by transmission through Fluids. 419 



meant by the deviation or rotation of the rays of polarized 

 light when transmitted through fluids said to possess circular 

 polarization. 



Assuming the general principles of the undulatory theory 

 of light, and making use of terms which, if not absolutely cor- 

 rect, are at least convenient, I shall first advert to what is 

 meant by rectilinear, or, as I would suggest it should be 

 termed, rectangular polarization. 



When a beam of ordinary light impinges on a polarizing 

 medium, it may be considered as dividing itself into two por- 

 tions, the waves of which respectively undulate in directions 

 varying from each other 90^, i. e. a right angle, in other words, 

 they are said to possess opposite planes of undulation, and to 

 be rectangularly polarized*. This effect may be produced 

 by five different modes, viz. refraction, reflexion, absorption, 

 dispersion, and double refraction. 



A number of pieces of thin glass f superimposed on each 

 other, and inclined at a suitable angle to a ray of light inci- 

 dent thereon, polarizes one portion of the light transmitted 

 through it by refraction, whilst the other portion reflected 

 from the surface is also polarized, but in an opposite direc- 

 tion. That the two portions into which the light is thus di- 

 vided are oppositely polarized, or may be supposed to undulate 

 in opposite directions, is established by the fact, that if trans- 

 mitted towards a similar bundle of glass, the two rays would 

 comport themselves differently ; that is, they would not both 

 be reflected or both transmitted by such second bundle if 

 held in \.\\e same direction or plane towards each respectively. 

 Calling the one portion of polarized light into which the ori- 

 ginal beam of ordinary light R (figs. 1 and 2, p. 420) is divided 

 O, and the other portion X, it will be found that the position 

 of the second bundle necessary to reflect X must be at right 

 angles to that in which it would reflect O, and similarly as to 

 retraction or transmission. This second bundle, serving to 

 distinguish the two rays, is termed the analyser. Figs. 1 

 and 2 serve to illustrate polarization by refraction and re- 

 flexion. In fig. 1 the bundles of glass are supposed to be all 

 in the same plane, or situated parallel to each other, whilst in 

 fig. 2 the bundles B and C are supposed to be situated at right 

 angles to the bundle A. The beam of light R impinging on 



* Consideration of the direction in which the waves of h'ght may be 

 supposed to undulate will materially assist the memory in studying the fa- 

 cility of reflexion or transmission of the respective rays by surfaces opposed 

 to them in different positions. 



t Tlie very thin glass sold for covering microscopic objects is extremely 

 convenient for this purpose, 



2 E2 



