SUMMARY OF CURRENT RESEARCHES. -"'81 



(6) Miscellaneous. 



Glass for Optical Purposes.* — S. D. Chalmers calls attention to 

 this important subject, and incorporates in his paper a series of historical 

 notes likely to act as useful hints to any manufacturers contemplating 

 improvements in British methods. In view of the importance to onr 

 industries and to the army and navy of an adequate supply of optical 

 glass of various types, it is most desirable that our optical glass should 

 be made in this country, and the author thinks that the manufacturers 

 should be encouraged to meet this demand. Success in this direction is, 

 however, most likely to be achieved by scientific experimental work 

 carried out in conjunction with those manufacturers who have already 

 acquired valuable and essential experience in the manufacture of optical 

 glass. The manufacture of the optical glass is not, indeed, to be 

 lightly undertaken. Great difficulties are associated with the purity of 

 the materials and their proper mixing ; the pots must be of clay, free 

 from impurities which might colour the glass. The preparation of these 

 pots requires skilled workmen of long experience, and the same may be 

 said of the melting temperatures, the proper period of stirring, the rate 

 of cooling, and the whole annealing process. At the same time small 

 variations of composition or treatment will affect the optical properties 

 quite considerably. The experimental work associated with the pro- 

 duction of special types of glass is expensive and troublesome, since the 

 principal difficulties arise when we endeavour to change from the labor- 

 atory to the works scale. 



The history of optical glass-making is to a large extent the history of 

 optical progress. Dollond's discovery of the achromatic combination 

 (1757) created a demand for flint glass suitable for optical purposes. 

 The demand for large disks of flint glass led Guinand (1748-1824) to 

 work out new methods of melting flint glass. Guinand in conjunction 

 first with Utschneider and later with Fraunhofer improved his process so 

 as to make good flint disks up to 10 in. in diameter. He afterwards 

 made them up to 14 in., and on his death in 1824 the secret passed to 

 his sons and through them to Bontemps in Prance. Bontemps' work 

 was carried on by the French house of Feil, now Paira-Mantois, while 

 Bontemps himself brought the secret process, in 1848, to the glass-works 

 of Messrs. Chance. The calculation of the Petzval portrait lens and its 

 successors led to a large demand for a glass intermediate in type between 

 the ordinary flint and the crown, and by 1<S80 it was possible to make 

 a complete series of glasses with their refractive indices ranging from 

 1*515 to 1*72. But these glasses had two special characteristics : as 

 the refractive index increased the dispersion increased more rapidly ; also 

 the dispersions of two glasses of different refractive index were not pro- 

 portional throughout the spectrum. The consequence was that all images 

 appeared coloured. The experiments of the Rev. W. V. Harcourt 

 (1789-1871) which extended from 1834-1S71 showed that this problem 

 could be solved ; he proved that the effect of substituting boric acid for 



* Nature, 2344, Oct. 1, 1914, pp. 171-2. 



