Notes and Comments. 353 
THE ISOLATION OF THE DIRECTIONS-IMAGE 
Dr. J. W. Evans discussed the different methods by which 
the interference figures of a small mineral in a rock-slice may 
be kept distinct from those of adjoining minerals. He recom- 
mended two. In one, which he believes to be new, a dia- 
phragm with a small aperture is placed below the condenser, 
which is lowered till the image of the aperture appears in focus 
on the rock-slice. In some microscopes the iris diaphragm 
provided for the Becke method of determining the refractive 
index may be employed. In others it is too near the condenser. 
The aperture should be sufficiently large to illuminate the 
maximum area of the mineral under investigation, but no 
portion of the others. The directions-image may then be 
observed in any of the usual ways. Unless the condenser and 
diaphragm revolve with the stage the aperture must be very 
carefully centred with the axis of rotation. 
OF A MINERAL IN A ROCK-SLICE. 
The other method was proposed by Becke in 1895, but is 
very little known. The diaphragm is placed in the focus of 
the eye-piece so as to shut out all except the mineral selected. 
The Becke lens, or system of lenses resembling an eye-piece, is 
placed above the eye-piece, when the directions-image of the 
mineral will be seen without any admixture of light from its 
neighbours. This method has the advantage that the dia- 
phragm is less highly magnified at the time of adjustment. 
When a rotating stage is employed, a very accurate centring of 
the nose-piece of the microscope is required, so that the co- 
incidence of the object with the aperture may be maintained. 
The common practice of placing a diaphragm for this purpose 
immediately below the Bertrand lens rests on no scientific 
basis, and is not effective in shutting out the ight of minerals 
other than that which is being studied. 
NORWEGIAN GRANITE. 
A rather interesting example of the value of the interchange 
of ideas on the heterogeneous mass of papers read at a meeting 
of the British Association arose in the Geological Section. 
There Professor R. C. Wallace described some of the salt pans 
of Manitoba, in north-west Canada, and showed that the salt 
had a peculiar disintegrating action upon certain volcanic 
rocks which were found as boulders in their neighbourhood. 
Professor Fearnsides imparted a topical interest by connecting 
the observations with a phase of the Tariff Reform agitation of 
a few years ago. At one time a great outcry was made against 
the importation of Norwegian granite for the building of Govern- 
ment dock walls. During this controversy an engineer de- 
clared that even if the price was equal, the Norwegian granite 
was to be preferred as the more durable. Professor Fearnsides 
1915 Nov. 1. 
