1912] 0)1 Sir WnUam Herschel 453 



diameter as the mirror to be ground. The two are then ground 

 together for a long time with emery powder and water until every 

 part of one surface tits truly to every part of the other. They must 

 then both be portions of a sphere of the same radius, because the 

 sphere is the only surface in which a universal fit is possible. The 

 concave iron is very precious because it furnishes the standard for 

 regrinding the convex grinding tools when they have become worn by 

 use. In order to make a plane mirror, three surfaces are ground two 

 and two. for if A tits B and C, and B tits C all over each surface they 

 must all be true planes. However, I shall only speak of the tiguring 

 of concave mirrors. 



The roughly hollowed glass disk is now laid on several layers of 

 Brussels carpet centrally on a massive horizontal turn-table. The 

 convex iron tool just described is suspended by a universal joint from 

 a lever, and it is counterpoised so that only a portion of the weight of 

 tiie tool will rest on the glass when it is in use. A complicated 

 system of cranks and levers is so arranged that the tool can be driven 

 by machinery to describe loops or curves of any arbitrarily chosen 

 size over the glass, and as these loops are described by the tool the 

 turn-table turns round slowly. In this way every part of the tool is 

 brought into contact with every part of the glass disk in a systematic 

 way. When working near the edge a large part of the tool projects 

 beyond the edge of the glass. 



Emery powder and water are supplied in a way I need not 

 describe, and the tool is lowered gently on to the glass. The motive 

 power is then applied, and the grinding is continued for many hours 

 until the preliminary rough depression has been hollowed to nearly 

 the desired shape — namely, that of the standard concave iron. 



For finer grinding a change of procedure is now adopted, and 

 very finely powdered emery is used. Another convex tool is formed, 

 by grinding with the standard concavity ; the working face of the 

 tool is, however, now cut up into small squares by a criss-cross of 

 narrow and shallow channels. Such channels are found to be 

 necessary in order to secure an even distribution of the emery and 

 water all over the surface. The grooved tool is now used for many 

 hours, and the surface is tested at frequent intervals with a sphero- 

 meter. The work ceases when it is no longer possible to detect 

 errors of curvature in this way. 



The next stage is polishing. The thickness of the layer of glass 

 worn off in polishing is to be estimated in ten-thousandths of an inch, 

 and can scarcely be detected even with the finest spherometer. For 

 polishing the iron tool is discarded and the work is carried on by 

 hand. As lightness is essential, the tool is built up by a stiff lattice- 

 work of wood with a continuous wooden working face. It is obvious 

 that however carefully the face may be turned it cannot be made 

 sufficiently true, and the requisite accuracy is obtained by means of 

 the plastic properties of rosin or pitch. A number of squares of 



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