188G.] on Telescopic Objectives and Mirrors. 417 



form, and are made of either brass or cast iron. I prefer cast iron, 

 except for very small sizes. They are grooved on the face, in the 

 manner suggested by the late Mr. A. Eoss, in order to allow the 

 grinding material to properly distribute itself. 



If two spherical surfaces be rubbed together they will, as may be 

 supposed, tend to keep spherical ; for the spherical is the only curve 

 which is the same radius every part of its surface. If fine dry 

 abrading powder be used between, the same result will be obtained ; 

 but if wet powder be used, the surface will no longer continue 

 spherical, but will abrade away more on the centre and edge than in 

 the zone between. It was to meet this difficulty that the late Mr."^A. 

 Ross devised the idea of the distributing grooves. The fine grinding 

 process is the first of the series which calls for any skill on the part 

 of the operator. 



That the modus operandi of the grinding be the more easily under- 

 stood, let me explain the principle of the process in a few words. 



When two surfaces of unequal hardness are rubbed together with 

 emery powder and water between the two, each little particle of the 

 powder is at any given moment in either of these conditions : — 

 (a) Imbedded into the softer surface ; (b) Rolling between the two 

 surfaces ; (c) Sliding between the two surfaces. 



Those particles which become imbedded in the softer material 

 do no work in abrading it, and but little in abrading the harder. 

 They generally consist of the finer particles, and are kept out of 

 action by the coarser which are rolling or sliding between the surfaces. 

 Further, those that are purely rolling do little or no work. The 

 greater part of the work is performed by those particles which are 

 facetted and which slide between the two surfaces. 



As the grinder is always of a much softer material than the 

 glass, there is much more friction between the grinder and these par- 

 ticles than between the glass and the same particles, and therefore 

 they partially adhere to the grinder and are carried by it across the 

 face of the glass. This being so, it is now easy to jDerceive w^hat 

 the best conditions for rapid grinding are. Not too little emery, 

 for then there will not be enough of abrading particles ; not too 

 much, for then the particles will roll on each other and tend to 

 crush and disintegrate each other instead of abrading the glass, 

 but just sufficient to form a single layer of particles between the 

 grinder and the glass surface. 



In the grinding of the small lenses, I mean up to five or six inches 

 diameter, it is usual to carry out the entire grinding processes by hand ; 

 above that size by machinery. Surfaces up to 12 or even 15 inches 

 can be ground by hand ; but the labour becomes severe, and for my 

 part I am gradually reducing the size for which the hand grinding 

 is used, as I find the machine work more constant in its eftects. 



The machinery used is the same as that emjDloyed for the polishing 

 operation, and I shall describe it under that head further on. 



In the fine grinding operation by hand, the glass is usually 



2 E 2 



