ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
691 
As regards the chromatic aberration, when the distance between ob- 
jective and object is within the normal focal length, with an over-cor- 
rected system the [image is surrounded by a violet border, passing into 
blue ; while if the objective is beyond the normal focal distance from 
the object, the image appears surrounded by a red border, passing into 
yellowish-red. With an under-corrected system exactly the reverse 
colour-effects are seen. Still more sensitive are bright lines on a dark 
ground with oblique illumination. A line at right angles to the light 
falling obliquely from the right, shows with an over-corrected system 
the left edge violet or blue, the right red or yellowish-red. With an 
under-corrected system the appearance is just the opposite. 
Introduction to Microscopy.* — This is an excellent book. The 
optical part is most clearly written, the whole of the chief problems 
being worked out by examples and diagrams, without a number of 
formulae, but yet scientifically. The chapter on the polariscope — not, as 
a rule, a strong point in the ordinary text-book — while to some extent 
perhaps recalling Nageli and Schwendener, is particularly good. The 
individual apparatus described is, naturally, entirely German, and there- 
fore the weakest part is that devoted to the substage condenser, Abbe’s 
chromatic being the only form dealt with. The practical sections appear 
equally satisfactory, and altogether it is one of the best works on 
general microscopy we have met with for some time. 
(6) Miscellaneous. 
Determination of the System of Microscopic Crystals.! — Dr. 
J. L. C. Schroeder van der Kolk describes the method he employs for 
determining the system of small crystals under the Microscope. The 
greatest difficulty is experienced in the case of minute needles with 
straight extinction. For such cases the author makes use of a glass 
hemisphere which rests with its convex surface in the opening of the 
Microscope-stage, while the flat surface serves as stage for the object. 
The radius of the opening in the Microscope-stage amounts to about 
9 mm., that of the hemisphere to about 15 mm. The hemisphere can 
now be turned in any way required, while the middle point remains 
stationary. 
The solution to be examined is allowed to crystallise on a thin 
cover-glass which is attached to the glass stage with some oil or Canada 
balsam. The needle to be examined is now centered and brought into 
coincidence with one of the cross wires, when it can be turned 
(1) about its own axis ; 
(2) about the horizontal normal to its own axis; 
(3) about both axes successively ; 
(4) about the vertical . 
The author then gives some practical examples of the uses of the 
hemisphere in distinguishing between isotropic and uniaxial pinakoidal 
plates, optically uniaxial and biaxial needles, rhombic and monoclinic 
pyroxenes, and in determining the oblique extinctions of the plagio- 
clases. 
* Dr. A. Zimmermann, ‘ Das Mikroskop. Ein Leitfaden d. wissenschaftlichen 
Mikroskopie,’ Leipzig and London, 1895, 8vo, 334 pp., 231 figs. 
f Zeitschr. f. wiss. Mikr., xii. (1895) pp. 188-92. 
2 z 2 
