GLASS FOR OPTICAL PURPOSES. 



By W. E. S. Turner, D.Sc, M.Sc, 

 Of the Department of Glass Technology, Sheffield LTniversity. 



Ifc is with great reluctance that I find I must forgo the pleasure 

 of being present at the Symposium on Wednesday next. 



I am glad to know that some members of the Society of Glass 

 Technology will be present and take part in the proceedings. The 

 fine array of papers is a tribute to the great efforts Sir Robert Hadfield 

 has made to direct the attention of scientific men to the importance 

 of encouraging the production of all-British optical instruments. 



Amongst the large number of papers, however, I do not find a 

 single one dealing with the manufacture of optical glass. It is, in 

 my opinion, unfortunati^ that there should not be as free and ready 

 a discussion of this subject as had taken place in America in these 

 past three years. 



I should like to make some remarks of a very general character 

 to indicate to makers and users of instruments the position in which 

 optical glass makers frequently find themselves. There is, in my 

 mind, not the slightest doubt that we can produce in this country all 

 the types of glass requisite for microscopes and other optical instru- 

 ments. The long experience of Messrs. Chance Bros., and the splendid 

 achievements of the new Derby Crown Glass Co. undoubtedly demon- 

 state this. 



The amount of optical glass in any one instrument is, as a rule, 

 quite small, whilst for the instruments of high precision, even the 

 total amount of glass called for by manufacturers is very small. The 

 glass maker, however, knows that in order to ensure homogeneity, 

 freedom from striae, etc., from the glass, he must make a melt on a 

 considerable scale. Further, some designers of optical instruments 

 will call for a new glass of special properties, may be, for example, 

 one which is successful in eliminating light rays between certain wave- 

 lengths. The production of such a glass calls for considerable research 

 on the part of the glass manufacturer, and he usually cannot expect 

 to sell but a very small quantity when produced, whilst the instru- 

 ment maker is not prepared to cover the cost. Such a demand cannot 

 always be met by the glass manufacturer; it is in no sense a com- 

 mercial proposition. For some reason, connected probably with, the 

 early days of German competition, the manufacture even of the well- 

 known varieties of optical glass has never been remunerative, although 

 with the recent American products the prices, I believe, have had a 

 more reasonable relation to the cost. One American manufacturer 

 stated that he was prepared to continue the manufacture of optical 

 glass if the loss was not greater than 10,000 dollars per annum, 

 although he was hoping, eventually, the factory costs might be 

 balanced by income. 



