MR. TALBOT ON THE OPTICAL PHENOMENA OF CERTAIN CRYSTALS. 27 
Having pointed out this resemblance, I may also mention another class of facts to 
which I think those I have described possess a considerable analogy. I mean the 
optical figures which Brewster has discovered in spheres of glass whose density was 
rendered variable by heating them. 
He says * that, “ if we take a cold sphere of glass and immerse it in a trough of 
hot oil, placed in a polarizing apparatus, we shall observe a black cross with four 
sectors of polarized light. If the sphere is turned round it will exhibit in every posi- 
tion the very same figure. If we now suppose the trough to be filled with such 
spheres they will exhibit the same phenomena in whatever direction the polarized 
light is transmitted through them, and even if they were in a state of motion. A 
fluid composed of such spherical particles would exhibit the same polarizing struc- 
ture in every possible direction, and even if it were in a rapid state of gyration. If 
the particles possessed the structure that produces circular polarization the fluid 
would develop the phenomena exhibited by oil of turpentine, &c.” 
And again “ The structure of the particles of a circularly polarizing fluid must 
be exactly the same along every one of its diameters ; that is, the structure must be 
symmetrical round the centre of the particle, or analogous to that which takes place 
in common polarization when a sphere of glass has its density regularly increasing 
or regularly diminishing towards its centre.” 
I have quoted these remarkable passages at length, because it appears to me that 
what is there advanced merely as a hypothesis, acquires a considerable degree of 
probability from the facts which I have stated, since I have succeeded in rendering 
actually visible circular particles of excessive minuteness, in each of which the micro- 
scope detects the very structure imagined by Brewster, viz. the black cross and four 
sectors of light. So that it appears not improbable that the circular-polarizing pro- 
perties of fluids may be owing to the presence of multitudes of particles similar to 
these, which they hold in solution. 
* Library of Useful Knowledge, art. “ Polarization of Light,” p. 51. 
t Ibid. p. 45. 
