14 
Transactions of the Society. 
ing in a furnace — was forbidden by their substance. The condition 
mentioned above, however, made it necessary to bring the cover- 
glasses to the required thickness by carefully grinding down somewhat 
thicker plates to 0*01 to 0*02 mm., and to polish them with care 
(like lenses of medium quality), which naturally makes such cover- 
glasses very expensive. 
This is not the place to dilate on what can be done with objectives 
of this kind. The results which Dr. van Heurck has already obtained 
in the use of them lead in any case to the hope that, in spite of the 
great difficulty of their use, they will afford valuable aid for certain 
problems of microscopy. It will be a question for connoisseurs to de- 
cide, whether in other branches of microscopical research besides that 
of diatoms (more especially cultivated by him) an equally marked 
advance on that previously reached can be obtained. 
The inquiry may be made whether and how far a further increase 
of the aperture beyond that of 1*60 here attained can be expected. 
The difficulty of such a construction consists (as was the special 
hindrance before) in the ivant of a suitable immersion liquid. This 
liquid must have an index of at least 1*8-1 *9 (in order to make an 
essential advance on the present objective), and it must besides possess 
those general properties which qualify it as an immersion liquid : i. e. 
not to attack the glass of either the cover or the front lens ; sufficiently 
transparent ; not too viscous ; not inflammable (like the phosphorus 
solutions), &c. If such a liquid were found, Professor Abbe would 
be at once prepared to undertake the calculation of an objective of an , 
aperture of 1 * 8 or 1 * 9, since glass of sufficiently high index for the 
front lens and the cover-glass could be provided without difficulty. 
