640 DEMONSTRATION OF ULTRA-VIOLET LIGHT [CH. XV 



rent may be employed, but direct current is to be preferred. 

 About 1 5 amperes will give the best results. When direct current 

 is used the lower carbon, which contains the salt to be studied, is 

 made the positive. 



With metallic electrodes or with metallic oxides contained in 

 sheet iron tubes, direct current only, can be used. About 6 

 amperes give the best results in this case. The metallic electrode 

 is made the lower, the upper electrode being carbon. Most 

 metallic electrodes will give different results when made the positive 

 than when made the negative terminal. The magnetite and 

 titanium ("Luminous arc") electrodes show the lines of iron and 

 titanium best when connected to the negative wire. 



Uranium oxide contained in an iron tube will give a line spectrum 

 when connected to the negative wire but will give a very nearly 

 continuous spectrum when connected to the positive wire 

 ( 88 S a). 



DEMONSTRATION ON A SMALL SCALE; DEMONSTRATION OF 

 ULTRA-VIOLET LIGHT AND PHOTOGRAPHY 



906. Demonstration on a small scale. Spectra may be 

 projected on a small scale for the observation of a few individuals 

 with the apparatus shown in fig. 368. This arrangement is similar 

 in every way to that for the projection of spectra on a large screen 



FIG. 368. PROJECTION OF SPECTRA ON A 



SMALL SCALE. 

 L Radiant. 

 C Condenser. 

 S Slit. 



Oj Objective before the prism. 

 O 2 Objective beyond the prism. 

 P Prism. 

 R V Spectrum. 



For the best definition there must be an objective (0 2 ) 

 beyond the prism to focus the spectrum (R V). 



