60 
No. 
Marks 
f) The iris diaphragm (Fig. 20), which is now included 
in the Abbe illuminator, is a very convenient substitute for the 
ordinary interchangeable diaphragms, as it affords a ready means 
of gradually increasing or diminishing the aperture with the 
greatest precision. Smallest aperture about 0.5 mm, largest — in 
the newest form — equal to the full aperture of the 
condenser system, so that it may remain in situ 
when either the central spot diaphragm or polari- 
se r is in use. 
Fig. 20. 
Iris diaphragm. 
The iris diaphragm may readily be adapted at any time 
to any of our recent stands provided with the large Abbe illu- 
minating apparatus (i. e. I to V^). It is only necessary to remove 
for this purpose the small screw which fixes an annular cap upon 
the diaphragm carrier and, after removing this cap, to insert in its 
place the iris diaphragm, which may then be fixed by means of 
that small screw. 
We do not supply the large ABBE illuminating apparatus 
in a detached state, as in most cases considerable trouble and 
annoyance is caused by having it fitted to stands of a differ- 
ent make, it being besides in many cases an utter impossib- 
ility to do so. 
Our own stands, from I to IV'' and are fitted with 
this apparatus, viz stands I, 1% II^" and the "photo-micrographic" 
stand with a condenser system of 1.40 num. apert. and iris 
