FOK EXHIBITING THE POLARIZATION OF LIGHT. 89 



the analysing prism is turned in its frame, we obtain the decrease ac- 

 cording to the law of Mains; if one of the laminae of mica is placed be- 

 fore, on turning the intensity of the light remains unchanged. If, instead 

 of the analysing Nicol's prism an achromatic double-refracting prism in 

 a similar frame is screwed in, the analogous phsenomena are obtained for 

 both images. 



When the polarizing prism e is bent on one side, a double-refracting 

 prism screwed into the ring / gives two mutually perpendicular polarized 

 images of the aperture in the diaphragm, the changes of intensity of 

 which are obtained by turning the analysing prism u. If the thin la- 

 mina of mica f is placed forwards, the images, when the principal sec- 

 tion of the double-refracting prism lies perpendicularly or horizontally, 

 become circular on the right and left, and an arrangement coinciding 

 with the apparatus proposed by Fresnel is obtained, consisting of three 

 rock-crystal prisms, of Avhich two belong to a crystal turned to the 

 right and the single one to that turned to the left. By turning the 

 analysing prism, the intensity of the images remains unchanged. If the 

 analysing prism be also a double-refracting one, on turning it, two 

 images with unchanged intensity (the mica plate lying between) move 

 round the two stationary images with the same property. 



If a mica or gypsum plate of a determinate thickness be in the ringw, 

 on its turning round the pin to which it is fastened we obtain the phse- 

 nomena of the so-called coloured polarization between the two Nicol's 

 prisms. The complementary colours appear of great intensitjr, and give 

 white where they overlap each other, when the analysing Nicol's prism 

 is exchanged for a double-refracting one. Should we wish to combine 

 ij two double-refracting prisms as above, the mica plate f must be ex- 

 changed for a thicker one. When the aperture of the diaphragm is 

 diminished the images separate from each other. If a plate of Iceland 

 spar, cut perpendicularly to the axis, is screwed upon the universal 

 setting of the Nicol's analysing prism, the corresponding modifications 

 of the system of rings in the separated and circularly polarized vacant 

 spaces are obtained, when the double-refracting prism is in /; if on 

 the contrary there is in the ring / a second plate of Iceland spar like- 

 wise cut perpendicularly to the axis, it is easy by turning this ring to 

 cause the centres of the second and first plates to coincide. In this 

 manner we may imitate the phaenomena (as described in the preceding 

 paper) of certain twin-crj'stals of Iceland spar by interposing a mica 

 plate of definite thickness in/. If / lies at the side, by turning the 

 i ring I the isochromatic curves originating from the combination of two 

 i plates of which the centres do not coincide are obtained*. In a similar 



j • In order to obtain the four mutually involved spirals of a rock-crystal plate 

 jtumed right and left, I combine aright-handed plate ground plano-concave 

 ■Willi a lefl-iianded crystal ground with parallel faces. 



