CORRESPONDENCE. 
299 
surfaces, and that which deals with the interior substance of tissues, 
will no doubt construct glasses to meet that difference. 
In common with all really interested in the perfection of micro- 
scope objectives, I cannot but greatly admire the beautiful perform- 
ance of recent high-angled lenses in the display of diatoms. There 
cannot be a doubt we owe very much to their constructors, and it 
is to my mind probable we shall owe more still. On the other hand, 
as a practical histologist I confess with regret that these lenses, 
whether equal to others or not in histological work, are at least greatly 
inferior in this respect to the power they themselves exhibit in the 
case of diatoms. If this be so, and high-angled glasses have so to say 
already done their best for histology, it is imperative that other 
constructions be fairly tried, and the catalogue exhausted if need be, 
in the attempt to arrive at the greater excellence, which is still so 
much to be desired. I see no other way in which this is likely to 
be done than by the encouragement and reward of makers in such a 
competition as I suggest. 
I am, Sir, obediently yours, 
R. Branwell, M.R.C.S.E., F.R.M.S. 
The Immersion Aperture Question: Reply to Mr. Wenham. 
To the Editor of the ‘ Monthly Microscopical Journal.'' 
224, Regent Street, London, November 11, 1875. 
Sir, — M r. Wenham states that the root of the present immersion 
aperture question is to be found in his contributions to the Quarterly 
Journal of twenty years ago : I must express my dissent. The question 
then discussed had reference to the illumination of balsam-mounted 
objects, — had no reference whatever to the matter now in hand, which 
concerns immersion lenses only, in relation to their image-forming 
aperture capacity. 
The argument maintained by Dr. Woodward is confirmed by the 
use of the Reflex Illuminator with those immersion lenses which claim 
from their construction to refract via water image-forming rays of 
greater obliquity than those corresponding to the maximum trans- 
missible by a dry objective. In the experiment alluded to by me, 
the Illuminator is used as a means of providing rays incident on 
the object in balsam (and consequently on the field) at greater in- 
clination than the angle of total internal reflexion between balsam 
and air ; these rays emerge from the cover-glass conditionally on 
there being water contact between the cover and the objective front, 
but with air as the external medium they do not emerge. Mr. Wen- 
ham says that when fluid is introduced between the lens and cover 
“ . . . .all total reflexion is gone ; ” it is this very fact which gives 
its value to the experiment. For what is the inclination of those 
rays which pass through the water and are so refracted by the ob- 
jective as to produce a luminous field? Is it greater than corresponds 
to the maximum transmissible by a dry objective ? 
In describing the action of the Reflex Illuminator, Mr. Wenham 
said, “ . the most oblique [light] that we can obtain by ordinary 
VOL. XIV. Y 
