CH. XV] PROJECTION OF SPECTRA 629 



If a source is used which shows a line spectrum, this spec- 

 trum will be found approximately in focus at R V. By focus- 

 ing the objective and by rotating the prism the spectrum can be 

 sharply focused on the screen. The sharpness of the spectrum can 

 be increased by narrowing the slit or the brightness of the spectrum 

 can be increased by opening the slit. If the slit is adjustable in 

 width, judgment must be exercised to secure a spectrum which is 

 as sharp as the occasion requires and at the same time 

 sufficiently bright. 



OPTICAL SYSTEM FOR THE PROJECTION 

 OF SPECTRA. 



FIG. 353. 



L Light source, arc lamp. 



C Condenser. 



Slit. 



Objective. 



1 Image of the slit focused on the wall before the 

 prism is in place. 



R V Spectrum focused on the wall after placing the prism in the path of 

 the rays. 



The distance from P to R V must be relatively much greater than here 

 shown, 2 meters (6 ft.) or more, in order to get a sharply denned spectrum. 



Note that the red (R) is deviated less than the violet (7). Compare with 

 a grating spectrum, fig. 360. 



892. Slit. The slit for projection should be a fairly large one ; 

 12 cm. square, with jaws 5 cm. long. 



If one is limited by time or expense a very serviceable slit may be 

 made by soldering a couple of pieces of tinned iron on a larger piece 

 with a hole in it, or one of the pieces may be fastened so it can be 

 slid closer or farther from the other. The main point to be 

 observed is that the edges forming the slit opening must be per- 

 fectly smooth and straight so that when they are brought close 

 together there will be no unevenness in the light which gets 

 through. 



Large slits with adjustable jaws may be obtained of dealers in 

 projection apparatus. 



