Abstract
A BEAUTIFUL illustration of the laws of polarisation of light has lately been made by M. G. Govi. To understand it requires a somewhat careful explanation. Let a parallel beam of light be passed through a polariser, then through a thin slice of quartz cut perpendicularly to the optic axis, then through an analysing Nicol prism. It is seen, as is well known, to be coloured. This coloured light when passed into a spectroscope gives a spectrum marked by one or more dark bands, corresponding to the particular rays whose relative retardations in passing through the crystal slice have produced interference. These bands are not always in one place; they are displaced right or left (according to whether the crystal is a right-handed or a left-handed specimen) if either the analyser or the polariser be rotated. A slice of quartz about 4.3 millims. thick produces a single band. One of 8.6 millims. two bands at once in the visible spectrum, the number of bands being proportional to the thickness of the crystal. Now suppose a mechanical contrivance by which both the analyser and the spectrum can be rotated at the same velocity. A direct-vision prism attached to the front of the Nicol prism realises the optical portion of this combination. There will be seen on rotation a circular spectrum, having either red or violet at the centre and either violet or red at its outer circumference. Now since the dark band spoken of is displaced by a quantity proportional to the amount of rotation, interference will take place in this circular spectrum along points which form geometrically a spiral of Archimedes. The persistence of impressions on the retina will enable this dark spiral to be seen in its entirety, provided the rotation be sufficiently rapid. If a thicker piece of quartz be used, giving two, three, or four dark bands, the rotation-spectrum will present a most beautiful appearance, being crossed by a two-branched, or three-branched, or four-branched spiral, the separate lines of which proceed from the centre to the circumference. The sense of these dark spirals will change with the sense of the impressed rotation. The effects are very striking.
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Physical Notes . Nature 22, 595–596 (1880). https://doi.org/10.1038/022595a0
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DOI: https://doi.org/10.1038/022595a0