400 Obituary. 



without adjustment-collar. This paper informs us : — (a) that the 

 author had suggested to Professor Abbe the desirability of constructing 

 an object-glass corrected for use with some immersion medium optically 

 identical with the cover-glass, for the purpose of removing all 

 necessity for collar adjustment ; (b) that Professor Abbe, acting on 

 this suggestion, had measured the optical constants of nearly 100 

 different media, and had found that oil of cedar was the best for the 

 purpose ; (c) that Messrs. Zeiss had constructed a homogeneous im- 

 mersion I objective from Professor xibbe's formula. The lens was 

 exhibited at the time. 



Early in the next year (1879), Mr. Stephenson brought to the 

 notice of the Society a catoptric immersion illuminator which he had 

 designed a couple of years previously. This was an annular illuminator 

 for wide-angled lenses, which would necessarily give a dark ground 

 with narrower apertures ; by the employment of suitable stops oblique 

 light could be obtained in one or more azimuths. In 1885 he further 

 amplified the idea by a catadioptric illuminator, 10 but this new form 

 did not prove as serviceable as the older one. 



The next paper was a numerical aperture table giving, in addition 

 to the N.A., the theoretical resolving power for objectives when illu- 

 minated by oblique light ; it also contained three columns with the 

 equivalent angular apertures of dry, water, and homogeneous immer- 

 sion objectives, in this respect being an enlargement of Dr. Pigott's 

 table n . 



There are two valuable papers by Mr. Stephenson upon mounting 

 in dense media ; among the substances experimented with were bi- 

 sulphide of carbon, phosphorus, sulphur, biniodide of mercury, iodide of 

 potassium, and oil of cassia ; and in 1882 there is a statement that 

 some slides containing diatoms mounted in phosphorus 12 in 1873 were 

 still perfectly good. 



10 J.R.M.S., v. (1885) ser. 2, pp. 207 and 523, figs. 44 and 112. 



11 M.M.J.. iv. (1870) p. 26. 12 Op. cit., x. (1873) p 1. 



