vii.] DIFFRACTION SPECTRA. 85 



A grating as now used is made by ruling, with a diamond and 

 dividing machine, a great number of very fine, parallel, equi- 

 distant lines on a piece of highly polished speculum metal, or 

 glass which is afterwards silvered on the scratched surface. 

 The manufacture of such gratings is carried to great perfection, 

 and some made by Mr. Eutherfurd contain as many as 17,000 

 lines to the inch. 



When light falls normally on such a grating, and we obscure 

 the light reflected from it, we find that a portion of it is re- 

 flected back unaltered, as it would be from an ordinary mirror, 



/ 

 FIG. 534. Angstrom's spectrometer. 



while on each side of this, at some distance from it, a 

 spectrum is formed (called a spectrum of the first order). Out- 

 side this, again, on each side, at a greater angular distance from 

 the normal, another spectrum appears (second order) of much 

 greater dispersion than the first. This is succeeded by a spec- 

 trum of the third order, and this again by a fourth, and so on, 

 each time with a greatly increased dispersion and decreased 

 luminosity. The observing telescope can then be directed to 

 the grating at such an angle as to enable either of these spectra 

 to be observed as it would be in an ordinary spectroscope. 



