98 
CORRESPONDENCE. 
constructed whicli shall be at the same time both aplanatic and 
achromatic." An object-glass may therefore be achromatic and not 
aplanatic ; so that all chromatic aberration does not involve spherical 
aberration, and it necessarily follows that Mr. Slack's theory is not 
coherent, or consistent with actual observation and mathematical for- 
mula. This he will the more readily admit from the fact that in 
the objective which commands his admiration — the Powell and Lea- 
land's new ^th — the point of best adjustment for achromatism is not 
coincident with that of the best adjustment for aplanatism. I may say, 
I have been unsuccessful in my endeavours to discover an optician who 
believes " in any theoretical necessity for leaving considerable chro- 
matic error in order to ensure sharp definition." I have, however, 
heard opticians confess to the enormous practical difficulties in the 
way of constructing a high-power objective combining aplanatism and 
achromatism. 
Mr. Slack is in error in supposing that "large-angled glasses" 
were ever considered " nearly or quite useless for general purposes 
of natural history and physiological research:" physiologists were 
among the first workers with the microscope to recognize the value 
and importance of high-angled powers,* and in their investigations 
constantly employ them to obtain a magnification of from one to ten 
thousand diameters. 
I remain, Sir, your most obedient servant, 
Jabez Hogg. 
Keply to Mr. Mayall and to Mr. Hogg. 
To the Editor of the ^Monthly Microscojpical JournaV 
AsHDOWN Cottage, July 14, 1875. 
Sir, — I am obliged by a sight of Mr. Mayall's letter, as it enables 
me to correct the misapprehension it might occasion without delay. 
The first part is a wordy misrepresentation of my paper on angular 
aperture. The question raised by me related to the smallest aper- 
tures capable of showing lined objects. This would not be supposed 
from the totally irrelevant comments of Mr. M. I showed that Zeiss' 
C, angle 48°, resolved P. hippocampus, and that his D, angle 68°, 
sufficed to exhibit Surirella gemma in dots. 
I do not think Professor Abbe's remarks will suffer from Mr. M.'s 
erroneous statement that "he is simply saying what he can as an 
advocate for the work Zeiss is producing under his direction." I feel 
no doubt of their value, and believe our opticians will find them well 
worth consideration. I can only attribute Zeiss' higher powers 
"breaking down at about 1000 x " to bad arrangement. His |^th, 
-j'^^th, and ^^^^th worked well in my hands, and in others', with Ross's 
1) eye-piece, and even E. 
Mr. M. is right in supposing me to mean that the demand of 
English observers for extreme angles has encouraged opticians to 
* [\Vc certainly <lisagree with Mr. Hogg as to this point.— Ed. 'M. M. J.'] 
