76 SUMMARY OF CUKKENT llESEARCHES RELATING TO 



MICROSCOPY. 



A. InstrumeSts, Accessories, etc. 

 (3) Illuminating- and other Apparatus. 



The Light scattered by Gases : its Polarization and Intensity. 



R. J. Strutt {Proc. Roij. Soc, Series "A," 95). The present investi- 

 2:ation is a development in various directions of that described in the 

 Proceedings of the Royal Society, "A," 94, 453, 1918. It is there 

 shown that the light scattered by air and other gases in the direction 

 perpendicular to the beam is almost completely polarized. It is of 

 importance to determine whether or not the polarization of the scattered 

 light at right angles to the beam is complete. Sky light is by no 

 means completely polarized, but in that case the incident light is not 

 all in one direction. The question is whether, apart from these compli-. 

 cations, the polarization would be complete. This cannot be decided 

 by observations on the sky, but the laboratory methods now described 

 allow of this investigation, not only in the air, but in other gases also. 



The simple theory, according to which the polarization should be 

 complete, assumes a spherical form for the molecules. That is to say, 

 it is assumed that, however the molecule may be turned with reference 

 to the direction of vibration in the primary beam, the result will be the 

 same, and vibration induced will be wholly in that direction. 



Any departure from complete polarization would indicate that the 

 molecule had certain preferential directions of vibration. The extreme 

 case would be that in which the molecule could only vibrate in one 

 direction fixed within it. The experimental arrangements for examining 

 the polarization are the same as that described in a previous preliminary 

 paper, except that slight modifications were introduced to enable the 

 investigations to be made under the conditions specified. 



Since the effect to be looked forward for is small it is desirable to 

 make the primary beam as intense as possible. An electric arc therefore 

 was used with " Kinarko " kinematograpb carbons. These carbons give 

 great intrinsic brightness. The horizontal core carbon charged with a 

 special preparation was IS m.m. in diameter, presented end on to the 

 condensing lens. The copper cord negative carbon, nearly vertical, was 

 10 m.m. in diameter. The current was about 25 amperes and the lamp 

 was a hand -fed one. 



The varying intensities of the polarized beam were recorded by 

 means of a photographic plate, so that reliance was not placed oil 

 observational methods, but the result was determined by the differing 

 capacities of the photographic deposit as shown by the resulting plate. 

 The results obtained may be summarized as follows : — 



1. The light scattered at right angles by gases and vapours is not 

 completely polarized. The vibration parallel to the exerting beam has 



