ON LIGHT. 327 



the "fixed lines," so often above referred to, may be 

 seen in them, and thus the wave-lengths, corresponding 

 to the most conspicuous of these lines, ascertained with 

 great precision. The violet ends of all the spectra are 

 nearest to the central point, and the more distant spectra 

 longer than the nearer, so that at length they overlap 

 and confuse one another by the intermingling of the red 

 end of one with the violet of that next in order. 



(no.) If the apertures of which the grating consists 

 be formed by removing with a graver portions of an 

 opake varnish covering a glass surface, spectra exactly 

 similar are seen accompanying the image of the lumin- 

 ous point reflected on the anterior surface of the glass 

 from the polished portions laid bare. The same is 

 observed, and with far more brilliancy, when a highly 

 polished surface of metal is furrowed in equidistant par- 

 allel grooves by a graver or diamond point (which de- 

 stroys the polish of those lines), and if the metal be 

 hardened steel, the furrows so formed are transferable 

 by violent pressure to the polished surface of a softer 

 metal, which then in its turn exhibits similar appear- 

 ances, and thus are produced the "buttons" above 

 spoken of. Mother-of-pearl, too, which consists of ex- 

 ceedingly thin layers of calcareous matter superposed, 

 and agglutinated or otherwise held together ; when 

 ground and polished, has these layers, which lie very 

 little oblique to the general surface, torn up at their 

 edges, where they crop out ; which remain rough and un- 

 polished, however brilliantly polished the general surface. 

 The polished surface, therefore, is lined all over with 



