THE WAVE THEORY OF LIGHT 281 



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 (FiG. 120), a light produced by a spirit- 

 lamp with salt in it. The sodium vapour is heated up to 

 several thousand degrees, when it 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 

 grating, and there is the first spectrum. You will note that 

 the violet light is least diffracted and the red light is most 

 diffracted. This diffraction of light first proved to us 

 definitely the reality of the undulatory theory of light. 



You ask why does not light go round a corner as sound 

 does. Light does go round a corner in these diffraction 

 spectrums; and it is shown going round a corner, since it 

 passes through these bars and is turned round an angle of 

 thirty degrees. The phenomena of light going round a cor- 

 ner seen by means of instruments 1 adapted to show the 

 result and to measure the angles through which it is turned, 

 is called the diffraction of light. 



I can show you an instrument which will measure the 

 wave-lengths of light. Without proving the formula, let 

 me tell it to you. A spirit-lamp with salt sprinkled on the 

 wick gives very nearly homogeneous light, that is to say, 

 light of one wave-length, or all of the same period. I have 

 here a little grating which I take in my hand. I look through 

 this grating and see that candle before me. Close behind 



