3 
On a Double Hemispherical Condenser for the Micro- 
scope. By the Key. J. B. Reade, F.R.S. 
The hemispherical condenser, consisting of a single lens 
with suitable diaphragms, was described to the Society in 
May, 1861 ; and at the Exhibition in 1862 it received 
“ honorable mention” by the appointed jury. This con- 
denser was proposed as a cheap and not inefficient substitute 
for more expensive apparatus, inasmuch as it enabled my 
■^-inch object-glass to bring out with great distinctness the 
markings on Pleurosigma angulatum and other similar tests. 
This fact was proved by ocular demonstration to Mr. Boss, 
whose skill had overcome the optical difficulties in producing 
a large angle of aperture ; but my assertion of its power was 
at first received with a smile of doubt. 
The single hemisphere has only a small angle of illumina- 
tion when the pencils converge from near the margin of the 
lens to the object. Hence the value of its application to 
deep powers was questioned by many, though, so far as my 
own experience goes, the -|hh an ^ Tyth are not beyond the 
scope of its illumination. Still, something like tact may be 
required — to use the language of objectors unskilled in its 
manipulation — in order to put the single lens through all its 
paces with all the powers. That the work can be done, but 
done with difficulty, is, perhaps, somewhat less than the merit 
which an inventor aims at, for, like poetry of a certain kind, 
if the sense does not stare you in the face no one cares for it. 
The felt want, then, is greater obliquity of the illuminating 
pencil. With the single “ kettledrum,” the angle of illumi- 
nation is rather less than 90°, and we all allow that this is 
too small for the easy exhibition of the more difficult tests. 
In the coarser lined objects the shadows are easily obtained 
when the pencils of illumination have a comparatively small 
angle ; but where the lines and markings are extremely thin 
the angle must be nearly doubled, that the shadows may be 
well defined and sufficiently intense. 
I propose to obtain this greater obliquity by placing a 
smaller hemisphere upon the larger one, and then the two 
kettledrums, turned to fifths, may be made to play ujion any 
scale. For practical purposes it is near enough the mark to 
say that one lens, when placed upon another, adds its own 
obliquity to the obliquity of the rays which fall upon it. 
Neglecting, therefore, the smaller effects due to thickness and 
density, it is certain, as a matter of fact, and proved by 
