328 Transactions of tJie [Sess. 



greatest of these are undoubtedly due to Mr Joseph Jackson 

 Lister, the father of Sir Joseph Lister. From many experi- 

 ments made by him with combinations of glasses, he was able 

 to devise certain forms for low- and high-power glasses, which 

 gave very flat fields, splendid definition, and wide angles. These 

 formulae he communicated to ,the three principal London makers, 

 and I believe that glasses are still made by them upon these 

 formulae without alteration. Besides Mr Lister, Andrew Eoss, 

 who has been termed the father of the microscope, was one of the 

 earliest experimenters in regard to the perfecting of the instrument. 

 By confining his methods to these three firms, Mr. Lister certainly 

 insured the manufacture of good glasses, but it became, and was 

 for long, a real monopoly. The prices charged were very high, 

 and after-experience of other makers has shown that they were 

 excessively so. However, this enabled them to be manufactured 

 with a high standard of excellence and progressive improvement. 

 Mr Thomas Eoss made an importa,nt alteration on the lower 

 powers, by separating the two lenses of which these are constructed, 

 and placing them about an inch apart, thus flattening the field con- 

 siderably, and improving the definition. Andrew Eoss was, I 

 believe, the first to increase the usefulness of the higher powers 

 by widening their angle, and thus increasing their light. A steady 

 advance in this respect, which began about the end of the third 

 decade of the present century, was continued for many years, when 

 for a considerable time the improvements seemed to stand still. 

 At last. Professor Amici of Modena startled microscopists by intro- 

 ducing the principle of water-immersion. By a glass constructed 

 on this principle, he was able to resolve with ease a Diatom, the 

 striae of which had not been seen before — viz., Navicula rhomboides, 

 or, as it has been named after him, the Amician test. The prin- 

 ciple was rapidly adopted on the Continent, Hartnack having made 

 water-immersion glasses by which he was enabled to resolve a 

 more difiicult test — viz., Surirella gemma. As in other things in 

 this country, we were slow to adopt the improvement ; and it was 

 only after a comparison of glasses in the first French Exhibition 

 showed our inferiority in their manufacture, that our leading firms 

 began to make them. It was not so with the principle of oil- 

 immersion, the last and greatest of all the improvements in the 

 microscope. We owe this to the suggestion of an Englishman, 

 Mr J. W. Stephenson, but he could only get his idea carried out in 

 practice by a foreigner — Dr Zeiss of Jena. There is one English- 

 man still alive whose name I must mention in connection with 

 the improvements of microscope object-glasses — viz., Mr F. H-. 

 Wenham. He has done more, perhaps, than any other after Mr 

 Lister, to improve the microscope. To Mr Lister we owe, I be- 

 lieve, the triple back, but to Mr Wenham we owe the single 



