[ 456 J 



XXXTX. Resonance Radiation of Sodium Vapour excited by 

 One of the JD Lines. By B. W. Wood and F. L. Mohler*. 



IN a paper on ".Resonance Radiation of Sodium Vapour "f, 

 published by one o£ us in 1905, it was shown that a glass 

 tube containing the vapour o£ metallic sodium at a low tem- 

 perature (about 200°), when illuminated by the light of a 

 sodium flame, re-emitted a yellow light, which spectroscopic 

 analysis showed vvas identical with the exciting light, in other 

 words the two D lines. 



In 1914 Wood and Dunoyer t showed that the vapour 

 when excited by the light of one I) line only, emitted only 

 this same wave-length, in other words, that it was possible 

 to have sodium vapour emit only one of the D lines. This 

 showed that the mechanisms producing the D lines could be 

 separately excited. 



Owing to the very long exposures which were necessary in 

 these experiments, and the extreme faintness of the images 

 obtained, it was impossible to tell whether the other D line 

 was in reality completely absent, or merely greatly reduced 

 in intensity. Moreover, in the few cases in which a trace of 

 the other D line appeared, it was never quite certain that 

 this wave-length was completely eliminated from the exciting- 

 beam, as the temperature of the room always varied consi- 

 derably, and this altered the optical properties of the quartz 

 block used in the polarization method of separating the two 

 lines. 



We have accordingly repeated the work, using a spectro- 

 scope of higher resolving power, and controlling the tempe- 

 rature of the optical system used for separating the D lines, 

 with a thermostat. 



We have, moreover, taken extreme precautions to remove 

 completely the occluded hydrogen, and have examined the 

 effect of the presence of small traces of hydrogen on the 

 nature of the radiation. 



This seemed very important, as it has been shown by one 

 of us that when iodine vapour in an atmosphere of helium at 

 low pressure is excited to resonance by the light of the 

 mercury arc, there is a transfer of energy from one vibrating 

 system to others, and the emitted spectrum is much more 

 complicated than is the case when the iodine is in a high 

 vacuum. 



* Communicated by the Authors. This communication is an abstract 

 of a paper which appears in full in the Physical Review. 

 t R. W. Wood, Phil. Mag. [6] x. p. 513*(1905). 

 % R. W. Wood & L. Dunoyer, Phil. Mag. [6] xxvii. p. 1018 (1914), 



