POLARISATION OF LIGHT. 



the one in an azimuth 45 to the right 

 of the plane of reflexion, and the other 

 45 to the left. 



If one of the two circularly polarising 

 rays is transmitted through a thin crys- 

 tallised film, and parallel to its axis, it 

 \vill be divided by the double refraction 

 of the film into two pencils, exhibiting 

 the complementary colours ; and these 

 colours differ by an exact quarter of 

 a tint, or order, from those that would 

 have been produced by the same film 

 with common polarised* light. 



M. Fresnel had the good fortune to 

 discover another method of producing a 

 pencil possessing all the properties of one 

 of those formed along the axis of quartz. 

 He allowed a common polarised ray, R P, 

 (fig. 50) to suffer two total reflexions at 

 Q and S in a glass parallelepiped : then 

 if the plane of reflexion P Q S is in- 



Fig. 50. 



c 



clined 45 to the plane of primitive polari- 

 sation of the ray R P, it will emerge at 

 T, possessing all the properties of one of 

 the rays formed along the axis of quartz. 



Circular Polarisation of Fluids. 



THE property of circular polarisation 

 is not confined to quartz. M. Biot and 

 Dr. Seebeck discovered it nearly about 

 the same time in certain fluids which 

 possessed it in a feeble degree. If we fill 

 a tube, six or seven inches long, with oil 

 of turpentine, and expose it to polarised 



Right to Left. 



Rock crystal 



Oil of turpentine 



Ditto another kind 



Ditto purified by 

 distillation 



Solution of 1753 



artificial camphor in 17359 

 of alcohol . . -j~ 



Essential oil of laurel 



Vapour of turpentine 



light in the apparatus, (fig. 39,) the ob- 

 server will perceive the complementary 

 colours similar to those in rock crystal. 

 Some fluids turn the plane of polarisation 

 from right to left, and others from left 

 to right. The following Table shows the 

 results of M. Biot's experiments. 



0.018 



M. Biot found that when oil of turpen- 

 tine was mixed with sulphuric ether, 

 which has not the rotatory property, it 

 gave, \yhen mixed, the very same tints 

 which it would have produced alone in 

 the same tube ; and that when a ri^ht- 

 handed was mixed with a left-handed 

 fluid, in quantities reciprocally propor- 

 tional to the intensity of their action, they 

 neutralised each other. He likewise 

 found that oil of turpentine required to 

 have a thickness of 68^ millimetres to 

 give the same polarised tint as a single 

 millimetre of quartz ; that a thickness of 

 38 millimetres of oil of lemons was equal 

 to a thickness of 66 millimetres of oil of 

 turpentine, and that a thickness of 4^ 

 millimetres of concentrated syrup of 



sugar was equal to one millimetre of 

 quartz. 



The very remarkable phenomena above 

 described might, at first sight, be attri- 

 buted to certain polarities in the particles 

 of fluids, in virtue of which they assume 

 some regular arrangement analogous to 

 that of "crystalline structure ; but Dr. 

 Brewster has remarked that this cannot 

 be the case, for the same phenomena are 

 seen in whatever direction the polarised 

 ray is transmitted through the fluid, so 

 that the structure of the particles of a 

 circularly polarising fluid must be exactly 

 the same along every one of its diame- 

 ters, that is, the structure must be sym- 

 metrical round the centre of the particle, 

 or a structure analogous to that which 



