( 101 ) 
tube 7 is fixed, which again carries the ocular 8. The slide 
13 has two projections of half-cylindrical form (g), each containing 
a cylindrical hole. Two rods (h) occupy the central lines of these 
holes. These rods are fixed to the base-plate at 7, 7. Their other 
extremities project through openings in the slide, which can thus 
move freely in either direction. Round these rods are two spiral 
springs which are compressed when the slide is moved towards 
the right, and therefore push it back towards the objective when 
the rope is relaxed. Round the part of each rod, which comes outside 
the hollow pieces g, is a brass tube which guides the spring and 
prevents it from bending. This detail is well shown in fig. C. 
On the slide is fixed a metal cross-piece which carries at its ends 
two cylindrical blocks / in which the ropes are fixed. These ropes 
run over the pulleys m,m and are joined together at a short distance 
above the apparatus. The combined rope then runs upwards and 
through the lecture-room to the disc 15 of fig. G, of which the 
diameter is 4.2 c.M. The diameter of the larger disc (16) is 19.4 c.M. 
The rope is made of twisted metallic wire +) of 2.1 m. M. diameter, 
which is very strong and inextensible. This is necessary with a 
view to the strong counteraction of the springs of the slide. 
In fig. H the handle for the demonstrator is shown. The required 
mean position of the slide can be arrived at, as already explained, 
by adjusting the length of the rope and the height of the pin p. 
But it may naturally be necessary afterwards to alter that adjust- 
ment, either because the rope may have been stretched, or if in a 
special case it is desired to use another mean position than usually. 
For this reason the handle is made hollow. The rod » can be slid 
in and out, and can be fixed at any required position by the screw o. 
During the course of lectures given in 1899—1900 the original 
wooden arrangement was used, and in !900—-1901 the metal apparatus 
here described. Experience has led me during this time to consider 
the apparatus as an indispensable auxiliary for the projecting- 
microscope. 
The apparatus was made according to my plans by the firm 
P. J. Kirep & Sons, J. W. Gimtay Opvolger at Delft; the rigid 
mounting for the microscope bar, the double disc and the adjustable 
handle were constructed by the amanuensis of the botanical laboratory, 
Mr. J. VEENHOFF. 
Groningen, 1901 June 22. 
1) „Verzinktes Drahtseil” of C. F. Rocuurrz, Berlin. 
Proceedings Royal Acad. Amsterdam, Vol LV. 
