THE WAVE THEORY OF LIGHT 283 



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 spectrum of vaporised so- 

 dium, 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 centi- 

 metre, say the 2i,oooth of a centimetre. Then measure 

 for the red and you will find something like the ii,oooth 

 for the lowest of the red light. 



Lastly, how do we know the frequency of vibration? 



Why, by the velocity of light. How do we know that? 

 We know it in a number of different ways, which I can- 

 not explain now because time forbids, and I can now only 

 tell you shortly that the frequency of vibration for any par- 

 ticular ray is equal to the velocity of light divided by 

 the wave-length for that ray. The velocity of light is 

 about 187,000 British statute miles per second, but it 

 is much better to take the kilometre which is about six- 

 tenths of a mile for the unit, when we find the velocity 

 is very accurately 300,000 kilometres, or 30,000,000,000 

 centimetres, per second. Take now the wave-length of 

 sodium light, as we have just measured it by means of the 

 salted spirit-lamp, to be one I7,oooth of a centimetre, and 

 we find the frequency of vibration of the sodium light 

 to be 510 million million per second. There, then, you 

 have a calculation of the frequency from a simple obser- 

 vation which you all can make for yourselves. 



Lastly, I must tell you about the colour of the blue 



