ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
363 
To make an observation with the Klein’s lens, the part of the section 
under examination is first adjusted with the Czapski eye-piece ; the iris- 
diaphragm of this eye-piece is then closed up so far that only the part 
of the section to be examined remains visible, and the Klein’s lens is 
put on and adjusted so that the objective diaphragm is clearly visible ; 
the interference picture is then plainly seen. Finally the scale is either 
raised or sunk until it also is sharply defined against the interference 
picture. 
To determine the optic axial angle with this apparatus, the formula 
of Mallard D = M sin E must be used. In this formula E denotes the 
half optic axial angle in air, D the number of the scale divisions for E, 
and M a constant to bo determined for the system by observation of a 
known optic axial angle. Instead of making use of the Mallard formula 
in each particular case, the author plots out a curve with the scale 
divisions as abscissae, and the corresponding angles as ordinates. 
Reichert Demonstration-Lens.* — Dr. W. Behrens describes a lens 
lately brought out by the firm of Reichert, which is useful for showing 
microscopic preparations to a large audience, since the apparatus can 
be conveniently passed from hand to hand without disturbing the 
preparation. 
It consists (as seen in the figure) of a black ebonite plate, with 
central aperture, on which the slide is held by means of two clips as 
Fig. 58. 
on a Microscope stage. The continuation of the plate behind forms a 
handle which is held in the hand during the observation of the 
preparation. 
A short upright between the spring-clips forms the lens-holder. At 
the top it carries a short horizontal arm which can rotate about the 
vertical axis. This arm supports a spring socket in which slides the 
lens, so that it can be adjusted by hand on the preparation, while 
the apparatus is directed towards the sky or an artificial source of light. 
(3) Illuminating: and other Apparatus. 
Illuminating Apparatus.']' — Dr. S. Czapski remarks that the arrange- 
ments which have hitherto been proposed for the quick passage from 
convergent to parallel light, i. e. for the rapid removal of the condenser, 
either require a radical change in the stage and other parts of the 
Microscope (Fuess method), or have the disadvantage that the removal 
of the condenser is not momentary, since the whole illuminating apparatus 
* Zeitschr. f. wiss. Mikr., xi. (1895) pp. 458-9. f Tom. cit., pp. 433-40. 
