A New Illuminating Apparatus for the Microscope. 79 
is to displace the disk with the entire diaphragm from the centre 
outwards, while the aperture in the diaphragm, which is thus thrown 
into an excentric position, admits of being rotated some 120° round 
the axis of the microscope, by using the handle as a lever to effect 
a horizontal rotation. The centre of the range of the lateral dis- 
placement that the rack admits of, or, in other words, the central 
position of the diaphragm in use, is indicated to the finger with 
accuracy by means of a spring-catch. The head of the rack and 
pinion serves also as a handle for moving the body of the dia- 
phragm-frame forwards or backwards. 
Firstly, with regard to the use of the apparatus for ordinary 
observation with transmitted light. For this purpose a series of 
circular disks is employed of about 30 miUimeters in diameter, 
having circular apertures ranging from 1 to 7 millimeters in width. 
With a central position of the rack and pinion, when inserted in suc- 
cession, they afford every desirable gradation of the ordinary direct 
illumination. It is true that there is not brought about thereby the 
continuous alteration of the aperture of the incident cone of rays, 
which is obtained by employing cylindrical stops provided with an 
up-and-down motion, yet as the free diameters of the apertures in 
these disks may differ as little as we please, and as they can be 
changed with great rapidity when one acquires the knack of using 
them, the present arrangement for the normal use of the microscope 
does not appear to stand in an unfavourable light when compared 
with Oberhauser's arrangement of the stops, more especially as for 
central illumination the loss of Kght in the condenser is inappreciable. 
The transition from direct to oblique light at any inclination de- 
sired is simply effected by manipulating the handle already spoken 
of, without in any way touching the mirror, and we are thus 
enabled thereby not only to divert the radiating surface away from 
the axis as far as the limit of the free aperture of powerful object- 
glasses, but also at the same time — by a movement of the excentric 
diaphragm in azimuth — to allow the obKque light to fall in the 
direction of the object from any side. Hence with the illuminator 
now described the rotation of the microscope upon its vertical axis 
may be dispensed with for this purpose. This arrangement allows 
of the control of the oblique illumination being effected with great 
ease and certainty, in particular it presents the advantage of en- 
abling us to pass from one modification to any other at our pleasure, 
without having to trouble ourselves again and again with the ad- 
justment of the light. When the mirror is once so placed that the 
condenser gives its full illumination, this is kept up without change 
as long as the source of light remains unaltered. 
On the other hand, it must be stated clearly that for ver^/ 
obhque light — as, for instance, when the matter in hand is to bring 
into play upon test-objects the utmost resolving power of an object- 
