248 PROCEEDINGS OP THE AMERICAN ACADEMY 



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 uni- 

 form 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 posi- 

 tion 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 interven- 

 ing 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 onJij. This 

 action is secured by two shafts set at right angles, and connected with 

 the required supports by three universal joints. B\' 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. In order to give a motion in revolution to 

 the holder, for the purpose of grinding circular points, a Hook joint is 

 used to connect it with a driving pulley. 



It may be proper at this point to offer a few observations, derived 

 from experience, on the kind and quality of gla^s best suited to receive 

 delicate lines. I have previously made some remarks before the 

 Academy on what, for the want of a better term, was described as the 



