Page:Scientific Memoirs, Vol. 1 (1837).djvu/101

the analysing prism is turned in its frame, we obtain the decrease according to the law of Malus; if one of the laminæ of mica is placed before, 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 phænomena are obtained for both images.

When the polarizing prism $e$ is bent on one side, a double-refracting prism screwed into the ring $l$ 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 lamina of mica $f$ is placed forwards, the images, when the principal section 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.

If a 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 phænomena 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 exchanged 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 $l$; if on the contrary there is in the ring $l$ a second plate of Iceland spar likewise 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 phænomena (as described in the preceding paper) of certain twin-crystals of Iceland spar by interposing a mica plate of definite thickness in $f$. If $f$ lies at the side, by turning the ring $l$ the isochromatic curves originating from the combination of two plates of which the centres do not coincide are obtained^[1]. In a similar

1. In order to obtain the four mutually involved spirals of a rock-crystal plate turned right and left, I combine aright-handed plate ground plano-concave with a left-handed crystal ground with parallel faces.