July 26, 1912] 



SCIENCE 



99 



One room in the house was kept tidy for 

 pupils, and the rest of the 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 tele- 

 scopes, 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 mir- 

 rors are not specified by the diameter of 

 the reflecting surface, but by the focal 

 length. Thus, whatever may be the diam- 

 eter of the reflecting surface, a 20-foot 

 telescope means that the mirror is approxi- 

 mately 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 mak- 

 ing of reflectors it is necessary to go to 

 original memoirs^ on the subject, and even 

 of them there are not many. I feel, there- 

 fore, 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 specu- 

 lum 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 ac- 

 count of the process of which I know is 

 contained in a paper by Professor G. "W. 



' Sir Howard Grnbb 's lecture at the Eoyal Insti- 

 tution in 1887 is one of these, Vol. XI., p. 413. 

 Lord Rosse's papers are amongst the most im- 

 portant. 



Eitchey in Vol. 34 (1904) of the Smith- 

 sonian Contributions to Knowledge. He 

 there gives a full description of the great 

 reflector of the Terkes 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 opera- 

 tions 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 grinding 

 of a shallow depression on one of the faces 

 by means of a leaden tool and coarse emery 

 powder. 



It will be well to begin by an account of 

 the manufacture of the tools wherewith 

 the finer grinding and polishing is effected, 

 and then I shall pass on to a short descrip- 

 tion of the way they are used. 



Two blocks of iron are cast with the de- 

 sired radius of curvature, the one being 

 concave and the other convex. The cast- 

 ings are then turned so that the concavity, 

 and convexity fit together as nearly as may 

 be. For the large mirror these blocks are 

 a little over 2 feet 6 inches in diameter, 

 but for small ones they are made of the 

 same 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 fits 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 sur- 

 face 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 



