CORRESPONDENCE. 
209 
An Error in Mr. Dod's last Letter. 
To the Editor of the * Monthly Microscopical JournaV 
Memphis, Tenn., U.S.A., August 17, 1875. 
Dear Sir, — Please do me the favour to correct a typographical 
error in my note published in the August number of ' M. M. J./ 
p. 100. The ToUes' " four-system " glass of ^^^th of an inch, should 
he ^jjth. of an inch, only. 
Yours truly, 
Albert F. Dod. 
A Query as to the Luminous Field in the Immersion Lens. 
To the Editor of the ^Monthly Microscopical Journal' 
Boston, August 24, 1875. 
Sir, — Your correspondent, Mr. Mayall, in the ' M. M. J.,' p. 96, 
alluding to the testimony of Mr. Wenham's Keflex Illuminator as 
deciding the " aperture question," says : " If he will try the experi- 
ment on Moller's Probe-Platte with the Eeflex Illuminator and a 
high-angled immersion lens, he will see a luminous field ; whereas, 
with a pneumo-lens he obtains a darh field. Whence comes the 
luminous field in the immersion lens if not from its having the power 
to collect rays which are totally reflected when the pneumo-lens is 
used?" 
I beg to suggest that this is not quite so definite a statement as is 
needed to make the conditions of the case clear. Will Mr. Mayall 
tell us what is the very least angle of such " a high-angled immersion 
lens " as will give the effects he names ; that is to say, of a dark field 
when dry (thus a " pneumo-lens "), and a luminous field when " wet," 
and becoming thus an immersion lens ? I hope for an explicit state- 
ment of the fact. 
Yours respectfully, 
R. B. TOLLES. 
English and Foreign Preparers of Microscopic Specimens. 
To the Editor of the ' Monthly Microscopical Journal.^ 
August 25, 1875. 
Sir, — Although in some instances foreign preparers supersede the 
English, it is but fair to state that the English excel in several 
instances the foreign preparers. 
Among the English preparers, Barnett, Cole, Enock, Norman, and 
Amos Topping take the lead in their specialities, although several 
amateur preparers produce preparations of the highest quality. 
Among foreign preparers, Bourgogne's (Charles, Eugene, and J. 
