186 PROCEEDINGS OF SECTION A. 
in that city, neither of which was of any value, the cutting 
edges being much too blunt for fine work. My own method 
of preparation is to carefully break the stones so as to ensure 
fracture parallel with some of the numerous cleavage planes. 
The fragments so obtained are examined under the microscope 
as to the perfection or otherwise of the angles or edges and 
faces forming them, the promising pieces being put aside for 
trial. Good results have also been obtained with stones 
upon which Jarge facets had been ground on the outer, or 
natural face and afterwards broken so that one face of the 
knife edge was artificially formed, while the other followed the 
line of cleavage. Excellent cutting angles have been obtained, 
too, in the case of stones, one face of which forms the outer 
coating, or skin as it is termed, of the uncut gem. 
I always set or mount the diamond so that its cutting edge is 
perfectly parallel with the line to be cut, and slightly raised in 
the direction in which it is to travel. This is contrary to what 
one would expect, comparing the action of a diamond with a 
steel graver or other cutting instrument for like purposes, but 
when it is remembered that the faces, the junction of which 
from the cutting edge wear more rapidly than the edge itself, 
one sees the analogy no longer holds good. In the setting and 
adjustment of the diamonds it is important to remember that, 
in the case of test rulings at any rate, the lines after being 
ruled must on no account be rubbed or polished, consequently, 
the material removed must be deposited on one side or the other 
of the groove formed, and this involves the utmost nicety of ad- 
justment of the cutting edge, and not infrequently is a consider- 
able tax upon one’s time and patience. The finer the ruling, the 
ereater is the importance to be attached to this particular fea- 
ture. The J/ength of the cutting edge is also of moment. The 
longer the edge within certain limits, soon ascertained by ex- 
perience, and providing it is perfectly straight, the longer will 
it endure, but as depth and breadth of line are important fac- 
tors, too long an edge implies too great a pressure strain to 
produce a line of given depth and width. The pressure upon 
the diamond to produce a line of a certain depth and breadth, 
I apply, in the case of micrometric rulings, by means of a 
spring controlled by a screw; this gives good results up to a 
rate of 20,000 lines per inch, but beyond this the friction 
involved is detrimental. The variation of pressure requisite 
in test plate ruling is obtained by means of a series of weights, 
ranging from 20 grammes or more down to a fraction of 1 
eramme. 
In the matter of spacing, it is of the utmost importance that 
a correct standard should be obtained as a basis for all micro- 
metric measurements. At the outset, I obtained copies of por- 
