1869.] 



and on the Polarization of Light. 



233 



to the plane of reflection, when the suspended particles became finer, and 

 therefore the beam more nearly continuous." 



Through the courtesy of its owner, I have been permitted to see and 

 to experiment with the piece of plate glass above referred to. Placed in 

 front of the electric lamp, whether edgeways or transversely, it discharges 

 bluish polarized light laterally, the colour being by no means a bad imita- 

 tion of the blue of the sky. 



Prof. Stokes considers that this deportment may be invoked to decide 

 the question of the direction of the vibrations of polarized light. On this 

 point I would say, if it can be demonstrated that when the particles are 

 small in comparison to the length of a wave of light, the vibrations of 

 a ray reflected by such particles cannot be perpendicular to the vibra- 

 tions of the incident light ; then assuredly the experiments recorded in 

 the foregoing communication decide the question in favour of Fresnel's 

 assumption. 



As stated above, almost all liquids have motes in them sufficiently nu- 

 merous to polarize sensibly the light, and very beautiful effects may be 

 obtained by simple artificial devices. "When, for example, a cell of dis- 

 tilled water is placed in front of the electric lamp, and a slice of the 

 beam permitted to pass through it, scarcely any polarized light is dis- 

 charged, and scarcely any colour produced with a plate of selenite. But 

 while the beam is passing through it, if a bit of soap be agitated in the 

 water above the beam, the moment the infinitesimal particles reach the 

 beam the liquid sends forth laterally almost perfectly polarized light ; and 

 if the selenite be employed, vivid colours flash into existence. A still more 

 brilliant result is obtained with mastic dissolved in a great excess of alcohol. 



The selenite rings constitute an extremely delicate test as to the 

 quantity of motes in a liquid. Commencing with distilled water, for 

 example, a thickish beam of light is necessary to make the polarization of 

 its motes sensible. A much thinner beam suffices for common water ; 

 while with Briicke's precipitated mastic, a beam too thin to produce any 

 sensible effect with most other liquids, suffices to bring out vividly the 

 selenite colours. 



January 21, 1869. 



JOHN PETER GASSIOT, Esq., Vice-President, in the Chair. 



The Chairman stated that Sir John Macneill and Mr. Edward Solly, 

 who, by reason of non-payment of their annual contributions, ceased to be 

 Fellows of the Society at the last Anniversary, had applied for readmission. 

 Extracts from their letters, explaining the circumstances under which non- 

 payment had occurred, were read, and notice was given that the question 

 of their readmission would be put to the vote at the next Meeting. 



The following communications were read : — 



VOL. XVII. 



s 



