Page:Outlines of Physical Chemistry - 1899.djvu/99

 Power of Botating Polarised Light

The determination of the rotatory power of a substance is made by means of an instrument whose function is based on the properties of polarised light. 1 The simplest apparatus consists of two nicol prisms (polariser and analyser) placed one behind the other, so that their axes lie in a straight line.

Suppose that the nicols are crossed, and that we send a pencil of parallel light rays along the axis of the polari- scope. It is well known that the light, already weakened by the polariser, will be completely extinguished by the analyser, and that the field of vision will be quite dark.

If now we interpose between the crossed nicols a mono-refractive solid, for example a plate of glass or of a substance which crystallises in the regular system, 2 the extinction of the rays remains as before.

The darkness is not even diminished by the inter- position of a crystalline substance belonging to the hexagonal or to the quadratic system, so long as we make use of a section cut perpendicularly to the optical axis and so placed between the nicols that this axis is parallel with

If, however, the experiment had been made with a quartz plate (and if, to avoid rotatory dispersion, mono- chromatic light had been used), the following would have been observed. In spite of the nicols being crossed, the field of vision would be more or less clear according to the thickness of the quartz plate ; and in order to re-establish the original darkness the analyser would have to be turned

1 In order to understand the theory of the polarimeter a previous knowledge of optics is necessary, and particularly the phenomena of reflection, refraction, interference, and polarisation.

2 The faces of the plate must, of course, be parallel.

• In the direction parallel with their optical axis the crystals in question (Iceland spar, &c.) behave as mono-refractive substances. It must be kept in mind that the rays from the polariser are supposed parallel. With convergent rays we observe the well-known coloured ring phenomenon.

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