Polarimeter half-shadow

A half-shadow polarimeter (Lippich type) is illustrated diagram-maticaUy in Fig. Ill, 169, 1. Here two polarized rays are produced by means of the main Nicol prism P and a smaU Nicol prism P the latter... 

In practice, observations are made easier and more exact by the introduction into the polariser of arrangements and devices such that the field visible through the eye-piece is divided into two or more zones, these being equally illuminated only for a definite position of the analyser. This position is easy to fix exactly, since the line of separation of the zones is then scarcely visible, whereas a slight displacement causes the immediate appearance of the contrast between the differently illuminated zones. Such position is taken as the standard, and since in it there is neither darkness nor a maximum of light, the apparatus is known as a half-shadow polarimeter. 

Polarimetric measurements were carried out on a polarimeter with very large half-shadow angle (5°) that could not give sufficient accuracy in the estimation of the optical rotation of products. 

Apparatus.—The Polarimeter.—Of the various forms of polarimeter which have been invented, only two need be considered here, viz. the Lippich and the Laurent half-shadow polarimeters and these differ essentially only in the method of producing the half-shadow. 

The lippich polarimeter differs from the Laurent form only in the method of producing the half-shadow. Instead of the quartz plate, there is a small Nicol prism which covers half of the opening at the end of the polarizer tube. The effect produced is the same as with the quartz plate, and the apparatus possesses the advantage that it can be used with light of any wave-length. 

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