6 Reade, on a Double Hemispherical Condenser. 
the test lines under examination require. It is absolutely 
necessary to have this entire command over the length of the 
apertures and the strength of the illuminating rays, and 
this is one method, among others, which might suggest them- 
selves to different observers, for securing this control. Two 
narrow strips of brass having small V apertures may also be 
conveniently used for the independent regulation of the two 
lower and larger apertures, over which they should be moved 
by a fine adjustment. The two hemispheres themselves 
are, as it were, the mere raw material ; the secret of success 
lies in the proper management of the condensed and conver- 
gent pencils. In the diaphragm-cap the apertures are always 
of one and the same size, and hence arises the only practical 
difficulty in using it for the most delicate tests. By the present 
arrangement the precise amount of light from either aperture 
is obtained at once. 
As cheapness is the order of the day, I may state that the 
cost of the material for these diaphragms is “ three a penny.” 
I cut the apertures with a pair of scissors, and after filing the 
edges, when necessary, I blacken them with oxide of copper. 
A few crystals of sulphate of copper must be dissolved to 
saturation in two measures of strong nitric acid and one 
measure of water. The diaphragm, raised to a temperature 
of about 300° over a spirit lamp, is dipped in the solution 
and immediately dried over the flame. On rubbing the 
frost-like surface with a small brush a clean and permanent 
black stain remains. 
No light can fall upon the upper hemisphere but that 
which passes through the apertures of the diaphragm on the 
lower ; and as the apertures are generally about -jAths of an 
inch deep, while the diameter of the lens itself is one inch 
and three quarters, it is evident that the outer portion of the 
hemisphere is alone called into use. There is, however, a 
limiting circle, beyond which parallel rays of light are not 
transmitted to the upper hemisphere. It may be an improve- 
ment to bring this portion into play, and Mr. Ross is con- 
structing a condenser of three lenses, in which every ray that 
falls upon the first lens will be available for illuminating the 
object. 
What I have aimed at in the double kettledrum is a dis- 
tinct and well-defined pencil of light, falling at right angles 
on the lines of the object, and at the necessary obliquity for 
resolution and definition ; and I trust that my experiments 
will not be considered fruitless. I have often used the hemi- 
spherical combination with much pleasure and satisfaction, 
and, at the request of the Council, I now place R before this 
Royal Society. 
