( 90 ) 
COEEESPONDENCE. 
The Measurement of Angular Aperture. 
To the Editor of the ^Monthly Microscopical Journal.^ 
Boston, November 28, 1874. 
Sir,— By the same mail that takes this I send you three object- 
slides, to illustrate the mistaken measurement of the angle, taken in 
air, of Mr. Crisp's ^-inch objective, and publicly promulgated through 
your Journal. You will readily ascertain that the diatoms mounted 
on the cover over the " slit " of each slide are part in the balsam and 
part dry mounted, while a portion of the slit is uncovered and there 
gives the very narrow angles, under circumstances described in either 
of two articles sent you the present week. Please bring these 
slides to the notice of those interested, in your discretion, and good 
pleasure. 
Respectfully yours, 
Robert B. Tolles. 
Note hy Mr. Wenham. 
Having seen a proof of the above, I beg to append a few lines, as 
a formal reply is not called for. The subject has been thoroughly 
explained and need not be repeated ad nauseam. That discussion 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. As^for " the mis- 
taken measurements of the angle taken in air " of 180° in the ^th, 
surely he does not intend to cast aside his laurels, which he can wear 
so complacently — may they never wither ! 
The slit, if properly used, is an essential adaptation for measuring 
large angles accurately. Its use is to prevent rays that give false 
indications from entering the front lens behind the focal point, and 
even if this 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. 
"The Society's Universal Screw." 
To the Editor of the ''Monthly Microscopical JournaV 
January 5, 1875. 
Sir, — As you have been so good as to quote my remarks on the 
above from ' Science Gossip,' you will permit me, perhaps, to add a 
few words on the subject. 
The gauge of the Society's " universal screw " seems to have been 
considered as forming — (1) the narrow gauge adopted by Beck, and 
