FOR EXHIBITING THE POLARIZATION OF LIGHT. 89 
the analysing prism is turned in its frame, we obtain the decrease ac- 
cording to the law of Malus; if one of the laminz 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 pheenomena 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 w. 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 which 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. 
Ifa mica or gypsum plate of a determinate thickness be in the ring m, 
on its turning round the pin to which it is fastened we obtain the phe- 
nomena of the so-called coloured polarization between the two Nicol’s 
prisms. The complementary colours appear of great intensity, 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 
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. Ifa 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 J; 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 phenomena (as described in the preceding 
paper) of certain twin-crystals of Iceland spar by interposing a mica 
plate of definite thickness inf. If f lies at the side, by turning the 
ring / the isochromatic curves originating from the combination of two 
plates of which the centres do not coincide are obtained*. In a similar 
* In order to obtain the four mutually involved spirals of a rock-crystal plate 
turned right and left, I combine a right-handed plate ground plano-concave 
with a left-handed crystal ground with parallel faces. 
