﻿690 Prof. R. TV. Wood on Selective Reflexion, Scattering 



the mercury vapour is mixed has heen determined. A very 

 remarkable secondary resonance radiation of the gas not 

 directly illuminated by the primary beam, but stimulated by 

 the light which comes from the directly excited molecules, 

 lias been discovered, the study of which has enabled me to 

 draw the conclusion that there is no true absorption except 

 when the pressure is raised by the presence of air. 



I have also succeeded in passing by gradual stages from 

 the condition in which the resonance radiation is diffusely 

 scattered to that in which, owing to the proximity of the 

 molecules, it is regularly reflected from the surface of the gas 

 mass. A resonance lamp has been constructed which emits 

 light probably more homogeneous than that obtained from 

 any source hitherto known, by the light of which I have 

 succeeded in photographing the vapour rising from a drop of 

 warm mercury into the air of the room like black smoke. 

 So sensitive is the light of this lamp to the presence of 

 mercury vapour, that I have been obliged to ventilate my 

 room before each experiment, owing to the presence of 

 mercury in the cracks on the floor, for the light of the lamp 

 is reduced to one-half of its intensity by traversing a layer 

 of mercury vapour at room temperature only 5 mm. in 

 thickness. 



The following topics will be discussed : — 



Introduction. 



Resonance radiation of mercury vapour at room tem- 

 perature. 



Energy diverted from the primary beam. 



Primary and Secondary Resonance Radiation. 



Destruction of the Secondary Radiation by molecular 

 collisions. 



Probability that the molecules continue to emit lig'ht 

 after they leave the region traversed by the primary 

 beam. 



Ratio between Scattered and Absorbed Light as a 

 function of the pressure of the gas. 



Transition from Diffuse Scattering to Regular Re- 

 flexion. 



Absence of all traces of Polarization. 



Experiments with the Resonance Lamp. 



Introduction. 



The scattering of light by gas molecules has been the 



subject of mathematical treatments, but practically no 



experimental work has been done upon the subject, and we 



have no data whatever regarding the actual amount of energy 



