The Anomalous Dispersion of Sodium Vapour. 167 



the turned-over seam hammered until a tight joint is formed. These 

 tubes can be heated and cooled any number of times, and can be kept 

 in operation for an hour or two, at the end of which time the sodium 

 is generally used up, a moss-like deposit of oxide gradually filling up 

 the tube. These tubes can be used over and over again without 

 deterioration, and are most satisfactory in every respect. Their only 

 fault lies in their conductivity, the sealing-wax softening and the glass 

 plates falling off, but this can be prevented by wrapping a strip of 

 cloth around each end, and wetting it from time to time. I have had 

 one tube made with water-jackets at each end, but it seems to have no 

 especial advantage, and is more complicated. Porcelain tubes are 

 quite satisfactory, but I prefer the iron on the whole. 



By employing a tube of about 5 cm. diameter I have succeeded in 

 projecting the anomalous spectrum, but the appearance is so very 

 inferior to that of the phenomenon when seen subjectively that I 

 should never think of employing the method. 



No trace of any influence of the fluted absorption bands on the dis- 

 persion has been detected. 



It is a matter of some interest to know what becomes of the energy 

 absorbed by a sodium flame, or by non-luminous sodium vapour. 

 Most of it is undoubtedly spent in raising the temperature of the 

 vapour ; this rise of temperature I have shown experimentally by 

 making the dispersion tube the chamber of a radiophone, and illu- 

 minating it with intermittent light. It was found that the radiophone 

 sang loudly when illuminated by an intense beam of light which con- 

 tained waves of the frequency of the sodium radiations, but became 

 almost silent when these frequencies were removed. 



Prismatic analysis was found to weaken the light too much, con- 

 sequently a very thin film of cyanine was used which was quite 

 opaque to the region of the D-lines while transparent to the rest of 

 the spectrum. Sunlight concentrated by a large mirror was employed 

 for the illumination. 



It has always appeared to me thinkable that the absorbing sodium 

 ions, when in resonant vibration, might emit a certain amount of light 

 laterally. Kayser mentions in his new work on spectroscopy an 

 experiment made by Miller, in an attempt to detect this phenomenon 

 if it existed. The possibility of such a lateral radiation is discussed 

 by Drude in his ' Lehrbuch der Optik " (page 493), and is shown to 

 depend on a small value of the constant r in the equation of motion of 

 an ion vibrating under the influence of a periodic electric force. For 

 sodium vapour r is calculated to be something between 10 and 100. 

 If it were equal to one we should expect a lateral radiation of light, 

 which, says Drude, has never been observed. 



While experimenting with the very intense beam of sunlight in the 



N 2 



