388 



On the Successive Polarization of Light. [Mar. 23, 



plate and the two parallelopipeds may be turned round the transmitted 

 pencil without changing the colour of the images. It is this analogy be- 

 tween the optical properties of this little apparatus and those of plates of 

 rock-crystal perpendicular to the axis which enabled M. Fresnel to foresee 

 the peculiar characters of double refraction that rock-crystal exerts on 

 rays parallel to the axis." 



It does not appear that Fresnel, in any of his published memoirs, has 

 given any further modifications of this experiment, the importance of 

 which has been almost entirely overlooked in elementary treatises on light. 

 He does not seem to have remarked that similar phenomena of successive 

 polarization are exhibited when the light incident on the crystallized plate 

 is plane-polarized, nor that the order of the succession of the colours 

 depends on the position of the principal section with respect to the plane 

 of polarization. These circumstances are indeed necessarily included in 

 the beautiful theory established by this eminent philosopher ; but I am 

 not aware that they have hitherto been specifically deduced or experi- 

 mentally shown. 



IX. 



The apparatus (fig. 1) affords also the means of obtaining large surfaces 

 of uncoloured or coloured light in every state of polarization — rectilinear, 

 elliptical, or circular. 



It is for this purpose much more convenient than a Fresnel's rhomb, 

 with which but a very small field of view can be obtained. It must, how- 

 ever, be borne in mind that the circular and elliptical undulations are 

 inverted in the two methods : in the former case they undergo only a 

 single, in the latter case a double reflection. 



For the experiments which follow, the crystallized plate must be placed 

 on the diaphragm E between the silver plate and the analyzer, instead of, 

 as in the preceding experiments, between the polarizer and the silver 

 plate. 



By means of a moving ring within the graduated circle D the silver plate 

 is caused to turn round the reflected ray, so that, while the plane of pola- 

 rization of the ray remains always in the plane of reflection of the glass 

 plate, it may assume every azimuthal position with respect to the plane of 

 reflection of the silver plate. The film to be examined and the analyzer 

 move consentaneously with the silver plate, while the polarizing mirror 

 remains fixed. 



In the normal position of the instrument the ray polarized by the 

 mirror is reflected unaltered by the silver plate ; but when the ring is 

 turned to 45°, 135°, 225°, or 315°, the plane of polarization of the ray 

 falls 45° on one side of the plane of reflection of the silver plate, and the 

 ray is resolved into two others, polarized respectively in the plane of reflec- 

 tion and the perpendicular plane, one of which is retarded on the other by a 

 quarter of an undulation, and consequently gives rise to a circular ray, 

 which is right-handed or left-handed according to whether the ring is turned 



