228 THE MICROSCOPICAL NEWS. 
but both elements under the obvious supposition, that the whole 
pencil of light which is collected by the objective, is under com- 
parison, or the full area of the aperture effective at the same time. 
If in any particular case a portion only of the clear aperture should 
be utilized by the delineating pencils, the actual dissipation of the 
light will of course be confined to more or less small spots than 
would correspond to the angles w and U, and would accordingly 
become less apparent. In the practical use of the Microscope we 
always have very variable conditions, according to the illumination 
in use and the structure of the objects under observation. With 
very low apertures the range of difference is not so great, it is true, 
because the illuminating cone of light will generally fill the whole 
aperture, or at least the greater part of it. But with wide-angled 
objectives the incident beam from the illuminating apparatus is— 
and in most cases must be—confined to a much narrower angle 
than the aperture of the system. How much of the aperture is 
actually utilized by the delineating pencils will entirely depend on 
the dissipation of the incident rays by the structure of the object 
—in particular the diffraction effect of the structure; and accord- 
ing as the illuminating cone, after its transmission through the 
object, is spread out to a smaller or greater angular extension, 
smaller or greater aberration-circles will disturb the image. ‘Thus 
it may happen that with one kind of preparation an objective may 
bear.a deep ocular very well, whilst with other objects a great 
deterioration of the image becomes visible under even a lower 
ocular. Objects which show a regular striation, and in general all 
regular periodic structures, are particularly zwsenszble to the residuary 
defects of the objectives, because they produce only a limited 
number of isolated diffraction-beams, and thus leave the greatest 
part of the objective’s aperture entirely unemployed. In observing 
an object with only one set of parallel lines which: are near the 
limit of the resolving power of the objective, only two small 
portions of the aperture are simultaneously utilized, one by the 
direct beam, the other at the opposite edge of the opening by the 
diffracted beam, as may be ascertained by a glance at the objective’s 
clear aperture. All defects and aberrations of the system which 
inhere in the inactive portions, do not exist for the image in that 
case, whilst they will at once become effective when those objects 
of a very complicated and irregular structure, which produce a con- 
tinuous and widely spread out diffraction-pencil, are observed. 
This consideration will show that the ordinary test-objects of the 
Microscope—particularly lined objects, and in a somewhat less 
degree all kinds of diatom markings—are the most unsuitable pre- 
parations for a proper judgment of the performance of the instru- 
ment in regard to the general conditions of scientific work inasmuch 
as the latter are always much less favourable than those of diatom 
observations. 
