CORBESPONDENCE. 
99 
make lenses tliat do not give the best attainable images, but the term 
" bad images " would be too strong. The lesson I have learnt from 
Zeiss' glasses is, that small and moderate-angled objectives can be 
brought to a point of perfection far beyond what is generally sup- 
posed, or what has been accomplished by any other optician whose 
work I have seen. I have no doubt English opticians will excel with 
small angles and great working distance when their customers ask 
them to do it. 
I remain, Sir, yours faithfully, 
Henky J. Slack. 
P.S. — Since writing the above I am further obliged by a sight of 
Mr. Hogg's letter. I regret that I am unable to understand some 
parts of it, but it seems as if he had not borne in mind the effect of 
the two kinds of aberration. In Ganot's ' Physics,' Atkinson's trans- 
lation, he will find that " when the aperture is larger (than 10° or 12°), 
the rays which traverse the lens near the edge are refracted to a point 
nearer the lens than the rays which pass near the axis. The pheno- 
menon thus produced is named spherical aberration by refraction." 
In chromatic aberration the violet rays are refracted to a point nearer 
the lens than the red ones, and the effect is a distortion of the image. 
The maximum of distortion will be where most rays are out of place, 
from failure of either or both corrections. 
The accuracy of my remark about considerable chromatic error is 
proved by the fact that Messrs. Powell and Lealand's last objectives, 
and those next preceding them, are much more perfect in chromatic 
correction than their earlier high powers, and likewise much sharper 
in definition. 
Mr. Hogg's last sentence will astonish physiologists and many 
others. With reference to the angles of the highest powers. Beck's 
2Vth was made with an angle (as per catalogue) of 140°, Powell and 
Lealand's -^^th. with 160°, 16° less than their old y^^th, and their J^th 
has an angle of 150°, moderate in proportion to its focal length. 
Mr. Wenham has informed us that the usual mode of estimating 
angles of aperture considerably exaggerates them, and I find my. 
J^th of Beck to be about 128° when measured with Mr. Stephenson's 
scale, and the angle taken when the edges of two real objects are 
distinctly seen. 
High-angled versus Low-angled Objectives.* 
To the Editor of the ^Monthly Microscopical JournaV 
Memphis, Tbnn,, June 25, 1875, 
Dear Sib, — Eeferring to your remarks "f on a brief paragraph of 
mine in a late number of ' American Naturalist,' I beg to offer the 
following : not to enter into any controversy regarding it, but merely 
as a fuller statement of the facts set forth in the previous note. 
* This letter was by a curious mistake addressed to Dr. Henry Slack. 
t ' M. M. J.; p. 258. 
