84 On a Possible Explanation of the Method employed by Nobert 
How are Nobert'a Lines of Varying Degrees of Fineness ruled 
on Glass ? 
First of all, tlie evidence seems quite clear that they are ruled 
with a diamond having a knife-edge. In all of the cases which I 
have examined the lines start in with a curvature, which is main- 
tained throughout the whole extent of the band. I have been 
able to produce this result only by setting the cutting edge of the 
diamond slightly inclined to the direction of the line ruled, and 
this inclination seems to give a decided improvement to the 
character of the lines. 
I assume that Nobert uses a prepared diamond, instead of a 
natural crystal. It is everywhere assumed by writers on the 
subject, that only the natural crystal possesses perfect cutting 
qualities. While this is probably true where a deep cut is wanted 
for the purpose of fracture, it does not seem to be true where 
distinct, smooth, and uniform lines are desired. I believe this is 
also the experience of Mr. Rutherford, who long ago abandoned the 
natural crystal, either unbroken, or broken into chance fragments. 
A circular point is objectionable for several reasons, mainly on 
account of its lack of durability. 
Starting with the theory that Nobert's lines are ruled with a 
highly polished knife-edge diamond, I had constructed from my 
own designs an apparatus for preparing diamonds in this way. 
The machine does not differ from the ordinary tool of the 
lapidary, except in two particulars ; but these are vital to success. 
It is well known that diamonds can be ground and perfectly 
polished only in the direction of the cleavage planes, of which there 
are twenty-four in every perfect stone. A skilful diamond-worker 
will locate the position of these planes by simple inspection. I 
found myself obliged to employ the more tedious, but not less sure, 
method of finding them by a tentative process. The machine was 
therefore so constructed that the direction of the cleavage planes 
could be detected after a few trials. 
Again, it is customary either to press the lap, on which the 
diamond dust is placed, up against the diamond, which is set in a 
rigid holder, or else to connect the holder to a rigid shaft by means 
of an intervening flat spring. In either case, the diamond is liable 
to crumble when it is reduced to a sharp edge. In the arrangement 
adopted, the holder containing the diamond is free in the direction 
of gravity only. This action is secured by two shafts set at right 
angles, and connected with the required supports by three universal 
joints. By weighting the horizontal arm or by lifting it with a 
spiral spring, the pressure can be regulated with great nicety. The 
lap has a circular movement, while the frame in which it rests has 
two motions in a horizontal plane, at right angles to each other. 
