NOTES AND MEMORANDA. 
261 
Mr. J. E. Smith said, that " some weeks since, when engaged in testing 
the extreme apertures of several objectives (including a ^th and y^th 
immersions, ToUes' new four-system formula), I used, for the purpose of 
cutting off the central ray, a simple piece of japanned iron plate — the 
ordinary ferrotype plate used by photographers. The usual diaphragm 
box and revolving plates were removed, leaving the under side of the 
stage entirely unobstructed ; the piece of ferrotype plate was fastened 
by a screw near its edge farthest removed from the illuminating 
lamp, so as to entirely close the ' well-hole ' of stage. Now, by hend^ 
ing the ferrotype plate it was easy to convert it into a <i shaped dia- 
phragm, with its open side adjacent to the mirror — and the included 
angle formed by under side of stage and the ferrotype plate depended 
entirely on the disposition (bending) of the plate. While using this 
rude and simple apparatus, with an objective of 170°, the ferrotype 
plate making an angle of 20° with under side of stage, the illumination 
being furnished by a German ' student's lamp,' I was surprised to 
find that I got strong views of transverse striae on No. 18 of the 
Moller Probe-Platte, the same shell having utterly defeated the objec- 
tive named on any previous occasion. Since I have made number- 
less experiments, using a variety of wide-angled objectives, and find 
the <i diaphragm of singular advantage when resolving severe tests. 
As to the percentage gained by the use of the diaphragm, I hardly 
know — and this will probably vary with the objective used. I detail 
a single experiment, and leave others to form their own conclusions. 
With the ToUes' four-system (yV^h immersion) German student's lamp, 
and using aperture, say of 160°, without diaphragm, I usually see A.jpeU 
lucida in Moller Probe-Platte (No. 20), exhibiting transverse striae in 
patches, and strongest near the end of valve ; turning the diaphragm 
into position and so as to include angle of 20° with under surface of 
stage, and little, if any, change made in the illumination, I now get 
easily a strong ' strice ' of transverse lines, and evenly distributed 
from end to end of the shell. Similar results obtain when viewing 
Nos. 18 and 19 on same Platte. This much for work with oblique 
light. Another fact is perhaps of still greater importance. The 
diaphragm may be bent so as to include any angle, say from 5° to 
50°, with lower surface of stage. We will assume, say an angle of 
45°. Now, by a proper disposition of the radial arm carrying. the 
mirror, arranging so that the diaphragm ' splits ' the illuminating 
beam, i. e. one half entering the wedge-shaped aperture while the 
other half is excluded, a beautiful effect of black-shadow illumination 
will at once be recognized, and the intensity of the shadows is easily 
graduated at the will of the observer, in two ways ; 1st, by a simple 
movement of mirror ; and 2nd, by combined movements of mirror and 
diaphragm. The effects thus obtained are innumerable and within 
the reach of any intelligent observer. This black-shadow illumina- 
tion gives promise of real value to those engaged in advanced in- 
vestigations of structure, in fact it will be found to be a new power. 
This simple and rude appliance has been for weeks a permanent fixture 
to the stage of any instrument, and the ferrotype plate seems to bear 
any amount of bend ng. Its fimctions are obvious, the iron plate 
V 2 
