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COEKESPONDENCE. 
An Explanation from Mr. Tolles. 
To the Editor of the ^Monthly Microscopical Journal.'' 
Boston, Ftbrm7-y 13, 1875. 
Sir, — In your last issue Mr. WenLam grievedly says : " That dis- 
cussion with Mr. Tolles is useless, is proved by his article, page 21 of 
this Journal for January, 1875, where, in order to show that I am 
wrong, he first tries the slit without and then with water contact, in 
the last case measuring not the internal cone, or immersion angle, but 
the increased emergent angle from the under surface of the slide," 
i.e. the air angle of the immersion objective. Just what and all I was 
talking about. 
Now drolly enough he says, in closing his commentary, that even 
if this, the focal point, "falls exactly on the under surface of an 
intervening plate of glass in water contact, it will still cut off stray 
rays within the glass, and give the true air angle, as one of final 
emergence." 
Very well said. Now let Mr. Wenham interpose the thin plate of 
glass, viz. "cover" so near to 0-013 of an inch thick as the ^th sent 
Mr. Crisp will work through with imter contact at " closed" adjustment 
of the objective and then putting the light down through the micro- 
scope tube accordingly as he says is the correct way, then measure 
the emergent pencil and report " the true air angle as one of final 
emergence." 
This will be very fair, and I pointedly invite it in accordance, if 
you please, with your own suggestion appended to my article of 
August last, ' M. M. J.,' p. 65. 
I repeat, — what seems unnecessary to say again, — I talk here (as in 
my last) of the emergent air angle of that immersion objective only, — 
and I promise him more than 112° 1 
But again,—" ONE HUNDRED AND EIGHTY DEGREES ! " 
In quoting from Mr. Wenham's article, p. 113 this Journal, March, 
1874, I, in my reply, alluded to the above-quoted inscription as being 
given by Mr. Wenham in " small caps," and Mr. Wenham thought I 
meant his " stops " over the ^th. — My fault. 
I suppose they are "screaming capitals" more descriptively. 
All this about 180° I abate for the present. If it should ever happen 
to be proven to Mr. Wenham that more than 82° of corrected " balsam " 
angle existed in an objective, say 90°, I presume he would admit that 
such an objective must have an air angle " up to 180°." 
But now let all that be apart. Mr. Wenham admits the thin plate 
of glass which before he did not use, and, water contact. Let us know 
then (and thus) if the "true air angle" of "final emergence" is 112° 
only, or a " rational and wholesome angle " somewhat above that. 
Yours respectfully, 
R. B. Tolles. 
