166 THE NORTHERN MICROSCOPIST. 
In the above we readily concur, and though we would have 
seen the matter demonstrated with a little more grace than the 
author has inspired into his work, yet we must admit that the 
treatise is one which may enable students to get a firmer grasp of 
“ Angular Apertures” than they have had the chance of getting 
before. Even an unbiassed reader would have great difficulty in 
believing that the treatise was not one long tirade against Mr. 
Wenham, but the valuable information contained in it more than 
counterbalances this fault. Of course there is some excuse for 
this, if we remember the Aperture controversy which was carried 
on some years ago in the “ Monthly Microscopical Journal ;” still 
there are two methods of demonstrating a scientific fact, and we 
regret that men of science often take the wrong one. 
By far the most valuable portion of the paper commences at 
page 13, in which the question is discussed, ‘‘ How should angle 
of aperture be measured?” The author then proceeds to show 
(working with a student’s Y% inch by Tolles) how this operation 
may be performed, repeating the process with a duplex sixth by the 
same maker, having a balsam angle of 95°. Seeing that Professor 
Abbe has published quite recently a paper bearing upon the sub- 
ject of Apertures,* it is well to call our readers’ attention to 
it: the subject should be thoroughly understood by the student, 
and all we desire is to call attention to trustworthy works which 
stimulate progress. 
NORTHERN SOCIETIES. 
BOLTON Microscopicat Society. Hon. Sec.: Mr. W. Rideout. Meets on Friday 
evening once in each month. 
CHESTER Naturat SCIENCE ASSOCIATION. Hon. Sec. of Microscopical Section : 
Mr. J. D. Siddall. 
DONCASTER MrcroscopicaL Society. Hon. Sec.: Mr. M. H. Stiles. Meets 
twice in each month. 
HALIFAX. A Private Society. Members meet at each others houses. 
LEEDS. No Microscopical Society in existence. 
LIVERPOOL Microscopical Socrety. Hon. Sec.: Mr. I. C. Thompson. Meets 
First Friday in each month. 
MANCHESTER Mricroscoricat Society. Hon: Sec.: Mr. C. L. Cooke. Meets 
First Thursday in each month. 
MANCHESTER Cryptocamic Socrety. Hon. Sec.: Mr. Thos. Rogers. Meets 
Third Monday in each month, at Old Town Hall, King Street. 
MANCHESTER Science Association. Hon. Sec.: Mr. J. Percival Yates. Meetings 
Second and Fourth Tuesday in each month. 
NEWCASTLE-ON-TYNE. NortH or ENGLAND MicroscopicaL Soctety. Hon. 
Sec.: Mr. M. H. Robson. Meets Second Wednesday in each month. 
NOTTINGHAM Literary anp ParLosopHicaL Society. Natural Science Section 
pea oeaes Fortnightly on Wednesdays. Hon. Sec.: Mr. A. H. Scott 
e, I. 
* Journal of the Royal Microscopical Society, April, 1881. 
