12 
Transactions of the Society. 
1 • 56, but the cover-glasses are made of a crown-glass (blown) whose 
index is about 1 • 52, and the index of the immersion liquid is equally 
about 1 *52. If, then, we are confined to the use of these materials, 
no higher aperture is attainable than 1 • 45 N.A. as a maximum, as is 
also proved practically. If it is desired to go beyond this limit, then, 
as shown above, care must be taken that the substances used for cover- 
glass, front lens, and immersion liquid have indices higher than the 
desired aperture. 
The aperture which was first decided upon, and which was com- 
pletely realized, was 1 * 60. An immersion liquid which satisfied 
the required conditions was found in monobromide of naphthaline, 
whose index is about 1*66. For the cover-glass and the front 
lens was selected for reasons of construction (removal of spherical 
and chromatic aberration) a flint-glass of index 1*72, so that the 
objective was no longer, in the strict sense of the term, a homo- 
geneous-immersion objective. None of the existing kinds of flint 
glass appeared to be suitable for the front lens. Special fusions had 
to be undertaken (by Dr. Schott) in order to obtain a satisfactory 
glass. 
The work of calculation for establishing a suitable formula had 
been going on for a year previously, but was not brought to an 
end until August of this year. In the course of the calculation the 
favourable result was obtained, that in spite of the very difficult 
conditions, not only could the removal of the chromatic and spherical 
aberration in the ordinary sense be obtained, but in addition a 
correction of almost the same perfection as with the apochromatics. 
In the beginning of September the first objectives were completed. 
One was exhibited by the author to the Naturforscherversammlung 
in Heidelberg, from September 18-24, in the exhibition of newly 
constructed scientific instruments, as well as in the section for patho- 
logical anatomy. The other was placed at the disposal of Dr. van 
Henrck, of Antwerp, who had done excellent service for the perfecting 
of the objectives, in prejoaring new test-objects indispensable for their 
testing and final adjustment. As already mentioned, the objects must 
have special cover-glasses of flint glass. There must not be between 
cover-glass and object any medium whose index <C 1 ' 66. Thus, the 
preparation must either be melted on the cover-glass (which does not 
succeed with flint-glass) or it must be imbedded in a medium whose 
index is at least = 1 * 66. 
For the visibility of microscopic objects it is, however, well known 
to be advantageous if they are placed in a medium whose index differs 
as much as possible from that of the object itself. In the present case 
it is important that that index should be as high as possible, since the 
index of the diatom valve is scarcely higher than 1 • 55. How Dr. 
van Heurck in numerous readily arranged experiments overcame this 
double difficulty, and how he has been successful in finding a practical 
medium of index 2*4, I must leave to him to explain. 
