336 POPULAR LECTURES AND ADDRESSES. 



yellow light. I am now going to show you the 

 diffraction spectrum. 



You see on the screen, 1 on each side of a 

 central white bar of light, a set of bars of light 

 of variegated colours, the first one on each side 

 showing blue or indigo colour, about four inches 

 from the central white bar, and red about four 

 inches farther, with vivid green between the blue 

 and the red. That effect is produced by a grating 

 with 400 lines to the centimetre; engraved on glass, 

 which I now hold in my hand. The next grating 

 that we shall try has 3,000 lines on a Paris inch. 

 You see the central space and on each side a 

 large number of spectrums, blue at one end and 

 red at the other. The fact that, in the first 

 spectrum, red is about twice as far from the 

 centre as the blue, proves that a wave-length of 

 red light is double that of blue light. 



I will now show you the operation of measuring 

 the length of a wave of sodium light, that is 

 a light like that marked D on the spectrum 



1 Showing the chromatic bands thrown upon the screen from a 

 diffraction grating. 



