﻿698 Prof. R.W.Wood on Selective Reflexion, Scattering 



to act as resonators. Experiments on the dispersion and 

 magnetic rotation o£ metallic vapours and luminescent 

 hydrogen give evidence that but a small percentage of the 

 molecules are at any instant concerned in the production of 

 the phenomena in question. 



Primary and Secondary Resonance Radiation. 



Photographs of the luminous cone of mercury vapour at 

 room temperature contained in the quartz tube appeared to 

 prove that the vapour outside of the cone of vapour directly 

 excited by the primary beam was itself luminous. It was 

 observed, however, that the fused quartz phosphoresces with 

 a violet light under the influence of the ultra-violet light, 

 and I did not feel perfectly sure that the light did not come 

 from the wall of the tube. To eliminate such a possibility a 

 hollow box of brass was constructed (PI. X. fig. 2), two 

 adjacent sides of which were left open, and closed with thin 

 plates of quartz (crystal) which is not phosphorescent. The 

 inside of the box was heavily smoked, and the plates cemented 

 in place with sealing-wax. A drop of mercury was intro- 

 duced and the interior of the box put in communication with 

 a Gaede pump and exhausted. The ultra-violet light was 

 focussed at the centre of the box, entering throuo-h one of 

 the quartz plates, and the resonance radiation photographed 

 from the side through the other plate. It was found that, 

 unless the pressure of the air was less than three or four 

 millimetres, no trace of any secondary radiation was present. 

 On lowering the pressure of the residual air it developed 

 rapidly however, and after the pump had been in operation 

 for several minutes, the luminous glow filled the entire 

 interior of the box, the luminous cone being nearly lost on 

 the strongly luminous background. With correctly timed 

 exposures the directly excited resonance radiation is always 

 four or five times brighter than the secondary radiation. 

 Over exposure may, however, increase the effect of the 

 secondary until it equals that of the primary, causing the 

 outlines of the primary beam to disappear almost completely, 

 as in the first picture in fig. 4, PI. XL 



The intensity of the secondary radiation depends upon the 

 cross-section of the primary beam, as does also the rate at 

 which its intensity diminishes with increasing distance from 

 the primary rays. With an exciting beam of square cross- 

 section (5 mm. 2 ) the intensity of the secondary radiation 

 half a millimetre from the edge of the beam was found to be 



