NATURAL PniLOSOPHY. 139 



subject. He notices more particularly a specimen of plate glass 

 which, seen by reflected light, exhibited a blue whicli was ex- 

 ceedingly like an effect of tiuorescence, but which, when prop- 

 erly examined, was found to be an instance of false dispersion. 



*' It often struck me," he writes, while engaged in these obser- 

 vations, "that when the beam had a continuous appearance, the 

 polarization was more nearly perfect than when it was sparkling, 

 so as to force on the mind the conviction that it arose merelj^ 

 from motes. Indeed, in the fonner case, the polarization 'has 

 often appeared perfect, or all but perfect. It is possible that this 

 may, in some measure, have been due to the circumstance, that 

 when a given quantity of light is diminished in a given ratio, the 

 illumination is perceived with more difficulty when the liglit is 

 diffused uniformly than when it is spread over the same space 

 but collected into specks. Be this as it may, there was at least 

 no tendency observed toward polarization in a plane perpendicu- 

 lar to the plane of reflection, when the suspended particles be- 

 came 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 trans- 

 veisely, it discharges bluish polarized light laterally, the color 

 being by no means a bad imitation of the color 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 sa}^, 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 vibrations of the incident light; then 

 assuredly the experiments recorded in the foregoing communica- 

 tion decide the question in favor of Fresners assumption. As 

 stated above, almost all liquids have motes in them sufficiently 

 numerous to polarize sensibly the light, and very beautiful effects 

 may be obtained by simple artificial devices. When, for exam- 

 ple, a cell of distilled water is placed in front of the electric 

 lamp, and a slice of the beam permitted to pass through it, 

 scarcely an}^ polarized light is discharged, and scarcely any color 

 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 colorsiflash into existence. A 

 still more brilliant result is obtained with mastic dissolved in a 

 great excess of alcohol. The selenite rings constitute an ex- 

 tremely 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 sensi- 

 ble. A much thinner beam suiaces for common water; while 

 with Brucke's ])recipitated mastic, a beam too thin to produce 

 any sensible effect with most other liquids suffices to bring out 

 vividly the selenite colors. 



Note on the Formation and Phenomena of Clouds. — It is well 



