224 
CORRESPONDENCE. 
Note hy Mr. Wenham. 
Anyone taking the trouble to interpret the above curious letter 
will admit that controversy should cease concerning apertures pecu- 
liar to Mr. ToUes. I have tried the ^th as in paragraph 3rd with 
lenses closed, and the focus on the front of a thickness of glass in 
water contact. As might have been expected, the aperture is the 
same, whether the glass is there or not — a practical exemplification of 
the first rule in optics. I have done with the -^th, which is here ac- 
cessible for trial by others if asked for. I am confident that the 
apertures will be found as I have stated and that the extra immersion 
angles claimed have no more real existence than the ISO*^ engraved on 
the object-glass to the delectation of such as are ready to believe, in 
spite of the focal distance and small diameter of front lens. 
Angular Aperture. 
To the Editor of the ' Monthly Microscopical Journal.' 
Boston, March 11, 1875. 
Sir, — In your Journal for March . current, p. 131, Mr. Wenham 
says, "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 ulti- 
mate emergent pencil was alike with all." The clause, "as I have 
stated before," is a mistake. I deny, with challenge, that he has ever 
so stated before in the pages of your Journal. 
This is what he has said, 'Monthly Microscopical Journal' for 
November, 1874, p. 223: "In measuring varying angles of aperture 
by the usual method, we take theni at all points of the adjusting 
collar, and do not place in front a thickness of glass suitable for that 
correction, because with a parallel plate of glass there is no per- 
ceptible difference. The angle at the crossing point of the rays is the 
same whether it is there or not. I stipulate that the edges of the 
stop shall be in the crossing point. If anyone thinks proper to 
introduce an intervening plate of glass, serving no purpose, he must 
focus through it, so as still to get the stop in the focal plane." 
Here is strong implication at least that he did not use " cover " to 
fill up the interspace. I suggest that he try it, or tell us what hap- 
pened when he did try it. He has not reported accurately. 
Let him use cover, the thickest the ^ih will focus through at 
"closed" the edges of the slit thus "in the crossing point" of the 
" corrected " rays, and I will bide the result. Because I Jmow the 
state of the case from irrefragable proof, in the first place, in my own 
hands, and I naturally expect he will get like results. Something 
more than 112°! 
Yours respectfully, 
E. B. ToLLES. 
