16 



he detailed the work of the Committee of the Royal Society 

 appointed to investigate the subject : " Mr. Dollond, one of 

 the first of our opticians, has not been able to obtain a disc 

 of flint glass 4^ inches in diameter, fit for a telescope, within 

 the last five years, or a suitable disc of 5 inches diameter 

 within the last ten years." After eulogising the labours of 

 Guinand and Fraunhofer, he went on to say : " .... 

 whether it is that the knowledge they acquired was altogether 

 practical and personal, a matter of minute experience and 

 not of a nature to be communicated ; or whether other circum- 

 stances were connected with it : it is certain that the public 

 are not in possession of any instruction, relative to the 

 method of making a homogeneous glass fit for optical purposes, 

 beyond what was possessed before their time." 



However, Guinand's process was by that time being 

 brought to give regular results. His elder son was working 

 upon it in Switzerland, and his younger son, Henri, in con- 

 junction with Bontemps, in France. In 1828 there were 

 exhibited discs, made at Choisy-le-Roi, of 12 inches and 

 6 inches diameter. From that time, says Bontemps, pro- 

 duction went on regularly, if for a period without great 

 activity. 



Mr. R. L. Chance, of Birmingham, who was well acquainted 

 with Bontemps, took out an English patent for Guinand's 

 process in 1838, and Messrs. Chance Brothers then commenced 

 the manufacture of homogeneous glass. It was not, however, 

 until 1848, when Bontemps left Choisy in consequence of the 

 French Revolution and came to Messrs. Chance's works, that 

 they were able to surmount the difficulties which surrounded 

 the manufacture of optical glass. 



When John Dollond presented to the world his great 

 invention of the achromatic lens, he had but^two glasses from 

 which to shape its twin components the ordinary crown 

 and flint. These names survived for the half-dozen standard 

 types of optical glass made at Birmingham and elsewhere a 

 century later. Ihe different densities of these were obtained 

 by the use of oxide of lead in varying quantities ; the " flint " 

 glasses being those into whose composition no lime entered. 



Up to the time of the introduction of the new Jena 

 varieties, some six types of glass had sufficed for the use of 

 opticians. These had been termed hard crown, soft crown, 

 light flint, dense flint, extra dense flint, and double extra dense 

 flint. 



Such progress was made with the production of large 

 meltings of uniform optical glass that the firm quickly gained 



