CORRESPONDENCE. 
131 
Mode of Measuring Mr. Tolles' Objective. 
To the Editor of the ' Monthly Microscopical Journal.' 
Boston, January 26, 1875. 
Sir, — The ' Journal' for January is just to hand. I see Mr. 
"Wenham is heard from again, but not to the point. He quotes from 
a late editorial in the ' Naturalist/ * and saddles a quotation from my 
statement, upon the editor, who will not want the burden. I am the 
offender all the while ; the editor was only telling of it. " Some one " 
lies not concealed. 
And now to the point. Andrew Ross observed the " negative " 
aberration of the covering glass to be the cause of indefinition in his, 
as otherwise used, well-defining object-glasses. Andrew Eoss invented 
" adjustment for cover " by closing the systems to a point giving 
balancing amount of" positive" aberration as the remedy. 
Well, the wonder has come to pass that Mr. Wenham closes the 
systems in measuring angle, and persists in ignoring the aberration. 
He refuses to put in 'twixt slit and lens a compensating cover, and 
on the other hand does not give any results of measure at open point, 
or contact of lens with slit, nor, which is only fair, measurements at 
approximations to open point, uncovered, with water contact. 
If he would but do what is now just indicated, he would get an 
angle much above " 112°," say nothing about 180°, for the nonce, but 
a good approach to that baleful number nevertheless. 
Yours respectfully, 
R. B. Tolles. 
Note hy Mr. Wenham. 
I really cannot continue this aperture question with Mr. Tolles on 
the position and passage of rays, for when argument at length resolves 
itself into mere repetitions, it is a sign that it is well-nigh exhausted. 
I repeat (as I have stated before) that I did try the ^th of Mr. Tolles 
with several thicknesses of glass in front, and whether these were 
superadded in water contact or not, the aperture, or ultimate emergent 
pencil, was alike with all. There has been much quibbling about 
differences at open and closed points. This reminds me that I have 
omitted to mention that the aperture of Mr. Tolles' ^th at open point 
was 108°, closed 112° — a difference of 4° only. I have never argued 
that the slit cut off any degrees of true aperture, for it will admit rays 
up to 180°. My argument was that in the ^th Mr. Tolles' angles do 
not exist, but that the maximum in air was 112°, and in balsam 68° ; 
and whether the focal point lays on the front lens, or cover, or much 
beyond either, or is wholly immersed in fluid, it will still admit all 
rays from that point, and there I always place it. 
However the case may be in America, it is generally believed here 
that enormous errors have been promulgated, in the measurement of 
* Not " anonymous," as he makes it appear. 
