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



to me tbat it might be possible that the emission of light by 

 the vapour surrounding the primary cone might not be due 

 to light emitted by the \apour directly excited, but to the 

 circumstance that, owing to the great length of the mean 

 free path, the mercury molecules might continue to emit 

 light after flying outside of the limits of the primary beam. 

 The action of a small amount of air in destroying the 

 luminosity would follow at once as a result of the reduction 

 in the length of the free path, the luminosity being destroyed 

 by the collisions. 



Probability that the Molecules continue to emit Light after 

 they leave the region traversed by the Primary Beam. 



Fortunately this is an hypothesis which is very easily 

 tested by experiment, for we have only to divide the cell 

 into two compartments by means of a partition of quartz, 

 passing the exciting beam immediately below and close to 

 the under side of the horizontal partition. The partition 

 will stop the moving molecules, but will transmit the light 

 emitted by the vapour illuminated by the primary beam. 

 A small plate of quartz 1*5 mm. in thickness was cemented 

 to the front window of the cell as shown in PI. X. fig. 3, and 

 the cell exhausted until nothing but the green phosphor- 

 escence was visible in the discharge-tube. The resulting 

 photograph showed that there was considerable luminosity 

 in the upper chamber, but it appeared to be of somewhat less 

 intensity than that of the vapour in the lower chamber. 

 The photograph, however, was not very satisfactory owing 

 to the thickness of the partition. The cell was accordingly 

 made over again, with a partition consisting of a quartz 

 plate only J mm. in thickness, and the same phenomenon 

 was found, somewhat greater intensity of the secondary 

 radiation in the compartment traversed by the exciting beam. 

 Calculation of the reflecting power of the quartz plate from 

 its refractive index for the wave-length, in question showed 

 that a reduction of intensity of 10 per cent, was to be 

 expected from this circumstance, but this did not seem to 

 be sufficient to explain the observed diminution of the in- 

 tensity ; and careful measurements of the intensity of the 

 secondary radiation at equal distances from the centre of 

 the primary beam were made by comparison with a plate 

 exposed in strips by gradually increasing amounts of time, 

 which was developed with a photograph of the cell for 

 exactly the same length of time. These measurements 

 showed that the effect of the thin partition was to reduce the 



