64 



DIFFRACTION GRATINGS 



[Encyclopcenia Brltannica, New Volumes, III, 458, 459, 1902] 



The grating is an optical instrument for the production of the spec- 

 trum ; it now generally replaces the prism in a spectroscope where large 

 dispersion is needed, or when the ultra-violet portion of the spectrum 

 is to be examined, or when the spectrum is to be photographed. The 

 transparent grating consists of a plate of glass covered with lampblack, 

 gold leaf, opaque collodion or gelatine, the coating being scratched 

 through in parallel lines ruled as nearly equidistant as possible. When 

 the lines are to be ruled very close together, a diamond ruling directly 

 on glass is used. Other transparent materials, such as fluor spar, are 

 sometimes substituted for glass. For certain researches on long waves 

 the grating is made by winding a very fine wire, l-1000th inch in diam- 

 eter, in the threads of two fine screws placed parallel to each other, 

 soldering the wire to the screws and then cutting it away on one side 

 of the screws. As the value of a grating is dependent upon the number 

 of lines ruled, it is very desirable to have their number great. Glass is 

 so hard that the diamond employed for the ruling wears away rapidly; 

 and hence the modern grating is generally a reflecting grating, which 

 is made by ruling on a speculum metal surface finely ground and pol- 

 ished. On such a surface it is possible to rule 100,000 lines without 

 damaging the diamond, although its point even then often wears away 

 or breaks down. The lines are generally so close together as 15,000 or 

 20,000 to the inch, although it is feasible to rule them even closer 

 say 40,000 to 50,000 to the inch. There is little advantage, however, 

 in the higher number and many disadvantages. 



The grating produces a variety of spectra from a single source of 

 light, and these are designated as spectra of the first, second, etc., order, 

 the numbering commencing from the central or reflected image and 

 proceeding in either direction from it. The dispersion depends upon 

 the number of lines ruled in a unit of length^upon the order of the 

 spectrum, and upon the angle at which the grating is held to the source 

 of light. The defining power depends upon its width and the angles 



