10 
SIR WILLIAM CROOKES ON THE PREPARATION OF 
vertical tube is blown out and the edges ground Hat at the part where the mirror 
hangs; a flat piece of glass is cemented to it, forming a window through which pass 
the entering and emerging index beams of light. The end A of the horizontal tube 
is left open to allow of the adjustment of the arm in its stirrup, and then it 
is sealed with a flat piece of glass cemented on. The stopper, H, is lubricated with 
drops of burnt indiarubber so that it can be smoothly rotated to allow the arm to be 
brought accurately to zero. 
Fig. 2 shows the arrangement of the appartus fitted for testing the samples of glass. 
The radiometer balance is enclosed in a wooden box having two holes opposite the 
mirror and the end of the blackened disc at the torsion arm. Great precautions must 
be taken to avoid all extraneous radiations from acting on the black disc; a slightly 
conical card tube, as narrow as the angular movement of the ray of light will admits 
is attached to the window at K in front of the mirror, S, and another to the bulb at L 
opposite the black disc. 
F, G. Radiometer arm with blackened mica disc. 0. 
H. Ground glass joint. (Fig. 1.) P. 
I, J. Quartz fibres. (Fig. 1.) Q. 
K. Guard tube in front of mirror S. R. 
L. Guard tube in front of radiometer disc. S. 
M. Nernst slower. T. 
Shutter. 
Black mica screen. 
Carrier for holding glass specimen. 
Aluminium screen. 
Concave mirror, one metre focus. 
Glass specimen under examination. 
T1 ie heat radiation used in these tests is emitted from a Nernst glower, M, enclosed 
in a metal box with an open end. In front of the glower is an aluminium screen, N, 
pierced with a centimetre hole. A shutter, 0, can be moved up and down by an arm 
close to the observer. The shutter screen is made of a piece of cork an inch thick, 
having on each side a plate of polished aluminium. In this way the heating up of the 
shutter when it is obscuring the ray from the glower is effectually prevented. At P 
is a frame for supporting the piece of black mica, and at Q is a sliding carrier holding 
the piece of glass under examination. This is so arranged that it can be drawn out 
and another piece of glass put in without causing any jar. Behind the glass is an 
