210 THE NORTHERN MICROSCOPIST, 


tion must be intentionally left at the central part of the system in order to 
prevent an obnoxious accumulation in the marginal zone. It is obvious, there- 
fore, that with an aperture-angle of Say 90° the inmost cone of 45° cannot be so 
well corrected as it might be if the marginal zone could be left out of account, 
The effect is by no means inconsiderable, particularly in regard to the colour 
corrections. Owing to the chromatic difference of the spherical aberration the 
central portion of a somewhat wide aperture must always, even in a well- 
arranged objective, be perceptibly under-corrected chromatically, and in using 
this central part alone (the compensating influence of the over-corrected marginal 
zone being done away with), we have the performance of an inferior lens. In 
point of fact, no intelligent optician would ever make an objective of 30° 
aperture on the same formula as one of 60°, or one of 60° on the same formula 
as another of 100°, though this could be done by merely reducing the clear 
diameter of the lenses, 
There cannot, therefore, be a reconciliation between the pleasure of exhibiting 
mere optical accomplishment and the interests of the working microscopist. Bad 
lenses will certainly not meet the demand for low and medium powers affording 
the utmost possible economy of time and labour in scientific work, This can 
be done only by systems in which all advantages attendant upon the lower 
apertures are fully realized by constructions specially aiming at the best which 
can be obtained under the actual conditions of the case. 
The progressive increase of aperture in the higher powers, formally within 
the capabilities of the dry system, and at a later period by the development of 
the immersion method, is, without any reasonable doubt, the most important 
feature of the modern advance of microscopical optics. It has rendered pos- 
sible the successful extension of microscopical research to minuter and minuter 
objects, which otherwise would have been impossible by the ineffectiveness of 
all increase of amplification beyond certain low figures. The appreciation of 
that progress and the recognition of its true basis has led to a tendency to in- 
crease more and more the aperture of every kind of objectives. The fact has 
been disregarded that it is an entirely different thing whether the object is to 
promote the performance of the Microscope in the whole at the limits of its 
power, or to promote its performance for aims beyond these limits. The 
opinion has thus arisen that what is a benefit for one kind of lenses must also be 
a benefit for every other kind, Objectives of low and medium powers (1-inch 
to %-inch) of 15° to 60° are proclaimed at this time by many microscopists as 
old-fashioned and worthless things; 45° to 100°, or even 60° to 140°, are wanted 
for the same powers. Now as from a purely technical point of view, it is an 
accomplishment when the delineating power of an objective cannot be exhausted 
even with the deepest eye-pieces, opticians (notwithstanding the total bootless- 
ness of such a super-abundance) of course take pleasure inmaking such ‘‘superior” 
lenses, and the natural consequence is that the lower apertures required for use- 
ful scientific research are likely to be esteemed as second-rate work, no longer 
worthy of high technical art. : 
This opinion is a fatal mistake, and its practical effect, if not counteracted, 
will be a decided retrogradation of microscopical optics. Nobody, of course, 
can have the least objection to the construction of lenses of any description 
whatever for the personal pleasure of this or that microscopist. Strong opposition 
should, however, be made against all tendencies of captivating microscopical 
optics, in favour of such predilections, at the cost of the general usefulness of 
the instrument, 
Scientific work with the Microscope will always require not only high- 
Power objectives of the widest attainable apertures, but also carefully 
finished lower powers of small and very moderate apertures, 
from the Journal of the Royal Microscopical Society. 


