550 Major-General McMahon—Double-Refraction of Minerals. 
vision, we shall find that the black line, above alluded to, moves 
gradually along the analyzing wedge from its thin towards its thick 
end, and spectra (in inverse order) of the Ist, 2nd, 3rd, and higher 
orders of Newton’s scale, come in between the black line and the 
thin end of the analyzing quartz wedge. ‘These chromatic bands of 
colour, as they rise in the scale of Newton’s orders, become fainter, 
and fainter, until at last the eye fails to detect them and the spectra 
merge into white light. 
This experiment shows that, other things being equal, the distance 
of the black line from the thin end of the wedge is proportional to 
the thickness of the quartz on the stage of the microscope. 
If we now substitute for the quartz on the stage of the microscope 
a wedge of calc spar cut in precisely the same way with reference to 
its optic axis as the quartz wedge is cut, and of the same thickness 
as the latter, and placed in such a position on the stage that the axis 
of greater elasticity of the calcite is parallel to the axis of less 
elasticity of the quartz, we shall find that the thick end of the 
analyzing quartz wedge is insufficient to make the dark line and the 
spectra visible when we examine even the thin edge of the calcite. 
Indeed, so much more powerful is the double-refraction of calcite 
than that of quartz, that even the thick ends of two ordinary 
quartz wedges, superposed one above the other in the eye-piece,’ are 
insufficient for the purpose. 
If for wedges we substitute flat slices of different minerals of 
uniform thickness cut at the same angle to the optic axis, but differing 
from each other in the intensity of their double-refraction, we shall 
find the distance of the dark line from the thin edge of the analyzing 
quartz wedge, and the number of chromatic bands that come in 
between it and the thin end of the wedge, depends upon the intensity 
of the double-refraction of the mineral under observation. 
I find, in short, by noting the position of the dark line in the 
quartz wedge, and by observing the number of the chromatic bands 
that appear between it and the thin end of the wedge, that it is 
possible to estimate the comparative thickness of the slice, when we 
are dealing with slices of the same mineral; or the strength, or 
feebleness, of the double-refraction possessed by a mineral when we 
have several sections of the same mineral in a slice of rock prepared 
for the microscope. 
The application of the above principle to the examination of thin 
slices of rocks is complicated by the fact that the sections of minerals 
contained in them vary not only in thickness (for few slices, micro- 
scopically considered, are exactly equal to each other in thickness), 
but also in the angle to an optic axis at which they are sliced. 
This difficulty, however, must be faced, for it is one that is not 
peculiar to the method described in these pages, but is common to all 
methods of estimating the double-refraction of minerals scattered 
promiscuously in rock-sections. 
1 The eye-piece of my microscope is furnished with two slots, one above the other, 
so I can either use two wedges at once or combine one wedge with an eye-piece 
micrometer, 
