March 27, 1919] 



NATURE 



6^ 



OPTICAL GLASS. 



/^NE of the surprises of the g^reat war has been 

 ^^^ the revelation to the majority of people of 

 the extent to which a nation may be absolutely 

 ■dependent for the conduct of the war on an in- 

 dustry which, in its magnitude, may seem quite 

 insignificant, but, owing to the technical experi- 

 ence underlying it, cannot be acquired in a short 

 space of time. In the forefront of such vital indus- 

 tries is the manufacture of optical glass. However 

 great the other resources in men and material may 

 be. It would be quite impossible to wage successful 

 warfare without adequate supplies of optical glass 

 for binocular field-glasses, range-finders, artillery 

 eights, photc^raphic lenses for aircraft, etc. It 

 is of interest to trace briefly the history and 

 practice of this important " key " industry, and 

 to consider how the nation will 

 be situated as regards the supply 

 of optical glass when peace is 

 restored and it will again be in 

 ■competition with German glass. 

 The manufacture of homogene- 

 ous glass suitable for optical in- 

 struments dates practically from 

 the time when the achromatisa- 

 tion of lenses became possible. 

 Up to that time selected pieces 

 of sheet glass served for the 

 manufacture of the crude optical 

 appliances then in use. Switzer- 

 land was originally the home of 

 the optical glass industry, when, 

 at the beginning of last century, 

 its manufacture was undertaken 

 by Guinand, who discovered hew 

 to make glass homogeneous by 

 stirring. For a short time the 

 production of optical glass was 

 carried on by Fraunhofer at 

 Munich, and, about the middle of 

 last century, Messrs. Chance 

 Brothers, of Birmingham, with 

 the assistance of M. Bontemps, 

 who had worked in France with 

 Guinand 's eldest son, commenced 

 the manufacture of this material. Methods of 

 manufacture have not materially changed since 

 that date, but more uniform results are now ob- 

 tained, in what was formerly a very hazardous 

 process, by the use of gas furnaces, careful tem- 

 perature control, and attention to detail in 

 •every direction. 



Optical glass is made by melting the necessary 

 ingredients at a very high temperature in clay 

 crucibles or pots. When the mass has fused to 

 a clear liquid free from bubbles, the molten glass 

 is stirred for a long time by a thick fireclay rod. 

 The viscosity of the liquid makes perfect admixture 

 by eddy currents and diffusion a slow process, 

 and all the time the glass is exerting a corrosive 

 action on the sides of the pot and stirrer, and the 

 products of this action tend to contaminate the 

 grJass and produce striae. 



NO. 2578, VOL. 103] 



When It IS deemed desirable to discontinue the 

 stirring process, the pot is allowed to codi. When 

 cold, the glass in the pot is found to be much 

 fractured, and falls to pieces when the pot 

 is "broken down." The fragments of glass 

 are heated until they soften, and then reduced 

 to suitable shape by moulding. After being 

 ground and polished, the lumps are exam- 

 ined for striae, and perhaps one-quarter of the 

 mass may be found to be ojF good optical quality. 

 When optical glass is required in the form of 

 discs or prism blocks, selected plates are brought 

 to the required form by a second moulding 

 process. 



The discs, etc., must then be annealed, or very 

 slowly cooled, so that the stresses which would 

 be set up in the glass by rapid cooling may be 

 reduced to a minimum and have no harmful effect 



V^u *''''S\'" '*!= manufacture of optical glass. On the ritrlit-hand side Is a pot or crucible in 

 which glass has bfien fount^ed which has been allowed to cool and is now ready to be broken down. 

 On the left hand will be seen a pot of glass which has been partially broken down. 



on the performance of the optical component into 

 which it is fashioned by the optician. 



The production of good yields of high-quality 

 glass, even of the oldest crown and flint types, is 

 an operation which requires skill and long experi- 

 ence. Apart from this, the present very varied 

 requirements of opticians necessitate the produc- 

 tion of varieties of glass which are widely different 

 in composition, and the manufacture of glasses 

 having the optical constants requisite to meet the 

 needs of the lens computer calls for expert 

 scientific assistance. 



During the first half of last century the require- 

 ments of opticians were fairly satisfied by a limited 

 range of glasses. Scientific work on the produc- 

 tion of new types was carried out in this country 

 for many years by Harcourt and Stokes, but, 

 though admirable in its scope, this work had a 



