DIFFRACTION. 125 



a great number of parallel lines be engraved at very small and 

 equal intervals upon a polished surface, the light reflected 

 from the intervals of the grooves will interfere in a manner 

 precisely analogous to that admitted through the apertures 

 of the gratings ; and will, by their interference, produce the 

 most brilliant spectra. In some of the grooved metallic sur- 

 faces constructed by Mr. Barton, there are 10,000 lines to the 

 inch. With surfaces so minutely divided, the spectra pro- 

 duced are as perfect as those formed by the finest prisms ; and 

 the colours which they display are little inferior to those of 

 the diamond. 



Similar appearances may be observed on metallic surfaces 

 which have been polished with a coarse powder, the powder 

 leaving minute striae which produce the effects we have been 

 describing. They may also be very simply produced by pass- 

 ing the finger over the surface of a piece of glass moistened 

 by the breath. 



(138) The beautiful colours of mother of pearl are natural 

 instances of the same phenomena. This substance is com- 

 posed of a vast number of very thin layers, which are gra- 

 dually and successively deposited within the shell of the 

 oyster, each layer taking the form of the preceding. "When 

 it is wrought, therefore, the natural joints are cut through 

 in a great number of sinuous lines ; and the resulting sur- 

 face, however highly polished, is covered by an immense 

 number of undulating ridges, formed by the edges of the 

 layers. These strise may be observed by the aid of a 

 powerful microscope, although they are sometimes so close 

 that 5000 of them occupy an inch. That they are the 

 causes of the brilliant colours displayed by this substance 

 has been placed beyond doubt by an experiment of Sir 

 David Brewster. This experiment consisted simply in 

 taking the impression of the surface of the pearl on wax, 



