Refractometer, Pulfrich

The quantities of material employed by Lehfeldt were small about 30 c.c. of the mixture to be investigated were placed in the still and three fractions of about 1 c.c. each were usually collected and examined separately by means of a Pulfrich refractometer. 

In making the refractive index measurements, a thermostated Pulfrich refractometer (Zeiss) with a sodium lamp was used. Near the transitional region the concentration gradient in the solution produces the formation of shadows and rays on the image. 

More precise measurements can be obtained with commercial refractometers, such as an Abbe or Pulfrich. Both refractometers are based on measuring the critical angle of incidence between the specimen and a standard prism. The sample consists of a flat plate with one surface and an adjacent vertical edge well pohshed. The contact between the polished sample and the prism is enhanced with an approximately matching index oU. A split image is adjusted to coincide, and the refractive index is read directly. The precision here is generally of the order 10". ... 

Regulation of the Temperature.— As the density, both of the prism and of the liquid under investigation, alters with temperature, it is necessary, in measurements of any considerable degree of accuracy, to be able to maintain the prism and the liquid at constant temperature. In the newer form oi Pulfrich refractometer, made by Zeiss, to be described below,... 

Pulfrich Refractometer.—The Pulfrich refractometer (as made by Zeiss) is shown in Fig. 34, the instrument bemg viewed from behind in order to show the optical parts. This instrument is arranged for illumination either by a coloured flame or by means of a Geissler tube (Q). 

Correction for Temperature.— The values of the refractive index given above (p. 107) refer, strictly, only to the temperature of 20". When meastuements are made at any other temperature, a correction has to be applied. Tables of temperature corrections for the dififerent prisms are issued by the instrument makers. To illustrate the use of the tables, we may take the table for Prism 1., supplied by Zeiss with their Pulfrich refractometer —... 

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