266 
CORRESPONDENCE. 
me the only copy I possess, and for the courteous letter which accom- 
panied it. 
The other work I have mentioned, that by Otto Miiller, is one of 
smaller pretensions, and restricts itself to a comparison of fifty-one 
not very modern objectives ; amongst which are forty-seven German, 
three American (by Grunow), and one French lens (an old y^^th inch 
by Chevallier, 1847). 
The German glasses are confined to those by Hartnack, Beneche, 
Gundlach, Zeiss, and MoUer, while many eminent names are con- 
spicuous hj their absence, e. gr. Plossel, Seibert, Schroder, Schieck, 
Wappenhans and Merz ; to say nothing of Hasert, with whose lenses 
Dr. Schumann did all his admirable work. English objectives also 
seem to be utterly unknown to him, — at least he never mentions them. 
Some may remark that I have omitted Nobert from my supple- 
mentary list of names, who somehow enjoys a high reputation in 
England as a maker of objectives ; but the plain truth is, no one 
in Germany knows him in that capacity. As a ruler of test-lines he 
is well known, but his countrymen have hitherto shut their eyes to 
his merits as a maker of lenses. I believe also he has never exhibited 
such at any of the competitive exhibitions which take place every 
year at some one or other of the large German towns. Germany 
indeed is full of small men who put " Optiker " after their names, on 
the strength of their selling spectacles which they do not make. I 
might also have omitted Merz, as he figures rather as a maker of 
telescopes than of microscopic objectives. 
Yours faithfully, 
W. J. HiCKIE. 
KouGH Measures of Angular Aperture. 
To the Editor of the ^Monthly Microscopical Journal.'' 
Hartley Couet, Beading, May 14, 1875. 
Dear Sir, — On a former occasion I drew the attention of the 
Society to the possibility of measuring apertures by means of minia- 
tures. 
The evident principle on which this must be done is the detection 
of image-forming rays, and this should not be a blurred or confused 
but a really good image. 
The method which I adopt is somewhat novel. Eemoving the 
condenser, an adapter is fitted to its upper part so as to receive an 
object-glass front downwards. The microscope is then used with a 
low power for looking down the interior of the objective fixed below. 
A 3-inch giving a low power, armed with a C eye-piece, is generally 
sufficient. Thus arranged the instrument is for greater convenience 
placed horizontally. A board is erected on a level with its axis, ruled 
with radiating lines terminating in a divided arc. On this arc are 
two carriers for two small benzine lamps, regulated to give small, 
