LIGHT. 



123 



The lines are sometimes formed by cutting through a film of silver 

 deposited on the glass. Lord Rayleigh has succeeded in photo- 

 graphing these lines, and thus obtaining comparatively inexpensive 

 gratings. The number of these lines varies from two to six thou- 

 sand or even more in the inch. When light, admitted through a 



Becquerel's Phosphoroscope. 



narrow slit, is allowed to fall upon one of these a spectrum is 

 formed, or rather a series of spectra, of increasing width on either 

 side from the centre outwards. Of these the first two or three are 

 tolerably distinct, but beyond this they overlap so much that they 

 can only be disentangled by means of Fraunhofer's lines, or by 

 some other fixed lines or bands in the spectrum. The spectra 



