190 Dr. W. Watson on the Construction and 



disk at the rate of about five turns a minute a piece of flint 

 is pressed firmly against the edge. In this way the diamond 

 particles are driven into the iron and, at any rate after an 

 attempt or two, the disk will be satisfactorily armed. A 

 horizontal plate with a guide at one edge is attached to the 

 slide-rest of the lathe and the wooden base to which the 

 silica is attached is placed on this plate, and the silica pressed 

 firmly against the edge of the disk which is rapidly rotated. 

 Soap and water must be used as a lubricant, being supplied 

 by means of a brush. In this way a disk having a diameter 

 of one centimetre can be cut in a minute. 



A small rod of fused silica is then fused to the edge of the 

 disk by means of a small oxyhydrogen flame, and all but the 

 last two or three millimetres cut ofl", so as to leave a small 

 tab of silica. The disk is then roughly ground flat on either 

 side by rubbing it on a flat plate of brass freely supplied with 

 powdered carborundum (of such a size as to pass through a 

 sieve of 250 threads to the inch) and water. The disk is 

 then annealed by being heated to a bright red in a small 

 scoop of platinum or thin sheet-iron over a Bunsen flame for 

 about 5 hours. That surface of the disk which_, on inspection, 

 appears most free from bubbles is ground on a plate of thick 

 plate-glass with water and emery of gradually increasing 

 fineness. The disk is moved round and round in circular 

 sweeps^ the end of the finger pressing it lightly down on the 

 surface of the glass. About ten minutes^ grinding with each 

 of three grades of washed emery, finishing with the finest 

 which can be obtained, will generally be sufficient. The 

 grades of emery I use are such that the emery is deposited 

 from water in the following times : — No. 1 settles in 1 

 minute, No. 2 settles in 10 minutes, No. 3 settles in between 

 20 minutes and 60 minutes. 



The surface is polished with rouge on a pitch form. It is 

 essential to obtain opticians' rouge, which has been well 

 washed, and not jewellers^ rouge. The proper quality of 

 rouge I have obtained from Messrs. Cooke of York. To 

 form the pitch surface some pitch is melted, and when 

 thoroughly liquid is poured over the surface of a piece of 

 plate-glass and a second piece of plate-glass, the surface of 

 which has been thinly coated with dilute glycerine, is pressed 

 down on the top of the pitch. Weights are placed on the 

 upper glass till the pitch is cold, when the glass can be slid 

 off*. The polishing is continued till on examining the surface 

 with a low-power microscope no pitting can be seen. Quite 

 a light pressure of the tip of the finger must be used during 

 the polishing, which will take from 10 to 30 minutes. 



