﻿394 . Royal Society. 



as to force on the mind the conviction that it arose merely from 

 motes*. Indeed in the former 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 quan- 

 tity of light is diminished in a given ratio, the illumination is per- 

 ceived with more difficulty when the light 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 towards 

 polarization in a plane perpendicular 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 trans- 

 versely, it discharges bluish polarized light laterally, the colour being 

 by no means a bad imitation 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 vibrations of the incident light, then assuredly 

 the experiments recorded in the foregoing communication decide the 

 question in favour of FresneFs 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 example, 

 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 any po- 

 larized light is discharged, 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 late- 

 rally almost perfectly polarized light ; and if the selenite be employed, 

 vivid colours flash into existence. A still more brilliant result is ob- 

 tained 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 polariza- 

 tion 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. 



* The azure may be produced in the midst of a field of motes. By turning the 

 Nicol, the interstitial blue may be completely quenched, the shining and appa- 

 rently unaffected motes remaining masters of the field. A blue cloud, moreover, 

 may be precipitated in the midst of the azure. An aqueous cloud thus precipi- 

 tated reverses the polarization ; but on the melting away of the cloud the azure 

 and its polarization remain behind. 



