POLARISATION OF LIGHT. 



but we have not space for continuing the 

 subject. 



3. Tiro rays originally polarised in 

 rectangular planes may be afterwards 

 reduced to the same plane of polarisa- 

 tion, without acquiring the property of 

 interference. 



If in the experiment with the film 

 of mica we place between the eye and 

 the sheet of copper a doubly refract- 

 ing crystal, having its principal section 

 inclined 45 to either of the planes of 

 polarisation of the interfering pencils, 

 each pencil will be divided by the crystal 

 into two of equal intensity, and will be 

 polarised in two rectangular planes, one 

 of which is the principal section of the 

 crystal. In this state of things there are 

 no fringes seen in the union of R O with 

 L O, nor in that of L E with R E. 



4. Two rays polarised in ojyposite 

 planes, and afterwards brought into si- 

 milar states of polarisation, interfere 

 like common light, provided they be- 

 long to a pencil, the whole of which was 

 originally polarised in one and the 

 same plane. 



5. In th*, phenomena of interference 

 produced by doubly refracted rays, a 

 difference of half an undulation must 

 in certain cases be admitted. 



These two last results are deduced 

 from experiments analogous to those al- 

 ready described ; but it would occupy too 

 much of our limited space to describe 

 them as they deserve. We must there- 

 fore refer the reader to Mr. Herschel's 

 Treatise on Light, Part IV., viii., No. 

 960973. 



The doctrine of interference was first 

 employed by Dr. Young, in an article in 

 the Quarterly Review for 1814, to ex- 

 plain the colours of polarised light. In 

 that article he maintains " that such co- 

 lours are perfectly reducible, like all other 

 cases of recurrent colours, to the general 



laws of the interference of light which 

 have been established in this country ; 

 and that all their apparent intricacies 

 and capricious variations are only the 

 necessary consequences of the simplest 

 application of these laws. They are, in 

 fact, merely varieties of the colours of 

 mixed plates, in which the appearances 

 are found to resemble the colours of 

 simple thin plates, when the thickness is 

 increased in the same proportion as the 

 difference of refractive densities is less 

 than twice the whole density : the colours 

 exhibited by direct transmission, corre- 

 sponding to the colours of thin plates 

 seen by reflection, and to the extraordi- 

 nary refraction of the crystalline sub- 

 stances, and the colours of mixed plates 

 exhibited by indirect light to the colours 

 transmitted through common thin 

 plates, and to those produced by the or- 

 dinary refraction of the polarising sub- 

 stances." According to these views, 

 colours ought to be produced in common 

 as well as in polarised light, and it was 

 therefore left to MM. Fresnel and 

 Arago to show how the production of 

 such colours was dependent on the 

 primitive polarisation of the pencil, andl 

 its subsequent analysis before entering 

 the eye. 



The second of the preceding laws ex- 

 plains at once the reason why no colours 

 are exhibited by the transmission of light 

 through a thin plate possessing double 

 refraction. The two pencils are polarised 

 in opposite planes, and therefore inca- 

 pable of producing the periodical colours 

 by their interference. 



In order to explain how the polarised 

 tints are produced by interference in 

 ordinary cases, let us take the case shown 

 in/g-. 21, pp. 18, 19, where the neutral 

 axes are inclined 45 to the plane of 

 primitive polarisation. 



Let R A, fig. 40, be the polarised ray 



Fig. 40. 



