(30) 
5 e.M. in a direction parallel to the axis of the microscope. The 
distance between the objective- and ocular-glasses could thus be 
altered by this amount. The motion of the slide towards the screen 
could be effected by a rope, the other end of which was held by 
the lecturer at the screen. When the rope was relaxed, the slide was 
pulled back towards the objective by a weight. The first experiments 
with this arrangement gave very satisfactory results. The focussing 
could be effected with at least the same sharpness as in the ordi- 
nary microscope, and it was found easy to retain the slide for a 
considerable length of time in any position, when it was necessary 
that one particular levei should remain in focus for some time. 
Thus the principal featare of the arrangement was arrived at, 
and it was decided to erect a definitive apparatus. In order to secure 
the necessary stability for the permanent centring of the lenses, this 
apparatus was made of metal. It is represented in the fig’s A, G, H 
of the plate. 
Of course it was also necessary to provide a light-tight but movable 
connection between the objeetive- and ocular-glasses, and also a 
few other details required attention. Firstly it was found possible 
by a simple device materially to increase the accuracy of the control 
over the motion of the slide. 
The rope runs from the projecting-apparatus upwards, and then 
along the ceiling of the lecture-room to a place above and beside 
the screen. There it is fixed to the circumference of a small wooden 
dise (15 in the figure) of which the diameter in 4,2 c.M. If this 
disc turns on its axis in such a direction that the rope is wound 
upon it, the slide consequently is drawn away from the objective. 
This dise is fixed concentrically to another larger disc (16) of 19,4 ¢.M. 
diameter, to the circumference of which the descending rope is so 
fixed that, when it is pulled, the rope of the smaller disc is wound 
up!). When the rope is relaxed by the demonstrator, the slide is pushed 
back towards the objective by a pair of strong springs, which replace 
the weight of the first arrangement. As a consequence of the intro- 
duction if this second (large) disc, very little force is required for 
moving the slide, notwithstanding the very strong counteraction 
of the steel springs. At the same time the range of motion of the 
slide, which in the definitive apparatus is 5.5 c¢.M., is increased 
1) Of course it is by no means necessary to place the disc-system ‘exactly above 
and besiae the screen. It can be placed anywhere in the course of the rope, according 
to local cireumstances. 
