An Objective with Aperture 0 / 1 * 60 N.A. By Dr. S. CzapsM. 13 
A third and fourth difficulty for certain purposes also lie in the 
following : — If it is desired to use the most oblique illumination which 
the objective will allow, the same remarks are equally applicable to 
the incident pencils as to those proceeding from the object ; its aper- 
ture will not have its full value, if between object and condenser there 
is a medium whose index is less than the figure which represents the 
aperture, and the condenser itself must be so constructed that it also 
shall have the full aperture desired. In other words, the slide must 
also consist of flint glass of > I * 65, and between it and the con- 
denser must be interposed a medium of I ' 65, and the front lens 
of the condenser must in all cases be made of flint glass of at least the 
same n. 
A condenser of this character was constructed at the same time as 
the objective ; also slides of flint-glass, and between them and the 
condenser monobromide of naphthaline was used, just as between 
cover-glass and objective. This arrangement, as above said, is only 
necessary with preparations for which the most oblique illumination 
possible is required — as, for example, Amphipleura pellucida, or those 
which are to be observed with completely open illuminating cone. In 
all other cases, including axial illumination, ordinary slides of crown 
glass and the ordinary condensers suffice. According to the aperture 
of the latter, and according as a stratum of air is left between it and 
the stage, or water or oil is added, we obtain even in these cases an 
illuminating cone of an obliquity representing an aperture of I ' 0-1 *4. 
For most purposes the latter is quite sufficient. 
The type of construction of the objective is the same as that of the 
other apochromatic objectives of large aperture. To the more than 
hemispherical front lens of flint glass (index 1*72) succeeds a binary 
achromatic lens. Over this lower part is the (for apochromatics) 
characteristic upper part of the objective, on the peculiar composition 
of which depends the removal of the chromatic difference of the 
spherical aberration, i. e. first a single lens of crown and following on 
this two more achromatic lenses, the one composed of two and the 
other of three lenses. The focal length of the objective is 2*5 mm. 
(1 /lO in.). 
Since the objective, as already pointed out, is not really a homo- 
geneous-immersion one (cover-glass and front lens having an index of 
1*72, whilst the immersion liquid has an index of 1*66)^ and also 
because of the extraordinarily large aperture of the image-forming rays, 
it is exceedingly sensitive to changes of the cover-glass thickness, and 
also to every change in the index of the immersion liquid — almost as 
sensitive as a high-power dry system. It must therefore be used 
only with pure monobromide of naphthaline and with cover-glasses of 
the same thickness for which it is corrected, if the image is to be 
perfect. The cover-glasses themselves moreover must be made with 
great care and of the right glass. 
The production of these cover-glasses in the usual way — by blow- 
