( 100 ) 
necessary independently of the focussing-apparatus described in this 
communication. During the microscopic projection the automatic 
centring of the lamp must now and then be corrected by hand and 
this causes vibrations of the microscope, which are very troublesome. 
The block 3 can be moved along the bar 1 by means of a 
toothed wheel (4). This block 3 originally carried the whole 
microscope-tube, but now it only supports the tube 9 in which 
the objective is fitted by means of the tube 6. The rough focussing 
is done by turning the head 4, while the finer adjustment is 
effected by means of the screw 5. Every thing that has been 
described so far belonged to the original apparatus, with the only 
exception of the objective tube 6 in which the ocular tube 7 
can slide in and out, without even at the most outward position 
admitting any troubling rays of light. Both the objective- and the 
ocular-tubes have a wider and a narrower part. This is not essential; 
it was only fuund necessary to make them thus, in order to be able 
to use the existing ocular and the existing tube 9, into which 
originally the whole microscope was fitted. When constructing a 
new apparatus, it would be better to make both tubes of a uniform 
width throughout. 
I now proceed to describe the details of the new part of the 
apparatus. The ocular-slide is carried by the cross-beam (10) of 
cast iron. As it is often necessary in the course of one and the 
same lecture to use alternately projection of microscopic preparations 
and other forms of projection, the whole apparatus must be easily 
removable. For this reason the cross-beam is hinged at one extrem- 
ity, so that it can be lifted up with the whole apparatus connected 
with it. In this position the beam is held by a hook, so that the 
space before the lantern comes free for another front. The other 
extremity of the beam is in the working position kept down by a 
catching arrangement, of which a section is shown in fig. F. The 
block d is fixed to a projecting rafter of the wall, and carries a 
catch-spring 4 which keeps the beam in its horizontal position. By 
the adjustable screw f any play which might exist between the beam 
and the spring, can be abolished. In order to prevent the handle H 
from sliding away from the pin p when the beam is lifted up, a 
rather heavy weight is fixed to the rope immediately above the 
projecting-apparatus. 
The sliding-apparatus itself consists of a base-plate (11) which 
is cast in one piece with the cross-beam 10. This plate carries 
two ridges (12) forming a dovetail-guiding for the slide 13, which 
carries the ocular, This slide carries a ring (14) in which the ocular- 
