ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
237 
weighted. Each is furnished with one double plate-holder and a fine 
ground-glass focusing screen. 
Fig. 39. 
Itzerott, G., & F. Niemann — Mikrophotographischer Atlas der Bakterien- 
kunde. (Microphotographic Atlas of Bacteriology.) 
Leipzig, 1894, 8vo, xii. and 115 pp., 126 microphotographic figs, in 21 plates. 
(5) Microscopical Optics and Manipulation. 
The Birefractometer or Eye-piece Comparer.* — M. J. Amann de- 
scribes the instrument which he uses for determining the double refrac- 
tion of crystalline bodies which exhibit this property. 
The birefractometer or eye-piece comparer consists principally of a 
wedge of selenite or of quartz carrying a micrometer division, at each 
division of which the thickness has been very exactly determined. This 
wedge can be inserted beneath the ocular lens of the Huyghens eye- 
piece, just as a micrometer, in such a way that its longitudinal axis 
makes with the principal sections of the nicols an angle of ± 45°. 
* Zeitschr. f. wiss. Mikr., xi. (1895) pp. 440-54. 
