4 Reade, on a Double Hemispherical Condenser. 
measurement, that the illuminating pencils, after passing 
through the two hemispheres, may be made to fall upon the 
test objects at an angle of more than 150°, and thus produce 
illumination on a dark field even with the -fl^th. This 
obliquity, no doubt, increases the facility of dealing with 
such close fine lines as those of the Amician test and the 
Macrum. To check the latter has been described as “ a most 
blinding and heart-rending and brain-softening test but 
by using the double hemisphere, with the -y^th and C eye- 
piece, the lines and dots may be counted with ease. It is also 
not very difficult to make the whole battery of powers, from 
the T Vth to the -^-inch, available for the exposition of these 
tests. A ^-inch of exquisite workmanship, now in my pos- 
session, readily checks the Rhomboides, and a 4-th checks 
both this test and the Macrum. These powers were presented 
to me by my friend Mr. Wray, a Fellow of the Astronomical 
Society, who has devoted many years to the difficult task of 
annihilating the secondary spectrum in telescopic object- 
glasses of large dimensions, and I have witnessed his success 
with the highest gratification. 
The want of achromatism in the double kettledrum is not 
injurious ; on the contrary, the dispersion is beneficial rather 
than otherwise when the deep powers are used. With these 
the illumination is virtually monochromatic, and we may use 
at pleasure the red, yellow, or blue ray, by slightly altering 
the distance of the condenser. The blue ray, being the most 
refracted, and therefore falling on the test lines with greater 
obliquity, is decidedly the most effective on N. rhomboides. 
The pure light of a bright white cloud reflected from the 
plain mirror shows this very clearly. Webster’s condenser, 
somewhat similar to mine, and consisting of two lenses 
partially achromatized, is, in its present state, not more 
effective than my single hemisphere. Mr. Highley, who 
makes them, is now enlarging the angle of aperture, and thus 
extending the application of the condenser. 
The diaphragm-cap, properly constructed for one, two, or 
three apertures, as in the case of the single hemisphere, may 
be removed to the top of the second hemisphere, unless the 
fineness of the lines under examination require the condenser 
to be very close to the object. In this case a diaphragm of 
tin-foil or of thin brass properly pierced may be placed 
between the two hemispheres ; tin-foil has an advantage in 
its very easy management. By looking down the body of the 
microscope, when the eye-piece is removed, and examining 
the dimensions of the discs of light, it is seen, at a glance, 
whether one or other of the apertures require to be more or 
