ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
595 
teristics of an image, wliich often are only to a limited extent com- 
patible; that we have at command two methods of illumination, which 
respectively promote more especially each of such characteristics ; and 
that in most cases our problem is so to combine and balance these two 
methods as to produce the best result. Fidelity of contour will be 
secured in proportion as we are able to obtain our image by hetero- 
geneous illumination, approximating the object to a self-luminous con- 
dition. But this method may prove utterly unable to give us contrast , 
which we may therefore have to gain at the expense perhaps of infidelity 
in contour. Thus an opaque object, even of much minuteness, may be 
best shown by ground-glass illumination, or a very wide cone ; while 
a diatom, unless in a very dense medium, or dry in air, may require 
narrow pencils of approximately plane waves. Moreover, neither kind 
of pencil can be obtained absolutely pure. The narrowest pencil will 
only approximately consist of plane waves, and the widest cone will 
not be free from them. We thus understand why, in really critical 
work, a large cone from a good condenser usually gives us the best 
result; but why it may be impossible, even with a perfect objective, 
to use a cone of light which will fill its aperture completely. It may 
be necessary to intensify the image, while using as much heterogeneous 
light as we can. But this necessity depends on the nature of the 
object, and does not exist in all cases. 
There is a very obvious and simple, yet decisive test as to the cor- 
rectness of this view. According to the Abbe theory, the amount of 
cone light which can be used will depend upon the minuteness of the 
structure alone. According to the author’s view, the density or contrast 
of the structure is the chief factor in this question. All experience 
proves that the latter is the case. 
In conclusion, the author discusses the effect of these theories on 
the work of the Microscope optician, and on the prospects of further 
microscopical research. Devotion to the Abbe theory alone has even 
led Continental microscopists to challenge the importance of spherical 
aberration in objectives ; but Mr. Wright shows that the perfect correc- 
tion of spherical aberration of a lens determines how far we may go 
in the use of the heterogeneous cone with that lens. The well-known 
fact that individual similar lenses frequently possess marked superiority, 
shows a more perfectly corrected spherical aberration ; and this suggests 
the direction of further improvements. It is not so much further 
minuteness, but greater powers of resolution within the limits of our 
present lenses, that are desiderated. 
(6) Miscellaneous. 
Simple Apparatus for Washing Microscopical Objects.* — Dr. G. 
Cruz describes an apparatus (fig. 109), which he uses for washing tissues, 
drc., intended for microscopical examination. The apparatus consists 
of a glass funnel fixed to the upright of a laboratory stand. Above 
this is another funnel in the reverse position. The upper funnel, also 
fixed to the upright, is of less diameter than the lower one. Through 
its stem passes, to near the bottom of the hopper of the lower funnel, 
* Zeitschr. f. wiss. Mikr , xv. (1S9S) pp. 23-80 (1 fig.). 
2 s 
1898 
