DIFFRACTION GRATINGS 



589 



The telescopes are fixed, and the grating is turned on its axis to pass 

 to different portions of the spectrum. As the glass of the telescopes 

 absorbs the ultra-violet light, this portion of the spectrum is cut off 



FIG. 1. Method of using Flat Grating. A, source of light; , slit; C,<7, two tel- 

 escopes, movable or fixed; Z>, grating, movable about its centre; E, eye-piece. 



entirely, unless quartz lenses are used. The concave grating avoids 

 this trouble, and produces a spectrum without the aid of lenses, the 

 lines being ruled on a concave surface instead of on a flat one. Such a 



5<u-ee o 



FIG. 2. Method of using Concave Grating. A, source of light; B, slit; D, grating 

 mounted in beam C, movable along the ways E, E; F, camera-box or eye-piece. 



grating, properly mounted, produces what has been called a normal 

 spectrum, and is specially adapted to photographic purposes (Fig. 2). 



