October i, 19 14] 



NATURE 



117 



GLASS FOR OPTICAL PURPOSES. \ 



1 



''T^HE importance of an adequate supply of 

 -I- optical glass of all the principal types cannot 

 be overestimated. The improvement of the micro- 

 scope has been and is still retarded for the want 

 of suitable g^lasses, the construction of large tele- 

 scopes is limited by the capacity of the glass- 

 maker to supply suitable discs of glass, and the 

 improvements made in the design of small tele- 

 scopes cannot be extended to larger sizes for the 

 want of suitable glasi. 



Photographic lenses, binoculars, range-finders, 

 and telescopes of all kinds are dependent for their 

 performance on the good quality, both optically 

 and physically, of the glass used in them. The 

 magnitude of this special branch of the glass- 

 making industry may be gauged from the value 

 of the German exports. In the last year for 

 which figures are available the value of the exports 

 of unworked optical glass exceeded fifty thousand 

 pounds, and large quantities were exported as 

 finished lenses. The characteristic of optical, as 

 distinguished from other glass, is its great homo- 

 geneity ; veins of material of different composition 

 even in the form of very fine striae render glass 

 unsuitable for the better class of optical work. 



The history of optical glass-making is to a large 

 extent the history of optical progress. DoUond's 

 discovery of the achromatic combination (1757) 

 created a demand for flint glass suitable for opti- 

 cal purposes. At first this demand was met by 

 the selection of the most suitable pieces from the 

 glass manufactured for other purposes, but the 

 demand for larger discs of flint glass led Guinand 

 (1748-1824) to work out new methods of melting 

 flint glass. The essential feature of his method 

 was the continual stirring of the glass to prevent 

 the formation of striae of different density. 

 Guinand migrated from Switzerland to Bavaria, 

 and in conjunction, first with Utschneider and 

 later with Fraunhofer, improved the process, so 

 that larger blocks of good uniform glass could be 

 manufactured regularly. On Guinand 's return to 

 Switzerland, Fraunhofer continued his experi- 

 ments, and was able subsequently to make good 

 discs of flint glass up to 10 in. in diameter. 

 Meanwhile Guinand had increased the size of discs 

 up to 14 in. diameter, and on his death in 1824 

 the secret passed to his sons, and through them 

 to Bontemps in France. Bontemps' work was 

 carried on by the French house of Feil, now 

 Parra-Mantols, while Bontemps himself brought 

 the secret process, in 1848, to the glass-works of 

 Messrs. Chance. 



For some time the principal advance was in 

 the improvement of the physical properties of the 

 glass, greater uniformity, greater transparency 

 and more durability being aimed at. The calcula- 

 tion of the Petzval portrait lens and its successors 

 led to a large demand for a glass intermediate in 

 type bet\veen the ordinary flint and the crown. 

 For this purpose, and for use in the other types 

 of photographic lenses, the series of light flints 

 was worked out. In this series the percentage of 



NO. 2344, VOL. 94] 



lead oxide increases as a higher refractive index 

 is required. 



The progress of the microscope makers required 

 the extension of this series of glasses to denser 

 flints, i.e. glasses of higher relractive index; at 

 the same time small variations in the constitution 

 of the crown glass were made to give slightly 

 differing optical properties. In this way, by the 

 year 1880 it was possible to make a complete 

 series of glasses with their refractive indices rang- 

 ing from I "5 1 5 to i'72. But these glasses had 

 two special characteristics : as the refractive index 

 increased, the dispersion, i.e. the difference in 

 the refractive indices for light of two chosen 

 colours, increased more rapidly, so that it was 

 always necessary to use the glass of lower refrac- 

 tive index for the positive lens of an achromatic 

 combination ; also the dispersions of two glasses 

 of different refractive index were not propor- 

 tional throughout the spectrum, so that if a com- 

 bination of a crown and a flint lens were made to 

 bring the red light proceeding from an object to 

 the same focus as the blue light, the yellow and 

 violet light would not come to the same focus. 

 The consequence was that all images appeared 

 coloured, in spite of the choice of the most suit- 

 able curves for the two lenses; this defect is 

 termed "secondary spectrum." 



Many attempts had been made to obtain a pair 

 of glasses which would enable the lens-maker to 

 get rid of this secondary spectrum. The experi- 

 ments of the Rev. W. V. Harcourt (1789-1871), 

 which extended from 1834 to 1871, showed that 

 this problem could be solved ; he proved that the 

 effect of substituting boric acid for part of the 

 silica in the glass was to reduce the dispersion of 

 the blue end of the sj>ectrum, and so to make a 

 flint glass which more nearly matched the ordinary 

 crown glass. He was also able to modify the 

 crown glass by using phosphoric acid, but wrongly 

 attributed the result to the presence of titanic acid. 

 Unfortunately these experiments did not lead to 

 practical results, probably because of the expense 

 attaching to experimental meltings on a practical 

 scale. 



The next stage in the development of optical 

 glass was the Investigation by Schott and Abbe 

 {1881-1886) of the effects of using different 

 materials. With consummate experimental skill, 

 and assisted by generous grants from the Prussian 

 Government, they were able to determine the 

 effects of employing different materials. They 

 were able to confirm Harcourt 's results as to the 

 action of boric acid, and correctly to attribute the 

 effects observed by him to phosphoric acid. In 

 addition, they were able to determine the effects 

 of barium both with and without boric acid. 



Now the use of boric acid in the ordinary lead 

 glasses always leads to a glass which is more or 

 less liable to spot, but by the use of barium in- 

 stead of some of the lead this effect is reduced. 

 The boric acid barium glass is, however, of special 

 value, because in this case a high refractive index 

 is associated with a low dispersion. It behaves 

 as a crown glass as regards its dispersion, but 



