406 FRED, EUGENE WRIGHT 
greater than that for solids and in the present instance the liquid 
will have a lower refractive index than the mineral for red and a 
higher refractive index for blue, with the result that the red rays 
are concentrated along the one margin of the grain and the blue 
rays on the opposite side. The grain appears fringed with colored 
margins, red or orange on the one side and pale blue on the opposite 
side. If the intensity of illumination on both sides of the grains 
is about equal, the refractive index of mineral and liquid are equal 
for the central part of the visible spectrum. 
Obliquely incident light is most readily obtained by placing 
the forefinger between the substage reflector and the polarizer 
and casting a shadow over half the field. The error of such a 
determination in white light is less than 0.005 on good sections, 
while if monochromatic light be used and care taken to select 
clear single grains, the error may be reduced to +o.oo1. If the 
grain be anisotropic, the three principal refractive indices a, 8, y 
can be determined by placing the grain in such positions that the 
parallel polarized light waves from the lower nicol vibrate, in passing 
through the crystal, parallel to one of the three principal axes of 
the optic ellipsoid. 
The chief difficulty in the measurement of the refractive indices 
of minute grains or plates in the thin section, is one of mechanical 
subdivision; the grains occur frequently in fine, overlapping 
aggregates, often imbedded in glass; and it is not an easy task 
under these conditions to find a clear, isolated grain on which 
measurements can be made. 
The materials for the set of refractive liquids noted above can 
be readily obtained from dealers in chemical supplies and the 
entire series prepared for use in a few hours’ time. With the set 
of refractive liquids at hand the refractive indices of a mineral 
grain can be readily ascertained within +o.005 and one of the 
most important optical constants of the mineral thus determined. 
It is a matter of surprise, in view of the ease and facility with 
which this method can be applied, that it is so little used by petrol- 
ogists and by chemists. 
Birefringence.—For the measurement of birefringence, extinc- 
tion angles, and optic axial angles a specially constructed ocular 
