234 Mechanical Action of Light. [April, 
being sealed up in glass and perfectly exhausted. To the 
centre of oscillation a glass mirror is attached. 
Now a glass fibre has the property of always coming back 
to zero when it is twisted out of its position. It is almost, 
if not quite, a perfectly elastic body. I will show this by a 
simple experiment. This is a long glass fibre hanging verti- 
cally, and having a horizontal bar suspended on it. I hold the 
bar, and turn it half round ; it swings backwards and for- 
wards for a few times, but it quickly comes back to its 
original position. However much twist, however much 
torsion, may be put on this, it always returns ultimately to 
the same position. I have twisted glass fibres round and kept 
them in a permanent state of twist more than a hundred 
complete revolutions, and they always came back accurately 
to zero. The principle of an instrument that I shall describe 
farther on depends entirely on this property of glass. 
Instead of using silk to suspend the torsion beam with, I 
employ a fibre of glass, drawn out very fine before the blow- 
pipe. A thread of glass of less than the thousandth of an inch 
in thickness is wonderfully strong, of great stiffness, and of 
perfect elasticity, so that however much it is twisted round 
short of the breaking point, it untwists itself perfectly 
when liberated. The advantage of using glass fibres for 
suspending my beam is, therefore, that it always returns 
accurately to zero after having tried an experiment, whilst I 
can get any desired amount of sensitiveness by drawing out 
the glass fibre sufficiently fine. 
Here, then, is the torsion apparatus sealed on to a 
Sprengel pump. You will easily understand the construction 
by reference to the diagram (Fig. 4). It consists of a hori- 
zontal beam suspended by a glass fibre, and having discs of 
pith at each end coated with lamp-black. The whole is 
enclosed in a glass case, made of tubes blown together, and 
by means of the pump the air is entirely removed. In the 
centre of the horizontal beam is a silvered mirror, and a 
ray from the eleCtric light is reflected from it on to a scale 
in front, where it is visible as a small circular spot of 
light. It is evident that an angular movement of the 
torsion beam will cause the spot of light to move to the 
right or to the left along the scale. I will first show you the 
wonderful sensitiveness of the apparatus. I simply place 
my finger near the pith disc at one end, and the warmth is 
quite sufficient to drive the spot of light several inches 
along the scale. It has now returned to zero, and I place 
a candle near it. The spot of light flies off the scale. I 
now bring the candle near it alternately from one side to the 
