548 Major-General McMahon—Double-Refraction of Minerals. 
importance.! I may add that snow-beds appear to have, comparatively 
speaking, a protecting influence. Still, snow-fields and glaciers in 
the upper part of a range act as storehouses for the water, and so 
keep the torrents always full; the former also give rise to endless 
streams which have a plunging action, and form precipitous cliffs, 
as may be seen in cirques; the latter increase the erosive power of 
torrents by rendering their waters muddy. I believe, therefore, that, 
wherever there is unprotected ground, the process of excavation is 
aided and accelerated by the existence of snow-fields and glaciers 
in the upper parts of the ranges. Hence I should infer that the 
sculpture of the more elevated regions progressed most rapidly when 
that of snow and ice was least extended. 
V.—On a Mone or Usine tHE Quartz WepcE For HstTIMATING 
THE STRENGTH OF THE DousBLE-REFRACTON oF MINERALS IN 
Turn Stices oF Rock. 
By Major-General C. A. McManon, F.G.S. 
N some cases it is of practical importance to petrologists to 
estimate the strength of the double-refraction exhibited by 
doubly-refracting minerals in thin slices of rock under the microscope. 
Babinet’s compensator, a description of which will be found in 
Rosenbusch’s Microskopische Physiographie der Mineralien und Ge- 
steine, affords a means of accurately calculating the refractive indices 
of the rays into which light is divided in its passage through doubly- 
refracting crystals. Michael-Lévy also devised another exact method 
based on observations made on the tints presented by slices of doubly- 
refracting minerals, an account of which appeared in the Bulletin de 
la Société Mineralogique de France, and which is discusige in a work 
recently published by Michael-Lévy and Lacroix. bay 
As these exact methods of calculating the double-refraction of 
crystals require special apparatus and are somewhat complicated, it 
may be worth while to describe a rough and ready method based on 
the use of the quartz wedge, which I have employed for some years, 
and which has yielded me useful results. 
In order to explain this mode of using the wedge, I will suppose, 
in the first place, that we have our microscope arranged on our table 
with crossed Nicols, and that we have inserted a quartz wedge, made 
in the usual way (which is too well known to need description), in 
a slot in the eye-piece at an angle of 45° to the plane of polarization. 
If we look through the microscope with the apparatus fixed in this 
position, and before any object has been placed on the stage, a series 
of chromatic bands will be observed in the quartz wedge, each band 
consisting of a spectrum of colours in an ascending order; the colours 
of the first order of Newton’s scale being the nearest to the thin 
edge of the wedge. The width of these bands depends on the 
thickness of the quartz and on the slope of the wedge, being broader 
in a flat wedge than in one cut at a considerable angle. 
The usual way of employing the wedge for the purpose of estimat- 
1 Discussed in a part of the lecture not included in this extract. 
