CORRESPONDENCE. 
263 
never to have known what the word definition really meant till I saw 
this glass, so beautifully clear, sharp, and distinct were all the details. 
I certainly never saw any objective that even approached it ! 
I may add that, throughout the whole trial it was matched with my 
•j T th immersion, which is a glass of no mean capabilities. 
On the other hand, this description of objective can never 
seriously compete with English manufacture, and for the three 
following reasons : 
1. Its inordinately high price. 
2. The strong prejudice in England against all objectives unpro- 
vided with correction-arrangement. 
3. Its comparatively weak magnification. 
I ought to have mentioned that, though nominally a inch, it 
is really only a T ~- inch ; and that there is a great lack of neatness 
and finish in the brasswork. 
Yours faithfully, 
W. J. Hickie. 
Mr. Mayall’s Letter. 
To the Editor of the 1 Monthly Microscopical Journal.' 
London, October 11, 1875. 
Sir, — Mr. Mayall appears to have been afflicted with the desire 
that I should be wrong in the immersed angle of aperture question, 
and now relieves his mind by attempting to revive the controversy. 
In this I have said all that I consider necessary, or intend to say. 
Mr. Mayall admits his inability to deal with the question, and 
consequently submits it to “ one of the highest mathematical autho- 
rities in England.” If the high mathematical authority, instead of 
taking his information second-hand from Mr. Mayall’s version, had 
looked directly at my writings on the subject, he would have found 
at the root and foundation of it all, a paper dated more than twenty 
years ago, in the £ Quarterly Journal of Microscopical Science,’ wherein 
I pointed out that any angle in an object-glass must necessarily be 
reduced by immersion in fluid or balsam, and showed how by means 
of a special frontal adaptation full aperture on balsam objects could 
be obtained. Therefore, however consoling to Mr. Mayall the opinion 
of the high mathematical authority may be, his anonymous verdict 
will not be considered important by others. 
As Crito observed, Moller’s Probe-Platte used with the reflex illu- 
minator has so little to do with the question, that a slip of plain glass 
would answer the same purpose, supposing that there is anything 
important in the experiment. The diatoms in Moller’s Probe-Platte 
are mounted in balsam, and whether they adhere to the cover, or slide, 
or float in the medium, practically there will be no difference ; and 
