ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
103 
pieces of the plate P, and is connected with the plate P by the screws 
BB. 
On the spring D depends the proper functioning of the instrument. 
It is a steel spring in the shape of an arc, and is provided in the 
centre with a screw-bolt F which fits in a boring of the block P. The 
tension of the spring is regulated by the screw E. The pressure of the 
spring is transmitted to the plate P by means of the two angle-pieces 
WW, so that the plate is pressed down upon the Microscope-stage A A. 
The plate P with all its accessories is cast in one piece, and has a 
large aperture for the complete use of the Abbe condenser. The 
dotted line in P marks the outline of the aperture on the under side of 
the plate. The whole plate should be made of aluminium to diminish 
the weight, so that a slight spring pressure would suffice in order to 
prevent a sliding of the carrier in an inclined position of the stand. 
To prevent scraping in the sliding motion, about a third (the 
dotted part in the figure) of the under side of the plate P is covered 
with a thin woollen material. As regards the method of using the 
instrument, it is clear that the vertical rod x with the grooved head C 
always keeps the plate Z horizontal. It is otherwise with the plate P 
which is inclined by the pressure of the spring against the stage AA of 
the Microscope. This pressure, however, induces a pressure of the arms 
of the plate Z in the grooves of the head C, since here a variation from 
the horizontal plane is impossible. Since, further, the force of the spring 
acts on levers of unequal length — from the hinge BB to the rod x on the 
one side, and from the same hinge up to the place where the plate P 
comes in contact with the stage AA on the other — therefore the friction 
on the stage must be correspondingly smaller than that in the grooves of 
the head C. 
In the practical use of the instrument, first the plate P is set parallel 
to the stage A of the stand by means of the two screws SS. The neces- 
sary tension is then given to the spring, which will vary according to the 
inclination of the Microscope. 
By loosely holding the knob K between the thumb and first finger, 
the preparation on the carrier may be made to describe the same curve 
backwards and forwards. In this case the guiding hand makes no 
attempt to overcome the greater friction in the grooves, while the slight 
friction on the Microscope stage A is scarcely felt. A firmer pressure 
on the knob K is required to make the carrier move to and fro in the 
grooves C. By these two movements the whole preparation can be 
systematically examined. But if it is desired to follow the outline of 
an object or the course of a curve in a drawing, in this case, the hand 
holding the knob K must take no account of the resistance offered by 
the friction in the grooves C, but must follow the curves and outline of 
the object as they present themselves. 
The figure represents the instrument in two-thirds of its natural size. 
A new Drawing Apparatus.* — Dr. S. Czapski states that the 
following conditions should be satisfied by a good drawing apparatus : — 
(1) The light from the image must not to any great extent be 
weakened by the apparatus. 
* Zeitschr. f. wiss. Mikr., xi. (1894) pp. 289 -98. 
