place the absorbing substance just in front of the slit. In 

 this position any slight lack of planeness of its surfaces will not 

 cause any interference with the optical system nor reduce the sharp- 

 ness of the spectrum on the screen. The specimen may cover the 

 entire slit, in which case the entire spectrum will show the absorp- 

 tion bands of the substance, or the specimen may be made to cover 

 part of the slit, in which case, part of the spectrum will be that of 

 the light source and part will show the absorption bands of the 

 specimen. The advantage of having this comparison spectrum 

 of white light is to bring out much more clearly any faint absorp- 

 tion of one end of the spectrum as with dilute copper sulphate or 

 with amber glass. Liquids may best be shown by placing them 

 in hollow glass boxes (fig. 365). Many variations of this method 

 and many fascinating experiments will soon suggest themselves to 

 the experimenter once the apparatus is set up. 



902. Suitable substances. The following substances will 

 show interesting absorption bands: 



Colored glasses. Red, blue, purple, canary-yellow. 



Gelatines colored with solutions of ana- 

 line dyes, for example, methyl violet, 

 eosine (red ink), fuchsine. 

 Blood diluted with water. 

 Solutions of mineral salts, as cobalt ni- 

 trate in water, cobalt nitrate in alcohol or 

 FIG. 365. GLASS Box FOR concentrated HC1 ; potassium permanga- 

 ABSORPTION SPECTRA. nate. 



