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Bepolishing
should be done by the maker; (iv) another limitation is that a
refractive index greater than 1-80-1'90 cannot be measured. The values
of refractive index are usually given for sodium light. Herbert Smith
has produced two forms of refractometer. The new model is a larger
instrument and the delicacy of the readings has also been increased.
The construction, of the instrument is on the same principle as
Tully's. The scale reads the refractive index for yellow (sodium
light), directly to the second place of decimals, and in the higher
values the longer intervals allow reading to tenth parts and even to
the third place of decimals.
The
range extends from 1-300-1-795 approximately and just includes the
refractive index of corundum (sapphire and ruby), which adds much to
the utility of the instrument. Methylene iodide with 1-74 and the
liquid saturated with sulphur, R.I. 1-800, are also supplied in two
bottles japanned on the outside.
The
reading is to be taken at the part of the edge where the green passes
into the yellow in the coloured band seen. Towards the bottom of the
field another edge is seen, which corresponds to methylene iodide. If,
on the other hand, the saturated liquid is used, no edge will be
visible since its refractive index lies outside the range of the
instrument. The colouration of the edge depends on the dispersion of
the substance under examination.
Double Refractton
When
a ray of light passes into a cubic or amorphous mineral it passes out
as a single refracted ray. But in minerals of the other five systems,
it is not only refracted but is resolved into two rays, e.g. in calcite
into ordinary and extraordinary rays. Many of the gem species are
doubly refractive and when tested with the refractometer will show two
distinct critical edges. In white light the separation will not be
apparent unless the double refraction
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