CH. XV] 



PROJECTION OF SPECTRA 



631 



A convenient way of get- 

 ting a prism which has a de- 

 viation of but about 150 

 while having a very good dis- 

 persion is to immerse a hol- 

 low 60 prism, filled with car- 

 bon bisulphide, in a cubical . 



j- u 11 A -4.1. FIG- 355- HOLLOW GLASS 



glass dish filled with water PRISM WITH COVER AND 

 (fip ?cR} STOPPER FOR CARBON 



BISULPHIDE (CS 2 ). 



895. A transmission 



diffraction grating, either one ruled on glass or a 

 replica grating, held in front of the projection objec- 

 tive will give wide but rather faint spectra on the 

 screen. 



If a grating with rather coarse lines, 100-200 to 

 the centimeter (250-500 lines to the inch) is used 



FIG. 356. RUTHERFORD PRISM. 



This prism consists of a flint glass prism F, with an angle 

 of about 90 and two crown glass prisms C C, cemented to it. 

 The combination as here shown has a prism angle of about 

 30 and has the same deviation as a 60 flint prism, but has a 

 much higher dispersion than could be obtained with a simple 

 prism of even dense flint glass. 



F!G. 357. USE OF A DIRECT-VISION PRISM FOR THE 



PROJECTION OF SPECTRA. 

 5 Slit. 

 O Objective. 



Flint Flint prism to cause dispersion. 

 Crown, Crown, Prisms of crown glass to obviate the devi- 

 ation. 



V R Spectrum. 



