274 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
EXPERIMENTS WITH POLARISED LIGHT. 
259. I have long endeavoured to carry out a suggestion made by Professor 
Maskelyne during the discussion of the first part of my paper “ On Repulsion 
resulting from Radiation,” at the Royal Society, the evening of December 11, 1873, 
viz . — that I should suspend a plate of tourmaline in a vacuum, and see if different 
amounts of repulsion were produced by allowing light to fall on it after passing through 
another tourmaline, which could be rotated so as to alter the plane of polarisation of 
the incident fight. 
Three separate sets of experiments were tried. The results are the same in each, 
and as they have been obtained under different circumstances, and are interesting 
from a theoretical point of view, I will briefly describe each apparatus. 
Fig. 5. 
The apparatus first used consists of an ordinary torsion apparatus in an inverted 
T tube, as shown at fig. 5, a, b, e, f, made of glass tubing cemented together at e, 
and ground flat at the end a. In the centre is blown a circular hole c, and another 
hole, c', is at the end ; the edges of these holes are also ground quite flat, a, c, and c 
are sealed up by cementing plates of glass a, d, d', on them, g is the connexion to the 
mercury-pump. The beam h i is of glass drawn from square glass tube ; j Tc is the 
glass torsion fibre ; Jc is a silvered glass mirror. The plate of tourmaline is attached 
to the beam at i, and a glass counterpoise is fixed at h to balance the tourmaline. 
In the first experiments a plate cut from a crystal of tourmaline was used at i, but 
it was found that it became so electrical when a ray of fight fell on it in the vacuum 
