632 



PROJECTION OF SPECTRA 



[Cn. XV 



there will appear on the screen not only the 

 central image O, of the slit but also fainter 

 diffraction irr.ages i, 2, 3, on both sides of the 

 central one (fig. 357). These diffraction im- 

 ages are really short spectra. By using colored 

 glasses it can be shown that with red light the 

 images are farther apart than with green or 

 blue light. 



FIG. 358. LIQUID PRISM OF GOOD DISPERSION 



BUT SMALL DEVIATION. 



It consists of a hollow prism filled with carbon bi- 

 sulphide (CS 2 ) immersed in a glass box filled with 

 water. 



896. Grating spectra. If a grating with 1 



fine lines, 5,000 to 10,000 lines per centimeter 

 (12,500 to 25,000 lines to the inch), is used the 

 diffraction images are spread out farther and / 

 appear as extended spectra. In case the de- 

 tails of a spectrum are to be studied it is 

 necessary to turn the axis of the lantern to one 

 side as in the case where a prism is used. The 



F IG - 359- USE OF A GRATING WITH COARSE LINES. 

 5 Slit. 

 O Objective. 

 G Grating. 



o Primary image of slit formed without grating. 

 123,123 Diffraction images of the slit formed 

 by the grating. These images are short spectra. 



CO 



