Oblique Vision of Surface Structure. By F. E. Wenham. 159 
These prismatic mounting slips may be cheaply and easily made 
by any ordinary glass-grinder, in the following manner : Thin 
well-polished sheet glass (such as is used for microscope slides) is 
cut into pieces, three-quarters of an inch long by four-tenths wide. 
Eight or ten of these are cemented together with hard Canada 
balsam, and their step-like projecting edges adjusted against a bevel, 
set at the desired angle, say 35°, as shown by Fig. 4. They must 
be pressed in very close contact, as it is important to have the 
edges worked fine and clean, and any thickness of balsam stratum 
between them will prevent this, for during the work close edges 
mutually protect each other. Having got a sufficient number of 
blocks together in this way, any quantity of them are cemented on 
to a runner or metal plate, as in Fig. 5, with the usual cement of 
pitch and wood ashes. They are then ground and smoothed on a flat 
metal lap, till all the steps are gone and keen edges shown, and are 
then finally polished. Thus hundreds may be made at a time. 
Discussion as to the effects of this suggestion of mounting for 
oblique vision, and its probable value, is at present premature. 
The author has done but little more than enunciate the optical 
principle. It may, however, be stated that the object selected should 
either lie in a parallel direction with the faces of the incline, or at 
right angles to it. If it occupies an intermediate position, rather 
a curious confusion of cross striation results. Oblique mounting 
affords a very marked means of discriminating whether the ribbings 
are on the upper or lower side of the scale, two of which adjoining 
sometimes showing the series in one very distinctly, and in the 
other confused, simply because they are relatively inverted. 
