330 Prof. Wood and Mr. Kimura on Scattering and 



radiation (Plate VI. fig. 3: lower spectrum, mercury arc; 

 upper, resonance radiation). This radiation appears to be 

 wholly free from polarization, even when the exciting 

 radiation is plane polarized. If the density of the vapour is 

 increased, the diffuse radiation (which is at first a volume 

 radiation) is confined to a shallow layer of molecules lining 

 the front surface of the illuminated bulb, and, with further 

 increment of density, disappears entirely, being replaced by 

 regular reflexion, much as if the inner surface of the bulb 

 was silvered. 



In the present investigation an attempt has been made to 

 clear up some of the doubtful points mentioned in the 

 previous papers, and ascertain, if possible, how the diffuse 

 scattering passes over into regular reflexion. 



In the earlier work it was found that the resonance radiation 

 had its maximum intensity at the moment of starting the 

 quartz arc, and that it became almost negligible after the 

 lamp had been in operation for eight or ten seconds, as a 

 result of self-reversal of the exciting 2536 line, and conse- 

 quent absence of the exact frequency necessary for stimulating 

 the mercury molecules in the bulb. In our work, we have 

 used a water-cooled quartz mercury arc of the type described 

 by Kerschbaum *, in which the arc is driven against the 

 front wall of the tube by a weak magnetic field. This reduces 

 the self-reversal to a minimum, for the cooler, non-luminous 

 absorbing layer is " squeezed out," so to speak, the current- 

 carrying vapour being in contact with the quartz wall of the 

 tube. The spectrum of such a lamp is quite unique in 

 appearance; for the 2536 line is so much brighter than 

 any of the other lines, that it is enormously overexposed, 

 appearing much like a photograph of a distant arc light 

 taken at night. Bright diffraction rays radiate from it in all 

 directions, causing it to stand out in the photograph with the 

 conspicuousness of a first-magnitude star in the Milky Way. 

 A photograph of the spectrum is reproduced as a negative on 

 Plate VI. fig. 4. The wavy lines joining the two spectra 

 were caused by the elevation of the plate between the two 

 exposures. 



The lamp consists of a straight tube of quartz with the 

 negative electrode (mercury) below, and a positive electrode 

 of tungsten above. It was made to order by the Cooper- 

 Hewitt Co., and operates on 110 volts, with resistance 

 sufficient to hold the current down to about 3*5 amperes. 

 During operation the anode is at a full red heat, but the 



* ' Electrician,' Tl. p. 1074 (1914). 



