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



the resonance radiation contributed to the darkening of the 

 photographic p]ate, for there appeared to be no difference 

 between the case of a plate placed close against the cell and 

 one placed at a distance. There is reason to believe that the 

 radiations emitted by a resonator lag in phase behind the 

 phase of the primary wave by 180°, that is they should reduce 

 the intensity of the primary wave by interference. This is a 

 delicate matter to investigate experimentally, but I have 

 made one or two attempts. If we place a black screen per- 

 forated with two small holes in front of the cell, we can limit 

 the radiation which traverses the cell to two beams of small 

 cross-section, each compartment transmitting one of them. 

 The primary wave w T ill now record itself on the plate as two 

 small spots of the same diameter as that of the apertures. 

 The short column of vapour traversed by the narrow beam in 

 the high vacuum compartment gives off scattered resonance 

 radiation, and we should expect this to record itself as a faint 

 penumbra around the sharply defined image imprinted by the 

 primary beam. No trace of any such penumbra was found, 

 from which we can infer that the effect is negligibly small 

 in comparison with that of the primary wave. This made me 

 suspect that, even when the vapour is in a high vacuum, 

 some true absorption of energy must take place, for it is 

 obvious that the magnitude of the effect due to the resonance 

 radiation will decrease in proportion to that of the primary 

 wave as the factor of true absorption is introduced. It 

 appeared possible to determine in this way, just what propor- 

 tion of the energy diverted from the primary beam was 

 absorbed and what proportion was scattered. I accordingly 

 placed the quartz lens at the centre of the box, with the plate 

 and absorption cell at the end opposite to that on which the 

 perforated paper cone was fastened. This gave me a sharply 

 focussed image of the circular aperture in the cone on the 

 photographic plate, which was placed as before with the film 

 in contact with the anterior quartz window. I now got a 

 very intense beam of light from the resonance lamp, and a 

 very black spot upon the plate with a distinct trace of a 

 corona around it due to the scattered radiation (PL XII. 

 fig. 10). Air was now admitted to the cell and the experi- 

 ment repeated. No trace of the corona appeared as was to 

 be expected, as is shown in PI. XII. fig. 11. 



This experiment will be better understood by reference to 

 PI. X. fig. 5, in which the scattered rays which produce the 

 corona are represented by arrows. The question now is, how 

 great an effect from these scattered rays are we to expect in 

 comparison with the effect produced by the primary beam 



