338 POPULAR LECTURES AND ADDRESSES. 



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 corner 

 seen by means of instruments 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 homo- 

 geneous 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 it you see a blackened slip of wood 



