630 



PROJECTION OF SPECTRA 



[CH. XV 



893. Prism. A glass prism may be used, but it is much 

 better to use a hollow prism filled with carbon bisulphide as this 

 liquid gives a much higher dispersion than glass, thus enabling 

 one to obtain a more extended spectrum than would be possible 

 with a glass prism. 



Caution. In using carbon bisulphide remember that it is very 

 volatile and its vapor is easily ignited. Hence this liquid should 

 not be poured or left in unstoppered vessels in the neighborhood of 

 the lighted arc. Also be sure that the hollow prism has no leaks 

 around the stopper or elsewhere. 



FIG. 354. HOME-MADE SLITS FOR PROJECTION. 

 a Slit with stationary blades. 

 b Slit with one movable blade. 

 c Side view of b. 



894. Other prisms: gratings. The 60 prism, either solid 

 or filled with liquid is usually the most available, but other forms 

 are often at hand. 



The compound prism due to Rutherford (fig. 356), composed of a 

 dense flint glass prism of a large angle and two crown glass prisms 

 cemented to it, will give a much higher dispersion than will a single 

 prism of even very dense glass. Such a prism is used in practically 

 the same way as a simple prism. 



With a direct-vision prism (fig. 357) the axis of the spectrum is not 

 turned to one side. Such a prism may be constructed of pieces of 

 different kinds of glass or it may be made by filling the hollow cells 

 of a prism with different liquids. 



