COREESPONDENCE. 291 



scopists attaining this very desirable result. The introduction already 

 partially accomijlished, of a grading of the eye-j)ieces by comparison 

 with equivalent single lenses, 2 in., 1 in., ^ in., &c., may render this 

 part of the subject, which seemed almost unattainable, the easiest and 

 first to be accomplished. 



COEEESPONDENCE. 



Grinding Diamond Points. 



To the Editor of the ' Monthly Microscopical Juariiul.^ 

 Sir, — Mr. Wenham having been asked the best method of grinding 

 diamond points for fine ruling, or writing on glass, kindly gave the 

 following valuable information, with permission to publish it. 



Yours faithfully, 



Henry J. Slack. 



" Many years ago I tried some very fine-line ruling by a very 

 beautiful little machine, made by the celebrated Tully. At first I 

 used fragments and splinters of diamond, but found that I could not 

 do any good with them, after working till my patience was exhausted. 

 No two pieces acted alike : some with the lightest pressure that it was 

 possible to give tore and shattered up the sm-face of the glass, breaking 

 all the lines into a mass. Other j)ieces that required more pressure 

 gave irregular lines. A piece might be found that would suit for 

 ruling lines of one degree of fineness, but would not do for others. 

 I therefore had to give up broken splinters as impracticable, and use 

 turned points. 



" A fragment of diamond was imbedded in a short piece of copper 

 wire, -^ in diameter, in the way described in my paper ' On the Con- 

 struction of Object-glasses.' This was chucked in the bow lathe or 

 ' jigger,' and another splinter of diamond, similarly mounted, was held 

 against it as a turning tool. Both were, I suppose, about equally 

 ground away, and you could see the dust flying off ; in fact, diamonds 

 rubbed together abrade each other just like two pieces of slate jjencil 

 will do. It is very easy with a delicate touch at last to bring the 

 rotating diamond to a point as fine as a needle. TJiis is the right 

 thing for glass ruling, and I have no doubt that Nobert uses the same. 



"In Peter's writing machine turned points are employed, as these 

 only will mark in every direction. At fii-st he used to buy his turned 

 points from the diamond workers at one guinea each, and few of them 

 good even at that. I explained my way of turning the points, at 

 which he succeeded at the first attemjit, and ever after that made them 

 with his own hands. He told me afterwards that what before cost 

 him 21s. did not now cost him Is. He did not, however, momit them 

 quite in my way, but thus : he split the end of the wire with a fine 

 saw, then closed the split end on the fragment of diamond with pliers, 

 so as to nip it fast ; then wound the end round with a few coils of 



VOL. VI. Y 



