THE WAVE THEORY OF LIGHT. 341 



measure the wave-length of a candle flame himself. 

 I hope some of you will be induced to make the 

 experiment for yourselves. 



If I put salt on the flame of a spirit lamp, what 

 do I see through this grating ? I see merely a 

 sharply defined yellow light, constituting the spec- 

 trum of vaporised sodium, while from the candle 

 flame I see an exquisitely coloured spectrum, 

 far more beautiful than that I showed you on the 

 screen. I see in fact a series of spectrums on the 

 two sides with the blue toward the candle flame 

 and the red further out. I cannot get one definite 

 thing to measure from in the spectrum from the 

 candle flame, as I can with the flame of a spirit 

 lamp with the salt thrown on it, which gives as I 

 have said a simple yellow light. The highest blue 

 light I see in the candle flame is now exactly on 

 the line. Now measure to my eye, it is forty-four 

 feet four inches, or 532 inches. The length of this 

 wave then is the 532d part of the four hundredth 

 of a centimetre which would be the 2i,28oth of 

 a centimetre, say the 2i,oooth of a centimetre. 

 Then measure for the red and you will find 



