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II. — 071 an Objective with an Aperture 0 / 1 * 60 N.A. {Mo7iohromide 
of Naphthaline Lnmersion) made according to the Formulse 
of Frof. Abbe in the Optical Factory of Ca7'l Zeiss. 
By Dr. S. Czapski (Jena). 
{Read \Wi Decemher, 1889.) 
An advance in the increase of the capabilities of an optical instru- 
ment is always possible in two directions — the qualitative and 
the quantitative. The first point Prof. Abbe has kept in view, 
as fiir as the Microscope is concerned, in the construction of the 
apochromatic objectives. The quality of the objective was 
here augmented by a complete union of the rays, with- 
out that element on which the capacity of the objective primarily 
depends, viz. tlie aperture, being sensibly increased. In the ter- 
minology of optics it was properly only the “ definition ” of the 
objective which was improved. That by this means its resolving 
power was also increased was an indirect consequence of the first 
condition. For it is natural that an objective of given aperture 
should only have the resolving power prescribed by theory, if the 
assumption of this theory — perfect union of rays — is fulfilled. The 
earlier known achromatic objectives did not therefore reach the limit 
of the resolving power which the aperture allowed them ; the apo- 
chromatics approach extraordinarily near to this limit. This aper- 
ture itself, however, in the case of the strongest apochromatics, was not 
essentially greater than that which had been already obtained by Zeiss, 
as well as by other opticians, in their earlier objectives (1*40 against 
1*30 of the earlier). 
An advance upon that attainable with the apochromatics of 1886 is 
only possible if the aperture is increased in a marked degree. To this 
advance a special difiQculty opposes itself. In order to reach a given 
aperture a, for instance 1 * 60, it is necessary that all media between 
the object and the front lens of the objective, as well as this lens 
itself, should have a higher refractive index than a, therefore higher 
than 1 • 60 in the case we are supposing. For since the angle of 
aperture of the rays entering into the objective can practically 
scarcely exceed 150° (as cover-glass and focus necessitate a certain 
distance of the object from the front lens), therefore, since a = n 
sin u, must n ~> a, because sin u is necessarily < 1 ; and between the 
object and front lens there must be no layer of a medium, however 
thin, whose index n' is < a. For at such a layer the part of the 
incident pencil whose aperture is n' would, according to the laws of 
geometrical optics, be lost by total reflection, and there would remain 
only the part whose aperture a' is << n\ 
At the present time, however, not only do the front lenses of objec- 
tives consist for the most part of crown glass with a maximum index 
