1886.] on Telescopic Objectives and Mirrors. 421 



Polishing. 



The polishing process can be, and is often, conducted precisely 

 in the same manner as the grinding, except that the abrading powders 

 used (oxide of iron, rouge, an oxide of tin putty-powder) is of a finer 

 and softer descrijDtion, and the surface of the polishing tool is made 

 of a softer material than the metallic grinder. 



Very nearly all my polishing is done on the machine I shall pre- 

 sently describe ; but before doing so, I will, with your permission, 

 say a few words on the general principles of the polishing process. 

 Various substances are used for the face of the polisher — fine cloth, 

 satin or paper, and pitch. Pitch possesses two important qualities 

 which render it peculiarly suitable for this work, and it is a curious 

 fact that we owe its application for this purpose to the extraordinary 

 perspicuity of Sir Isaac Newton, who we may fairly say was the first to 

 produce an optically perfect surface, and that that material is not 

 only still used for the purpose, but is as far as I know the only sub- 

 stance which possesses the peculiar qualifications necessary to fulfil 

 the required conditions. "With skill and care, moderately good sur- 

 faces can be obtained from cloth polishers ; but it is easy to see why 

 they can never be perfect. There is a certain amount of elasticity in 

 cloth and in its " nap," and there is consequently a tendency to 

 round off the surfaces of the pits left by the grinding powder, and to 

 polish the bottom or floor of these pits at the same time as the upper 

 surface. It is easy to show mathematically that any process which 

 abrades the floors of the pits at the same time as general surfaces even in 

 a very much less degree, can never produce more than an approximation 

 to a perfect surface, and practice agrees with the theory. Paper is said 

 to be much used by the French opticians. I can say nothing about it. 

 I have tried it and failed to produce a perfect surface with it, nor 

 indeed should I expect it. It is of course open to the same objection 

 as cloth. Pitch possesses, as I said, two most important qualities 

 which render it suitable for the work ; the first, in its almost perfect 

 inelasticity ; the second, a curious quality of subsidence, which we 

 utilise in the process. 



If we watch with a microscope, or even a magnifier, the character 

 of two surfaces during the process of polishing, the one with cloth, 

 and the other with pitch, the difference is very striking. With the 

 cloth polisher, the polish appears much quicker, and it would at 

 first sight appear as if the same polishing powder abraded more 

 quickly on the cloth than on the pitch polisher, but I do not believe 

 that such is the case, for if we look at the surface with a magnifier 

 we shall find that while all the surface has assumed a polished appear- 

 ance, the surface itself has retained some of the form of the original 

 pitted character with the edges rounded off ; while in the pitch half- 

 polished surfaces, the floors of the pits are as grey as ever, and the 

 edges are sharp and decisive. In pitch polishing, too, a decided 



