154 
CORRESPONDENCE. 
College of Surgeons of Edinburgh, January 17, 1868, " On the Atmo- 
spheric Germ Theory," " I have employed for these investigations the 
^th of Eoss, and frequently a -j^th by the same maker, and the immer- 
sion No. 10 of Hartnack with varying powers of from 600 to 800 
diameters linear, and I have confirmed my observations with a lens 
made for me by Messrs. Powell and Lealand, of ^^th of an inch focus, 
whereby I obtained a magnification varying from 1250 to 2000 dia- 
meters linear. *My observations were made in 1841 and 1842, and 
carefully repeated and extended to October, 1864, and they have been 
since repeated by my assistant, Dr. Eutherford." 
The high powers used by Dr. Beale, Dr. Bristowe, Dr. Bennett, 
Dr. Payne, Mr. Lister, Mr. Tomes, Dr. Burdon Sanderson, Dr. 
Pritchard, Mr. Lankester, Dr. Michael Foster, Dr. Klein, &c., together 
with a host of French and German physiologists, are, it is well known, 
lenses of large angular aperture ; and they, in their published works, 
constantly write of Eoss's Jth and -j^^h, of Powell and Lealand's 
y^g^th, ^Qth, and gV^h, or of Hartnack's Nos. 10, 11, and 12 immersions 
(t55 T¥5 ^t)? ^® being the lenses with which their finest work has 
been done.* And, lastly, I would point to the wonderful series of 
photographs of muscular tissue, blood-corpuscles, &c,, by Dr. Wood- 
ward, of America, all of which were produced by the aid of high 
powers with the largest angle of aperture. Can Mr. Slack produce 
or indicate any work done with low-angled powers that will for a 
moment compare with these photographs ? 
I remain your obedient servant, 
Jabez Hogg. 
Observations on Mr. Slack's Opponents. 
To the Editor of the 'Monthly Microscopical JournaV 
SiK — " Trying Zeiss's Nos. 1, 2 and 3 immersions (=ith, yV^^j 
and sV*^) ^®®P oculars, I find .... the image with 
these comparatively low-angled objectives breaks up with any magni- 
fication beyond abuut 1000 diameters." 
Such is the verdict of Mr. John Mayall, jun., in his observations 
on Mr. Slack's paper on Angular Aperture, which appeared in your 
last number. • 
Would Mr. Mayall be " surprised to hear " that the linear magni- 
fying powers of a y^th and J^th are, with even a C eye-piece, 1500 
and 2500 respectively, and that an ith has, with a No. 4 eye-piece, 
a power of 1440, according to Smith and Beck's catalogue. 
* On the question whether an increase of angular aperture is a real advantage 
to the pliysiologist, Dr. Leopold Dippel, in his work ' Das Mikroscop und seine 
Anwenduiig,' vol. i. j). 36, says : " The result obtained by the immersion system 
is tljorefore equivalent to an increase in the angle of aperture. This arrangement 
is consequently a real step in advance, especially with regard to the most difficult 
physiological investigations, as the use of my Hartnack's object-glass convinces me 
more completely every day." 
