328 Address by the Presidetit. [Sess. 



Before any further advance could be made a new kind of 

 glass had to be found. This had long been the desideratum 

 — a new vitreous compound which would allow of all the 

 rays of the spectrum being brought to a point. In 1878 

 Professor Abbe of Jena made a report on the microscopes of 

 the South Kensington Exhibition. This report came under 

 the notice of a Dr Schott of Witten, in Westphalia, a chemist, 

 but who had a practical knowledge of glass-making. He put 

 himself into communication with Professor Abbe, and the two 

 in conjunction entered into a series of experiments in the 

 manufacture of different kinds of glass for optical purposes. 

 At first they worked apart, Dr Schott taking charge of the 

 chemical side and Professor Abbe that of the optical. 

 Latterly Dr Schott removed to Jena, where the experiments 

 were carried out on a larger scale. Then the Messrs Zeiss, 

 the firm of opticians, joined them, and a subsidy of £3000 

 having been obtained from the Prussian Government, a large 

 glass factory was erected. Enormous difficulties were en- 

 countered, as the optical properties of each new vitreous 

 compound had to be tested. The experiments extended over 

 about five years, and during that period something like 1000 

 prisms of different kinds of glass had been ground and tested 

 for their refractive and dispersive properties. At last a glass 

 was produced which allowed three rays of the spectrum to be 

 united for chromatism and two for sphericity. This prac- 

 tically gave a colourless image of the object. 



At the meeting of the Eoyal Microscopical Society in 

 March 1886, two object-glasses in which the new glass — now 

 called Jena glass — was used were exhibited. These were the 

 first produced. Each of the object-glasses had ten lenses 

 combined in five pairs, and their performance was pronounced 

 by the most competent judges to be of the highest order. 

 The name given to these object-glasses is apochromatic. At 

 first the new glass was found to be liable to deteriorate on 

 exposure to the atmosphere, — a piece so deteriorated was 

 shown to me in the workshop of Messrs J. Swift & Son, when 

 I was in London last June, — but, owing to a slight alteration 

 in the composition or manufacture, it is now said to be quite 

 stable in temperate climates. Owing to the extremely careful 



