CORRESPONDENCE. 
151 
a smaller aperture will do best, viz. about -f of its focus, (for any power 
less than 300 decimal standard,) but the magnifier requires to be more 
free from aberrations.” 
Thus far Mr. Slack has seemingly got with the help of the Reflex 
Illuminator. 
Sir D. Brewster then presages the most modern developments of the 
microscope : “ For transparent objects a larger aperture is absolutely 
necessary ; and for some tests it should be equal to its focal distance, 
to show the cross strice between the lines on many of the scales, when 
the power of the instrument or lens is considerable.” 
The finest English, French, and American lenses are practical 
examples of the truth of this statement. 
Sir D. Brewster then says : “ It is worthy of remark, that the same 
aperture that with advantage will develope one class of objects will not 
show another with the same success.” 
This is evidence that there is room for both large and small-angled 
lenses. 
Against this we have Professor Abbe quoted : “ that we should 
look for improvement in the direction of making objectives of 3 or 4 
millimetres focus do the work now done by higher powers.” I main- 
tain in opposition to Mr. Slack that Zeiss’s lenses made “ under the 
direction of Professor Abbe ” do not substantiate his position. 
High -power microscopy has been hitherto very little indebted to 
reasonings a 'priori, from the practical impossibility of working exactly 
to formulae ; but the lenses, when made, can be put to the test of com- 
parison : this is what I did in a series of trials which I endeavoured 
to make exhaustive, and I placed the statement of them as evidence 
for the consideration of your readers. 
That Mr. Slack should seize upon Professor Abbe’s view of the 
question does not surprise me ; it is novel, even if it be not suscep- 
tible of practical proof. In these days of eager competition, novelty 
is at a premium. 
I am, Sir, your obedient servant, 
John Mayall, jun. 
To the Editor of the 1 Monthly Microscopical Journal .’ 
Ashdown Cottage, Forest Row, August 8, 1875. 
Sir, — I am again obliged by your giving me an opportunity of 
preventing a month’s misconception arising from Mr. Mayall’s in- 
accuracy. 
The statement regarding Surirella gemma, as shown by me at the 
Scientific Evening referred to, does not correspond with facts. Nume- 
rous Fellows saw the markings distinctly divided into beads, as in 
Dr. Woodward’s photograph. My apology related to loss of time 
in getting the illuminating apparatus into fair action, and to not 
obtaining quite enough light for the D eye-piece. A public room and 
a tremulous floor are not good conditions for doing, or seeing, any- 
thing difficult. Several well-known microscopists have seen the object 
YOL. XIV. M 
