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16-1 .REFRACTIVE INDEX AND ITS MEASUREMENT
(The Dialdex Refractometer)
In 1972 Rayner succeeded in producing a refractometer of quite a new type although it acts upon the same principles as all others. in place of the customary scale of the conventional refractometer, the user (having placed
the usual small drop of contact liquid and the stone to be tested on the table of the instrument) sees through the eyepiece a blank screen crossed with the faint shadow edge or shadow edge due to the stone. the position of which will depend, as usual, on the refractive index or indices of the stone. by the turning the calibrated dial at the side of the instrument a black ribbon-like marker can be adjusted to coincide with each shadow-edge. the corresponding refractive index can than be read very easily from the dial hence the name 'dialdex'. reading accurate to
one or two units in the third decimals place can be assured on this instrument rather more readily than on the previous standard Reyner refractometer. A further advantage has been gained by the employment of a new type of glass for the prism of the instrument, which has a lower dispersion than is normal for such high-index glasses. this means that the shadow-edge are sharper in white light, and by using the dark yellow filter provided in combination with a strong light the edge are so sharp that the use of a sodium varpour lamp (see later) become an unnecessary luxury.... ☺
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