13 
it has, on the contrary, been constructed for the purpose of placing in the hands of 
those specially interested in the development of microscopical technique from an 
optical point of view, a means of determining to what extent lenses possessing an 
aperture exceeding the highest aperture obtainable under the usual conditions of 
application may successfully be used in certain departments of microscopical research. 
Every advance in this direction necessarily involves an increase of the external dif- 
ficulties attending the application of the lens; we are, in particular, referring to the 
preparation of objects ; the difficulties to be surmounted here will be readily appre- 
ciated if it be remembered that the aperture can never be made to exceed the 
refractive index of the embedding medium and the cover-glass. 
Originally only a few specimens of this new objective were made as a purely 
scientific experiment. The results, however, which have been obtained with it by 
several microscopists , despite of the difficulties attending its application, have 
encouraged us in including it in our list of objectives, and we shall continue to 
make this lens as long as nothing better exists in this direction. 
A detailed account of the principles of construction of this lens, its mode of 
application and its capabilities will be found in the Zeitsclir. fiir wiss. Mikroskopie, 
VI, 1889, p. 417 — 422 (also English translation in pamphlet form), Journ. of the 
Roy. Microsc. Soc. 1890, p. 11, and H. van Heurck, La nouvelle combinaison 
optique de Mr. Zeiss et la structure de la valve des diatomees, Anvers 1890. 
We shall be glad to forward these publications to applicants. 
CHO. 
