196 Extracts from Mr, E. E. Fri^pp's Translation of 
front lens from the object to a degree wliicli makes it practically 
useless. The application of the immersion principle renders it 
possible to overcome spherical aberration, where even the maximum 
angular aperture is used. It is in this power of using very large 
angles of aperture, and also in avoiding loss of light, that the real 
advantage of the immersion plan lies. It will indeed be seen from 
what follows, that these two facts fully explain the undoubted 
superiority of the immersion lens. 
Every appliance by which the amending of spherical aberration 
has been attempted — whether by correcting lenses placed above 
the objective or by construction of ocular — will produce no better 
result than what is already effected by changing the distance of the 
front lens of the objective from those behind it. They simply 
permit the existing residual aberration to be transferred — shifted 
backward or forward between the centre and outside border of the 
aperture — and by this means to keep, for a time, some particular 
zone of the objective more or less free from aberration, at the cost 
of the rest ! 
In an analysis of the conditions which belong to a perfect con- 
struction, it becomes obvious that the factors on which correctness 
of image in the centre of the field, and the maximum of good 
performance depend, namely, chromatic and spherical aberration, 
pertain to the functions of the objective alone, upon which no 
influence of the eye-piece, however constructed, can produce any 
marked effect. Arguments advanced in favour of a long tube or 
of a short tube are untenable in theory; and the supposed dif- 
ferences of effect have no real existence when examined under 
conditions which are truly comparable. There will be found in 
every objective a particular angular amplification obtainable at will 
by means of length of tube and strength of ocular, which must 
exactly suffice to enable any eye possessing normal capacity of 
vision to recognize all the details that can possibly be delineated in 
the virtual image formed by the objective. And this, which may 
be termed " necessary angular amplification," may be looked upon 
as the measure of the relative perfection of the objective. 
Theoretical study of the aberrations of the image-forming rays, 
and practical experience involving the application of methods to be 
hereinafter described, and the careful testing of a considerable 
number of objectives of recent date from the best workshops on 
both sides of the Channel, have led Professor Abbe to the conclu- 
sion that the numerical value of " necessary amplification " yet 
arrived at or attainable at present, is altogether much lower than 
might be supposed from the liberal way in which microscopists deal 
with thousands and tens of thousands. According to his experience, 
the capacity of the most perfect objectives, the usual forms of illu- 
mination being assumed, is exhausted with an eightfold angular 
