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to a range of + 26 e. M.!) for the motion of the hand. Thus 
a very fine adjustment of the focus is made possible, and it is 
easy, while pulling the rope, to bring the slide to rest at 20 or 
more different positions during its motion in one direction. The 
relative accuracy of focussing is thus here considerably greater than 
with the ordinary microscope. 
Before proceeding to the description of the details of the apparatus 
I will still call attention to another point. 
In the ordinary microscope, before the beginning of the observa- 
tions, the adjusting screw must occupy a mean position, in order 
to enable lower as well as higher levels of the object to be brought 
to focus. Similarly here the slide must occupy a mean position 
when not in vse. The assistant at the microscope focusses 
on the middle level of the object by means of the adjusting screw, 
and the demonstrator must then be able by means of the rope to 
move the slide in either direction. For this purpose there is fixed 
to the end of the rope a handle (fig. //) with a ring, which can 
be passed on a pin, which has been driven in the wall at a con- 
venient height. The length of the rope must be so adjusted that 
the slide is in the required mean position when the ring is on the 
pin. As the discs are high up in the room, the rope in the hand 
of the demonstrator is Jong, and he is free to move before the whole 
of the screen while holding it, in order to point out special details 
of the image. 
It will be seen that the arrangement is very simple and erfective. 
There is however one drawback. When the ocular-glass is moved, 
the magnification and the illumination of the field are altered, 
however so slightly that it causes no inconvenience to the audience. 
As the ocular moves away from the objective the magnification is 
increased and the illumination is reduced. 
Also the changes of level, which are produced by a definite 
change of position of the ocular, decrease as the ocular moves away 
from the objective. For this reason the correct optical mean position 
is not the middle point of the range of motion of the slide, but a 
point nearer to the objective. The best mean position of the slide 
has been found by experience to be such that a motion of 2.2 c.M. 
towards the objective and of 3.3 c.M. towards the screen is possible. 
In this position of the slide the distance between the objective- 
1) Actually the range is somewhat larger (29 c. M.) probably owing to the exten- 
sibility of the ropes. 
