THE WAVE THEORY OF LIGHT. 337 



(Fig. 48), a light produced by a spirit-lamp with 

 salt in it. The sodium vapour is heated up to 

 several thousand degrees, when ft becomes self- 

 luminous and gives such a light as we get by 

 throwing salt upon a spirit lamp in the game of 

 snap-dragon. 



I hold in my hand a beautiful grating of glass 

 silvered by Liebig's process with metallic silver, 

 a grating with 6,480 lines to the inch, belonging to 

 my friend Professor Barker, which he has kindly 

 brought here for us this evening. You will see 

 the brilliancy of colour as I turn the light reflected 

 from the grating toward you and pass the beam 

 round the room. You have now seen directly with 

 your own eyes these brilliant colours reflected from 

 the grating, and you have also seen them thrown 

 upon the screen from a grating placed in the 

 lantern. Now with a grating of 17,000 lines per 

 inch a much greater number than the other 

 you will see how much further from the central 

 bright space the first spectrum is ; how much more 

 this grating changes the direction, or diffraction, 

 of the beam of light. Here is the centre of the 



z 



