CH. XV] DEMONSTRATION OF ULTRA-VIOLET LIGHT 641 



except that the additional lens O 2 , brings the parallel rays of light of 

 each wave-length to a focus in the spectrum R V. The arrange- 

 ment in fig. 368, with the lens d, giving a converging beam, will 

 not give good results, and the second lens C>2, is required if any fine 

 details in the projected spectrum are to be shown. The two lenses 

 Oi and O 2 , should be achromatic. The two lenses from a symmetri- 

 cal photographic objective will give excellent results. Ordinary 

 spectacle lenses can be made to answer if no others are available. 

 The prism can be of any of the forms previously described. The 

 spectrum is received on either a white screen or one which is coated 

 with anthracene in order to show the ultra-violet. 



FIG. 369. PROJECTION OF SPECTRA ON A 

 SMALL SCALE. 



With this arrangement, where the distance from the 

 prism to the spectrum R V is relatively short, as here 

 shown, the definition will be poor: see fig. 368 for a better 

 method. 



L Radiant (arc lamp with vertical carbons). 



C Condenser. 



5 Slit. 



O Objective. 



P Prism. 



/ Image of the slit S when no prism is in place. 



R V Spectrum projected by the prism. 



907. Projection of ultra-violet. Ordinary glass prisms and 

 lenses if not noticeably yellow or green will transmit radiation in the 

 ultra violet to about .35^, which can be observed by the use of an 

 anthracene screen ( 899). If the far ultra-violet spectrum is to be 

 demonstrated it is necessary to use a quartz system, that is, all 

 condensers, lenses, and prisms between the source and the screen 

 must be made of quartz, either quartz glass or quartz crystal. 

 The apparatus is arranged as in fig. 370. 



The quartz prisms are usually made of two 30 prisms, as shown 

 in fig. 370, one of which is a right-hand crystal, the other a left- 



