THE POLARIMETER 485 



Lippich. — P. Lippich modification of nicol prism. K. Half prism 

 serving to give half shadow. C. Quartz wedge compensating system shown 

 as a double system. A. Analysing nicol. Fig. 315. 



Adjustable and Fixed Half-Shadow Angles. — In the original types of 

 polarizer the half-shadow angle is fixed, and generally lies between 5° and 8°. 

 Jellett himself in the first half-shadow analyser used a j- angle. The instru- 

 ment of Laurent is sent out with an arrangement such that the angle can be 

 varied, and the same is true of instruments designed for general use ; but 

 apart from the Laurent t\'pe, sugar instruments have usually a fixed angle. 

 The advantage of the variable angle is that with light-coloured solutions a 

 small angle and low intensity of light can be used giving superior sensibility. 

 With dark solutions and a greater angle more light can be transmitted, 

 facilitating their observation. 



Landolt^"* asserts that with technical instruments a fixed half-shadow 

 angle should be employed, since with every change of angle there is a change 

 in the zero which requires adjustment. While this reasoning may be correct 

 with regard to chemists of a certain mental type, it is quite inapplicable to 

 others of a superior intelligence. 



The instrument of Bates^^ as built by Fric, is provided with means to 

 vary the half shadow dependent on the colour of the solution under analysis 

 and this with automatic adjustment of the change in the zero. 



Source of Light used in Polarimetry. — Measurements of academic interest 

 are alwavs made with monochromatic light. That first used was obtained 

 from a bead of a sodium salt incandescing in a bunsen flame ; such light is 

 not spectrally pure and a closer approximation to homogeneity is obtained by 

 filtration through a cell of potassimn bichromate, to which Landolt^" later 

 added a cell of uranous sulphate; such measurements are referred to as^ijo. 

 More recentl}' measurements are made \Wth spectrally pure light obtained 

 bv passing the light from a mercur\' vapour lamp through a prism. Such 

 measurements are referred to as [a]jj^. Although homogeneous light 

 may be, and is, used in saccharimetr}-, it is more con\-enient to employ white 

 light and such light before use should be filtered through a cell of potassium 

 bichromate such that percentage of salt x length of cell in cms. equals 9. 

 Such light has a mean wave length of 600 /a/x and to it the loo-point of the 

 sugar scale is referred. Error may be introduced by neglect of filtration, 

 and, for example, Schonrock^" found a rotation of 100-12 \Wth unfiJtered as 

 compared with 100 for filtered light. The difference varies with the eye of 

 the observer and is probably connected with the pigmentation of the eye. 



The actual light used may be a flat-wick kerosene lamp, a fish-tail coal 

 gas or acetylene burner, the Welsbach mantle or any form of electric light. 

 The writer prefers a concentrated filament nitrogen-filled tungsten light of 

 50 c.p. The Welsbach and electric light require the interposition of a dis- 

 persing surface to eliminate the image of the mantle or filament. Ground 

 glass is usually employed, and in its absence colourless transparent paper, 

 which may even be represented by a grease spot, serves well. 



In instruments of German design the light filter is inconveniently placed 

 within the instrument and difficult of access. It should be located without 

 the instrument, and between it and the source of light. It may be carried on 

 an extension rod or on a separate stand. No objection lies to its replacement 

 by a glass light filter giving a light of the same wave length as that specified. 



