452 Sir George H. Darwin [April 26, 



house, including the bedrooms, was a litter of lathes and polishing 

 apparatus. He made reflecting telescopes not only for his own use, 

 but also for sale, for the purpose of providing funds to enable him to 

 continue his researches. His industry must have been superhuman, 

 for later in his life he records that he had made over 400 mirrors for 

 Newtonian telescopes, besides others of the Gregorian type. These 

 mirrors ranged in diameter from a few inches to 4 feet, in the case 

 of the great 40-foot telescope. I should say that mirrors are not 

 specified by the diameter of the reflecting surface, but by the focal 

 length. Thus, whatever may be the diameter of the reflecting sur- 

 face, a 20-foot telescope means that the mirror is approximately portion 

 of a sphere of 40 feet in radius, and this will give a focal length of 

 20 feet. You must, in fact, double the focal length of a telescope to 

 find the radius of the sphere of which it forms a small part. 



In order to learn anything of the making of reflectors it is neces- 

 sary to go to original memoirs * on the subject, and even of them 

 there are not many. I feel, therefore, that I shall not be speaking 

 on a topic known to many of the audience if I make a digression on 

 a singularly fascinating art. Mirrors are now made of glass with a 

 reflecting surface of chemically deposited silver : formerly they were 

 made of speculum metal, an alloy of copper and tin. Of whatever 

 substance the mirror is made the process of working it to the required 

 form is much the same. The most complete account of the process 

 of which I know is contained in a paper by Professor G. W. Kitchey 

 in Vol. xxxiv. (1904) of the Smithsonian Contributions to Knowledge. 

 He there gives a full description of the great reflector of the Yerkes 

 Observatory. The process only differs from that employed by Herschel 

 in that he worked by hand, whereas machinery is now required to 

 manipulate the heavy weight of the tools. The Yerkes mirror is 

 formed of a glass disk 5 feet in diameter, and it weighs a ton ; the 

 grinding tools are also very heavy. 



I must pass over the preliminary operations whereby the rough 

 disk of St. Gobain glass was reduced to a true cylindrical form, smooth 

 on both faces and round at the edge. Nor will I describe the grind- 

 ing of a shallow depression on one of the faces by means of a leaden 

 tool and course emery powder. 



It will be well to begin by an account of the manufacture of the 

 tools wherewith tlie finer grinding and polishing is effected, and then 

 I shall pass on to a short description of the way they are used. 



Two blocks of iron are cast with the desired radius of curvature, 

 the one being concave and the other convex. The castings are then 

 turned so that the concavity and convexity fit together as nearly as 

 may be. For the large mu-ror these blocks are a little over 2 feet 

 6 inches in diameter, but for small ones they are made of the same 



* Sir Howard Grubb's lecture at the R.I. in 1887 is one of these, vol. xi. 

 p. 413. Lord Rosse's papers are amongst the most important. 



