8 On the Aperture of Object-glasses. By F. H. Wenliam. 
phenomenon, especially in sucli a case as that under consideration. 
I am very glad that what I said in my address has been the 
means of drawing attention to this question, since we cannot expect 
to arrive at perfectly satisfactory conclusions, unless the subject 
is examined by various observers, using independent means of 
research. 
III. — On the Aperture of Object-glasses. By F. H. Wenham. 
The slit that I have employed for cutting off the lateral rays of an 
object-glass (which indicate an erroneous aperture beyond the true 
angle), consisted of a clean line cut through a thin coating of black 
varnish. As it is a difficult operation to lay this on uniformly, and 
it has besides an objectionable thickness, 1 now make use of a slit 
constructed as follows. A 1 x 3 slip of glass is moved about over 
the flame of an ordinary petroleum lamp, till the black deposit 
nearly ceases to be transparent. This coating has but little coherence, 
and particles are swept before a scribing point, so that a clean cut 
line cannot be made ; but if a drop of turpentine is caused to flow 
gently over the smoke deposit and then evaporated by heat, the 
film has some consistence, and a narrow clean cut slit can be made 
through it, with the keen point of a penknife drawn along a 
straight edge ; a thin glass cover is now laid over the slit, and 
Canada balsam run beneath it by capillary attraction aided by heat. 
A slit prepared this way has some advantages. It is protected 
from dust and injury. It enables the object-glass to be tested for 
either dry or immersion under the proper adjustment in either 
case, for the usual cover thickness. This adjustment can be easily 
made on the edges of the slit itself or any detached particle. 
Finally, the rays of the pencil are refracted outwards to a less 
angle through a slit mounted in balsam, which consequently may 
be narrower than an air slit, without the risk of interfering with 
very oblique rays. 
I am quoted in the last Journal * as having many years ago 
affirmed that Professor Kobinson's method of measuring apertures 
is by far the best. It makes no diflerence whether Professor 
Stokes " admits the validity of this " or not ; the question must 
be decided by fact, and not by force of opinion. If everyone else 
has hitherto been wrong, so have I ; and I cannot admit the con- 
cluding sentence in Mr. Hogg's review in the last Journal, that 
" with Professor Eobinson's modified method no slit is required, 
and most conclusive and reliable results will be obtained ; " for the 
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