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CORRESPONDENCE. 
A Monstrous Form of Aulacodiscus. 
To the Editor of the * Monthly Microscopical Journal .’ 
Baltimore, June 1G, 1875. 
Dear Sir, — Allow me to offer you, for publication if you please, a 
photo of a drawing of mine representing a disk of Aulacodiscus Ore- 
rjonus with two centres, conveying the idea of two half disks, each 
excentric, joined along a line of suture. 
I am aware that this class of objects is not extremely rare, but 
such a valve as the one in my possession must give rise to questions 
of interest with regard to the manner of their production. 
I have the honour to be, dear Sir, 
Yours very truly, 
Christopher Johnson. 
[The photograph sent is not sufficiently distinct to make an en- 
graving from. But it represents the two centres very well, and is 
certainly a curious departure from the ordinary form. We learn, 
however, from Mr. F. Kitton that monstrosities are not very rare in 
this department of botany. — Ed. ‘ M. M. J.’] 
Mr. Slack’s Paper on Angle of Aperture. 
To the Editor of the ‘ Monthly Microscopical Journal .’ 
224, Regent Street, London, June 23, 1875. 
Sir, — So long as the question of angular aperture was being dis- 
cussed by rival opticians — the one anxious to have it believed that 
everything worth knowing about the construction of objectives was 
known to, or discovered by, himself ever so many years ago — while 
the other, not content with having the splendid testimony of Dr. 
Woodward and Professor Rcnel Keith in his favour, must needs venture 
to speak in his own behalf with almost disastrous effect on his own 
lucidity, — I say, so long as the question of angular aperture was 
being discussed by rival opticians, we spectators might well stand by 
and be amused by the prodigious display of personalities. But when 
an Honorary Secretary of the Royal Microscopical Society comes for- 
ward with a paper “ On Angle of Aperture in Relation to Surface 
Markings and Accurate Vision,” we expect (at least some of us do) that 
interesting information will be given, and that some of the latest de- 
velopments in microscopy will be discussed with all the fulness of 
knowledge to which his official position gives him access. 
The interesting information in the Hon. Secretary’s paper seems 
to be that he “ has a microscope in his library, opposite a north 
window ; ” that the instrument “ is pointed like a telescope towards 
the clear sky,” and, “ with Powell and Lealand’s immersion i-th, their 
last but one, a perfect [?] definition is obtained of P. hippocampus.” (In 
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