Reade, on a Double Hemispherical Condenser. 5 
less deeply cut. In the one case the little lappet of tin-foil 
can be so doubled as to shorten the aperture, and in the other 
it may he cut deeper and thrown further back. To obtain 
these very slight but not slightly important variations, and 
in a moment, is an advantage which minute observers will 
readily recognise. It corresponds, in fact, to the gentle 
strengthening or diminishing the gas-lights when the pair of 
achromatic prisms is dexterously arranged by our friend Mr. 
Tomkins, in order to give to rectangular lines the force of 
illumination indicated by their thickness. Mr. Tomkins, to 
take the Rhomboides as an example, after placing it perpen- 
dicularly as to the major axis of the stage, proceeds to bring 
out the transverse lines with the one prism, and then, shut- 
ting off its light, acts on the longitudinal lines with the other. 
The same admirable method of working may be adopted in 
the case of the kettledrums by placing a bull’s-eye condenser 
in front of the lamp. By a little alteration in the position 
and height of this lens, the upper aperture, or that at right 
angles to it, may be easily and separately used for the inde- 
pendent exhibition of the transverse or longitudinal lines ; 
the two pencils of light are then worked together, as through 
the two prisms, for the final resolution of the valve. 
In my own condenser a slit has been made in the brass- 
work between the lenses for the insertion of the thin brass 
diaphragms without the removal of the condenser, an arrange- 
ment with which the Waterhouse diaphragms in photographic 
combinations has made us familiar. With respect to these 
diaphragms, I have made a new arrangement which gives 
immediate and ready command over the length of the V 
apertures, and therefore over the power of the illuminating 
pencil for rectilineal tests. The method is this — on the sur- 
face of the lower hemisphere place a circular diaphragm, 
having apertures at right angles to each other, and extending 
from the circumference to within a quarter of an inch from 
the centre. On another circular diaphragm of the same size 
draw diameters at right angles to each other, also draw lines 
from the extremity of one diameter to the two extremities of 
the other, and with a pair of scissors cut off the two lunes, 
thus forming a right angle upon a semicircle. This rectan- 
gular diaphragm is placed in the slit of the tube between the 
condensers, so that the vertex of the right angle divides the 
space between the two long Y apertures of the lower dia- 
phragm. When pushed home it nearly shuts up the apertures 
of the fixed diaphragm ; but by gently drawing it out, and 
moving it a little sideways, if necessary, we can obtain with 
the utmost nicety just that length of either aperture which 
