346 PROCEEDINGS OF THE SOCIETY. 



Mr. Crisp said that at the last meeting mention was made (see 

 p. 220) of some experiments which Professor Abbe was conducting 

 with the view of finding some immersion fluid that coukl be substituted 

 for oil, and chloride of zinc was referred to as a possible fluid. After 

 the meeting, however, some of the Fellows expressed the opinion that 

 chloride of zinc would diss(dve the brass setting of the objectives, 

 in consequence of which Mr. Stephenson had communicated with 

 Professor Abbe on the subject, and in reply he said that " what he 

 spoke of was not the ordinary chloride of zinc, obtained by dissolving 

 zinc in hydrochloric acid, but the chloride released from water 

 (anhydric) by distilling it over. The ordinary zinc salt would not 

 give sufficient refraction." 



Mr. Zeiss had also sent over four samples of the following solutions 

 (which were shown to the Meeting), viz. : — 



(1) Chloride of cadmium in glycerine (CdClo), 1"504. 



(2) Copaiva balsam oil, 1 "504. 



(3) Chloride of zinc in water (ZuCl^), 1-504. 



(4) Sulpho-carbolate of ziuc in glycerine, 1 "501. 



The chloride of cadmium in glycerine Professor Abbe describes 

 as somewhat too thick for convenient use, but very good in optical 

 respects. It is literally " fluid crown glass, its dispersion being almost 

 equal to that of ordinary crown. The oil of copaiva balsam he 

 pronounces to be " in every respect perfectly equal to oil of cedar-wood, 

 but not quite so fluid." 



Mr. Stephenson said that just before he came to the meeting he 

 had received a letter from Professor Abbe (7th April), in which he 

 further said, " As to the chloride of zinc, we have tried it repeatedly, 

 and have found no obstacle, but it does not allow a prolonged immer- 

 sion with the same drop. After ten to fifteen minutes' exposure, it 

 deposits small crystals, as it seems, on the slide and on the front lens, 

 whereby the optical efiect is considerably deteriorated, though all can 

 be cleaned off by water and alcohol. In using this solution, therefore, 

 the slide and objective should be cleaned after ten minutes' observation, 

 and a fresh drop taken. If the objective is well cleaned at the end of 

 the observation, wiping it finally with alcohol, there will be no damage 

 at all. With the glycerine, good cleaning of the preparations and of 

 the objective is also necessary, as glycerine has a very strong adhesion 

 to glass." 



Mr. Ingpen inquired if there would be any difiiculty in making 

 the ends of the fronts of the objectives of platinum ? 



Dr. Edmunds said that he had written to Herr Zeiss suggesting 

 that the front lens should be set in platinum, because of its incorrodi- 

 bility, and because, imder variations of temperature, its coefficient of 

 expansion was almost identical with that of crown glass. Herr Zeiss, 

 while admitting these advantages in platinum, pointed out that its 

 want of rigidity was fatal, inasmuch as for the fronts of these high- 

 angled lenses, the setting had to be turned out very hollow, and to an 

 edge little thicker than a sheet of writing paper. Such a setting 

 if in platinum would collapse under slight pressure, and the lens 



