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DR. A. E. H. TUTTON ON THE 
of the polariscope. On raising into position the cell containing the immersion liquid, 
cedar oil, of similar refractive index (n — 1'5102 for Na-light) to the mean index of 
the crystals ,* so that the crystal occupied the centre of the cell, and then rotating the 
goniometer circle (with the crystal-carrying axis rigidly fixed to it) first on one side 
and then on the other, the optic axial interference figure was seen, with the optic 
axes separated at their true obtuse angle, the more accurately true the more nearly 
identical the refractive indices of the liquid and the crystal. For the plane of the 
optic axes is the symmetry plane h {010}, which had been horizontally adjusted ; and 
the second median line, the obtuse bisectrix and axis a of the optical indicatrix as the 
sign of the double refraction is positive, is very nearly perpendicular to the basal 
plane c {001}, the plane of the crystal plate. 
Three different constants were then determinable:—(l) the exact position of the 
second median line with reference to the normal to the basal plane c {001}, which is 
the same as the normal to the glass mount-plate, for the circle reading for this normal 
could be directly observed ; (2) the true acute optic axial angle 2Y a , for this is the 
supplement of the true obtuse angle 2V 0 directly measurable, and is indeed itself 
directly measurable by further rotation of the circle; (3) the dispersion of the 
median lines. The course of these operations was as follows. 
The crystal plate was first adjusted so that the plane of the plate and of its mount- 
plate was approximately truly vertical, parallel to the goniometer axis, and the circle 
reading of the polariscopical goniometer was determined for the position when the 
plane of the plate was exactly normal to the axis of the polariscope, the crystal-holder 
and the steel axis carrying it and its adjustments being locked immovably to the 
circle axis. This adjustment of the plate to normality was rendered readily possible 
by use of the Becker transparent mirror-and-lens fitting, described on p. 34 and 
illustrated in fig. 13 of the author’s “Crystallography and Practical Crystal 
Measurement ” (Macmillan), the fitting being placed close up in front of the 
analysing Nicol of the polariscope. This combination, of a glass plate transparent 
mirror arranged at 45° and a lens fitted in an elbow side-tube, permitted the light 
from a goniometer electric lamp (provided with a copper cylindrical shade perforated 
by a suitable circular aperture and arranged to one side of the goniometer) to be 
reflected along the axis of the analysing tube of the polariscope to the glass mount- 
plate (the crystal being behind the plate during these operations), from which it was 
reflected back again to the Becker fitting, and passed through the transparent mirror 
and the small observing aperture in the front of the fitting to the observer’s eye. 
The images of the cross spider-lines of the analysing tube, a parallel pair of vertical 
lines and a single horizontal line, could be plainly seen with the aid of this fitting, 
* It was possible to forecast with considerable accuracy the optical constants of this salt from the 
general principles already established for the relations of the three alkali-metallic salts of each group. To 
be adequately certain as regards the refractive index, one of the two natural prisms was used for a 
determination of the intermediate index f3 before proceeding with the plates, as will be described later. 
