Obituary. 159 



ferred, was his Diffraction Theory. The second was the production 

 of the Jena Optical Glass. Abbe had come to the conclusion that 

 the main hindrance to further improvements lay in the paucity of 

 the kinds of glass available. Already in 1874, in a paper dealing 

 with measurements of dispersion and refraction,* he had drawn 

 attention to the fact that " makers of optical glass denominated 

 their products according to their specific weight, as though they 

 were destined for ships' ballast," and had gone on to show how 

 glasses having a different relation between the mean refractive 

 index and dispersive power were absolutely a necessity. In 187(> 

 he came over to this country to inspect the scientific apparatus at 

 the International Exhibition ; and a report which he wrote on the 

 Loan Collection of Microscopical Apparatus at South Kensington f 

 may be classed as one of the most important of his papers, for it 

 was in this treatise that he set himself to explain in detail the need 

 of producing suitable new kinds of glass, which had the effect of 

 attracting Dr. Otto Schott, a Westphalian glass-maker, who volun- 

 teered to assist in the work. 



Experimental work on a small scale was begun by Schott 

 in 1881. In 1882 he removed to Jena. The results were so 

 promising, that Abbe, by his efforts, was able to obtain a large 

 grant from the Prussian ministry in aid of further work in the 

 same direction, and in 1884 the manufacture was placed on a 

 wholesale industrial footing. A year later the first fruits were 

 seen, and Abbe had brought out Apochromatic Objectives and 

 Compensating Eyepieces. 



In a short memoir like this, a mere mention of some of the 

 other apparatus devised by Abbe must suffice. Amongst these 

 may be mentioned the Eefractometer and Spectrometer bearing his 

 name (1874), the Camera Lucida (1881), and the Microscope Illu- 

 minator (1872). The latter, curiously enough, was originally 

 designed only for testing Microscope objectives. The Demonstra- 

 tion Microscope, in which his theoretical analysis of the Microscope 

 is carried out into practice, should also not be forgotten. But, 

 apart from the instruments bearing his name, his influence may be 

 traced in the many productions of the firm of Carl Zeiss. The 

 history and development of that undertaking, as well as of the 

 < )ptical Glass Works, are indissolubly connected with Abbe, and 

 are a striking testimony, not only to the achievements of Abbe the 

 man of science, but also of Abbe the social reformer and the 

 capable organiser and administrator. For, great as Abbe showed 

 himself as a pioneer in the paths of science, he was equally great 

 as a pioneer in other directions. 



* Jenaische Zeitschr. f.|Naturwiss., viii. (1874) pp. 96-174. 



t " The Optical Means of Assisting Microscopy," published by A. W. Hofmanu 



1878. 



