PROJECTION OF SPECTRA 



[Cn. XV 



FIG. 361. USE OF A GRAT- 

 ING FOR PROJECTING A 

 HIGH ORDER SPEC- 



5 Slit. 

 O Objective. 

 G Grating. 



Image of the slit if 

 there is no grating in position. 

 R V Spectrum. 

 The grating is tilted as shown. 



slit than for the plane 

 grating, that is, the light 

 from the objective must 7 

 be diverging instead of 

 slightly converging when 

 it strikes the grating (fig. 363). 



898. Direction of the light. As 

 the direction of the light from a 60 

 prism is oblique to the axis of the lan- 

 tern (fig. 353), it is necessary to turn 

 the entire projection apparatus to one 

 side so that the spectrum will strike 

 the screen. 



899. Screen, white and anthra- 

 cene. A white screen such as is suit- 

 able for ordinary lantern projection 



FIG. 362. USE OF A PLANE REFLECTING 

 DIFFRACTION GRATING FOR THE 



PROJECTION OF SPECTRA. 

 i Primary image of the slit as it would be 

 reflected by a plane mirror. 



R V Diffraction spectrum produced by 

 the reflecting grating. 



These high order spectra are very faint. 



