Major-General McMahon —Double-Refraction of Minerals. 549 
ing the strength of the double-refraction of minerals is to compare 
the tint of the mineral with the corresponding tint in one of the 
chromatic bands in the wedge. The eye of the observer may thus 
be trained to determine whether the tint of the mineral under obser- 
vation belongs to the first, second, or higher order of Newton’s 
scale. This method is based on the rule that the stronger the double- 
refraction of a mineral, the higher will be the order of the tint exhibited 
by it when sections of different minerals cut in slices of uniform 
thickness and at the same angle to an optic axis are examined in 
transmitted light under a microscope. 
In working this method I employ a quartz wedge specially made 
for the purpose,! which is similar to the ordinary wedge in all 
respects but one; namely, my wedge only occupies half the depth 
of the slot, so that the observer is able to directly compare the tint 
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Fie. 1.—Diagram of quartz wedge used by the author. 
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of a mineral, say at d (Fig. 1), with the spectra seen in the wedge 
a-b-c, and so determine satisfactorily whether the observed tint 
belongs to the Ist, 2nd, or higher order of Newton’s scale. 
The method, however, which I desire to describe in the following 
pages differs from the above. It depends not on the order of the 
tint exhibited by the mineral under observation, but on the character 
of the phenomena produced in the quartz wedge by the passage of 
light through the mineral to the quartz. 
Having fixed our microscope in the manner above described, viz. 
with a quartz wedge inserted in the eye-piece at an angle of 45° to 
the plane of polarization, let us examine a second quartz wedge 
placed on the stage of the microscope in a position at right angles 
to the axis of the wedge in the eye-piece. In this position the 
velocity of the extraordinary ray is retarded in one of the two plates 
of quartz and accelerated in the other; and, at the point where the 
velocity of the extraordinary ray, on emergence from the upper quartz 
wedge, becomes the same as that of the ordinary ray, there is no 
double-refraction and a dark line appears. 
If we examine the thin edge of the quartz on the stage of the 
microscope with the wedge in the eye-piece, the black line will be 
seen near the thin edge of the analyzing wedge, with spectra of the 
Ist, 2nd, 3rd, and higher orders rising in succession beyond it 
towards the thick end of the wedge. If we allow the analyzing 
quartz wedge in the eye-piece to remain stationary, and move the 
wedge on the stage of the microscope so that thicker and thicker 
portions of the quartz are successively brought within the range of 
1 The wedge should be a tolerably flat one so as to give a wide spread of colour. 
