Wen HAM, on Illuminating Opaque Objects. 55 
On a Method of Illuminating Opaque Objects under the 
Highest Powers of the Microscope. By F. H. Wenham. 
(Read March 26th, 1856.) 
Repeated experiments have shown, that it is a matter of 
extreme practical difficulty to contrive any method of con- 
densing light directly down upon an object, when viewed 
under an eighth or twelfth object-glass of large aperture. In 
the first place, the close proximity of the front lens and its 
setting, will only allow a thin conical disc of light, to find a 
passage towards the object, at an angle of seldom less than 
100°, or at an obliquity far too great to be practically useful ; 
and secondly, when the object is covered with thin glass, con- 
siderably more than half the light will be lost by the reflection 
from, the surfaces, the rays from which enter the microscope, 
and occasion an amount of glare and fog sufficient to obscure 
the object ; for these reasons I think that there is but little 
chance of obtaining any useful result in this direction.* 
The methods that I now bring before the Society are based 
upon an entirely different principle, which is not applicable 
to dry objects, but only to those mounted either in Canada 
balsam, fluid, or any other refractive medium. An experience 
of nine months warrants me in the assurance of its complete 
success, as a means of investigation — objects being brilliantly 
illuminated in a jet-black field, with an objective of 170° of 
aperture or more. 
The principle of operation consists, in causing rays of light 
to pass through the under side of the glass slip upon which 
the object is mounted, at the proper angle for causing total 
internal reflection from the upper surface of the thin cover ^ which is 
thus made to act the part of a speculum, for throwing the light 
down upon the under-lying objects, immersed in the balsam or 
fluid. 
As there will be no total reflection from the planes of a 
parallel plate of a refractive material, it is necessary to adopt 
some method for allowing the rays to enter the medium at 
such an angle as to cause total reflection from the upper sur- 
face. There are many methods of effecting this ; those which 
I now describe I have found to be the most practicable and 
useful : a, a, fig. 1, is a glass slide containing objects mounted 
in balsam ; Z>, thin glass cover ; c, is a right-angled prism 
* Since the above, Mr. Ross has shown me his ingeniouslj'-contrived 
Leiberknhn, applied to the highest powers for iUuminating uncovered 
opaque objects, and which performs most admirably ; to my mind un- 
doubtedly proving the fact, that the minute scales from the wings of 
butterflies, &c., are perfect cellular structures. 
