EYE-PRESERVING GLASS FOR SPECTACLES. 
9 
A mean of ten similar observations gave a value which was taken as the diather¬ 
mancy of the glass to dark heat. Its athermancy is obtained by subtracting this 
value from 100. In this way a multitude of glasses were tested, and recently the 
results have been plotted on a curve which was found to correspond closely with the 
result subsequently obtained by the radiometer method. 
The Radiometer Balance. 
In early papers on “Repulsion Resulting from Radiation,”* I showed that the 
blackened surface of a radiometer was repelled by all the rays of the solar spectrum, 
from the ultra-violet to a distance at the red 
end extending far into the ultra-red, the maxi- 
mum intensity being a little distance below the 
spectrum line A. 
An instrument was accordingly made based 
on the principle of the radiometer, and some¬ 
what resembling the apparatus described in a 
paper read before the Royal Society in 1875: 
“ On Repulsion Resulting from Radiation.’ t 
It is a torsion balance in which the beam 
moves in a horizontal plane. Figs. 1 and 2 
show the details of the instrument, the letter 
references being the same in each figure. AB 
is a thin glass tube, with a bulb at the end, B, 
and ground flat at the end, A. To the centre 
of AB is sealed an upright tube, CD, having an 
arm, E, blown to it for the purpose of attach¬ 
ment to the pump. FG is a very light arm of 
aluminium carrying at the end, G, a disc of 
silver-flake mica coated with lamp-black. At 
the end, F, is a small counterpoise to allow 
the arm to hang level. H is a glass stopper 
to the upper part of the tube, CD, which is 
widened out to form a cup to hold mercury. 
The stopper in the cup is accurately ground 
in the tube, as long a surface as practicable 
being in contact. The horizontal arm is sus¬ 
pended from the stopper by a bifilar suspension 
of fine quartz fibres, IJ, which are attached to the arm FG by an aluminium 
stirrup holding at its upper end a silvered glass mirror of one metre focus. The 
* ‘Phil. Trans. Roy. Soc.,’ 1876, Part II., vol. clxvi., pp. 355-361. 
t ‘Phil. Trans. Roy. Soc.,’ 1875, Part II., vol. clxv., pp. 533-4-5. 
VOL. CCXIV.-A. 
Fig. 1. 
c 
