642 DEMONSTRATION OF ULTRA-VIOLET LIGHT [Cn. XV 



hand crystal. The space between the two prisms is filled with 

 glycerin. 



Aside from the material of which the lenses and prisms are made 

 there is but one thing which is different from the case with a glass 

 system. By using quartz alone, no achromatic lenses are possible 

 and the spectrum instead of focusing in a line at right angles to the 

 axis of the beam, focuses along a line oblique to the axis. Thus, 

 the far ultra-violet, UV, focuses nearer lens O 2 , than does the visible 

 spectrum. Tilting the screen to the position indicated will enable 

 one to get all of the spectrum in focus at once. 



The anthracene screen ( 899) will enable one to demonstrate all 

 of the lines of arc spectra which would appear upon a photograph 



FIG. 370. PROJECTION OF THE ULTRA-VIOLET 

 ON A SMALL SCALE. 



L Light source. 



C Quartz condenser. 



5 Slit. 



0, Plano-convex quartz lens. 



P Quartz prism, preferably made of two 

 prisms r and / from right-and left-handed quartz 

 crystals cemented together with glycerin. 



O 2 Plano-convex quartz lens. Turn the convex sides of the lenses towards 

 the prism. 



U V, V R Focus of the spectrum. 



Anthracene screen, fluoresces to ultra-violet. Note its oblique position. 



made by the use of a quartz spectrograph, the ultra-violet lines of 

 the aluminum arc at .217^ being easily seen. Demonstration of 

 fluorescence of other substances to ultra-violet may be shown by 

 substituting them for the anthracene. 



The demonstration of the far ultra-violet on a large scale is 

 hardly possible owing to the small intensity of the light emitted in 

 this region. 



