15 



obtained by melting the necessary constituents M a suitable 

 container and the liquid mass is withdrawn from time to 

 time by gathering or pouring, and brought by appropriate 

 methods of working, casting, rolling, etc., to the required 

 shape. 



Optical glass, on the other hand, is left, when melted and 

 refined, to solidify in the pot. The block that results, or 

 fragments of it, are reduced by reheating into forms suitable 

 for the completing work of the opticians. The manufacture 

 is of great nicety and difficulty, demanding the utmost care 

 and patience. It was first undertaken in England some 

 70 years ago by Messrs. Chance Brothers & Co., of Birm- 

 ingham, and, until quite recently, it has remained in the 

 hands of them alone in the United Kingdom. 



The function of lenses and prisms is to refract or reflect 

 the rays of light that traverse them. In order that they 

 may perform this function perfectly, the glass must be 

 uniform and homogeneous, so that the rays are not distorted 

 in their passage. 



In the early days of the telescope, very thin lenses were 

 used, and for this purpose selected sheet glass was sufficiently 

 satisfactory. On account of the chromatic effects of single 

 object-glasses, the refractor became completely displaced by 

 the reflector for large astronomical instruments, and it was 

 not until Moor Hall in 1724, and Dollond independently in 

 1757, invented the achromatic objective that the manu- 

 facture of large refracting telescopes became possible. A 

 demand then arose for homogeneous pieces of glass of much 

 larger size than had previously been required. 



The method by which such glass could be produced was 

 discovered through the work of a Swiss joiner, Pierre Louis 

 Guinand, at the end of the eighteenth century. Guinand 

 found that he could equalise the density of his molten glass 

 throughout its mass, and so attain his end, by long-continued 

 stirring. After slow cooling of the whole, he broke the pot 

 away and obtained his block of glass if not completely homo- 

 geneous, at least homogeneous in parts that could be selected. 



He himself failed to render his process reliable. Working 

 with scientific men at Munich, among them the celebrated 

 Fraunhofer, he was able to produce from time to time discs 

 suitable for working into object-glasses, one or two as much 

 as 13 or 15 inches in diameter. But, for their regular needs, 

 opticians for many years still had to depend on the rare 

 discovery of suitable fragments of ordinary glass. 



Faraday stated in his Bakerian lecture of 1829, in which 



