98 
CORRESPONDENCE. 
constructed which 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. 
Reply to Mr. Mayall and to Mr. Hogg. 
To the Editor of the ‘ Monthly Microscopical Journal' 
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 dow n at about 1000 x ” to bad arrangement. His ^th, 
Jg-th, and ^.th worked well in my hands, and in others’, with Eoss’s 
L 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 
* [We certainly disagree with Mr. Hogg as to this point. — Eu. ‘M. M. J.’] 
