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418 Proceedings of the British Association 



data of the problem. Under these circumstances, he perceived that 

 there was no resource but to improve* metalHc plane reflectors as much 

 as possible. Now, it was well known that in reflexion by silver much ||| 



less light was lost than by any of the other'metals,— but, unfortunately, 

 this metal was so soft, that great difficulties presented themselves in 

 giving it the requisite degree of high polish. He had tried by the elec- 

 trotype process to procure a surface with a high polish by depositing a 

 silver on a surface of speculum metal, and treating it by the same pro- 

 cess as that used by the distinguished officers engaged in the Trigono- 

 metrical Survey of Ireland- But, unfortunately, he soon found that, 

 use what precaution he would, either there took place an adherence of 

 the deposited silver to the surface on which it had been deposited, or 

 the polish was rendered imperfect by the means resorted to to prevent 

 this. He tried copper similarly, which did not adhere, but produced a 

 high degree of polish, — but of course its color and other properties 

 rendered it inadmissible as a reflector. He then determined to endeav- 

 or to grind and polish a plane surface of silver, the softness of that 

 metal having heretofore, however, caused the attempt to fail in the 

 hands of the most experienced who had tried it. The processes of 

 grinding and polishing are essentially different: in grinding, the sub- 

 stance, whether emery or other powder, must run loose between the 

 substance which is used to rub it against the other and that which is 

 to be ground ; and he soon found that he could not use emery or any 

 other grinding powder for bringing a surface of silver to a correct form, ^ 



for, from the softness of the metal and the unequal hardness of its 

 parts, the emery was found to confine its action to the softer parts, 

 leaving the harder portions in elevated rido-es and prominences, some- 

 thing in the way that the iron handle of a pump which has been long 

 and much used may be observed to be worn away. Hard steel he 

 found he could bring to a very true surface, and even impart to it a 



high degree of polish ; but the quantity of light it was capable of re- 

 flecting was by no means sufficient, nor could he succeed in imparting 

 to the surface of silver by compression with highly-polished steel sur- 

 faces the evenly and highly-polished surface requisite for his purpose. 

 At length, he found that he could by the use of good German hones 

 grind surfaces of silver perfectly true, and he had now no doubt that 

 he could with safety recommend for that purpose as the best material 

 the blue variety of the German hone- 



The next point was, to polish the surface to a true optical plane re- 

 flecting surface. This was by no means so easy a task as may be 

 supposed : — for although our eminent silversmiths do produce surfaces 

 of silver of an extremely brilliant polish, as in the magnificent plateaux 

 and other articles which they turn out, yet if any one will take the 

 trouble to examine these surfaces, they will be found to be so irregular 

 though highly polished, as to be entirely unfit for producing correct 

 images by reflexion. And it is a singular fact that, although in the first 

 part of the process of polishing, chamois leather of the finest kind was 

 used to rub the rouge on the silver surface, yet the finer finishing pol- 

 ish had always to be communicated by the human hand. Nor would 

 the hand of every individual answer: — the manufacturer had to select 

 those with the very softest and finest grain, — nor would the hand of 



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