64i B/EAPBj on a New Hemispherical Condenser. 
diaphragms to stop out the light in the right place of a 
Gillet^s, or perhaps still better, from its greater angle of 
aperture, a PowelPs condenser, we should approach perfection 
in resolving difficult markings under the deepest powers. 
My old black-ground illumination, which led to the forma- 
tion of valuable condensers by Messrs. Shadbolt and Ross, 
may be produced with very good effect by the hemisphere and 
a single aperture ; and I feel sure that the members of our 
society will be much pleased with the brilliant definition and 
detail of a scale of Podura under this illumination and the half- 
inch object-glass. I have in my possession the same scale 
Avhich my old and valued friend, Andrew Koss, saw with his 
first achromatic -^th, in his little workshop at St. John's, 
Clerkenwell, and I shall never forget the expression of his 
astonishment. But the present half-inch is superior in all 
respects to that -^th. 
It is now generally known that I offer the hemispherical 
condenser as the special adjunct of the new half-inch object- 
glass of 90° aperture. Mr. Thomas Ross sent me his first 
object-glass of this new construction, for examination and 
report ; and I believe, like many others, he hesitated to give 
implicit credence to my account of its working. As he was 
ignorant of the power of the " kettle-drum condenser,^^ he 
thought that the asserted resolution of that old microscopic 
nebula, the P. angulatum, under so low a power as a half-inch, 
even of large aperture, indicated the partiality of friendship 
rather than the severity of honest criticism. Accordingly I 
was summoned before a microscopic jury, consisting of Messrs. 
Leonard, Millar, Lobb, and Roper; and after sufficient and 
careful examination, Mr. Leonard, as the judge, decided that 
I might '^take a rule nisi'' 
As the half-inch and the condenser had not only not flinched 
from any fair work, but had even trespassed on the domain of 
the ^th and the 4^th, I thought that I would show at last what 
they could not do; and therefore, without the slightest expecta- 
tion of taking anything for my pains, I placed on the stage of 
the microscope a slide of the Amician test, the Navicula 
rhomboides, which was kindly presented to me by Mr. Powell, 
whose fine -Y^i^ with its unequalled achromatic condenser, 
reveals the exquisite skill which is bestowed on this almost 
invisible work of the great Creator. It does one good, both 
mentally and morally, to review such a work as this ; and, to 
my astonishment and delight, I witnessed its resolution under 
ray new arrangement. It is necessary, in this instance, to 
use a deep eye- piece for attaining the requisite amplification ; 
and as eye-pieces are instruments for measuring the imper- 
