124 MR. W. CROOKES OK REPULSION RESULTING PROM RADIATION. 
or disks are capable of rotation. When exposed to light the movements take place 
in accordance with theory. 
THICKNESS OF THE LATER OF MOLECULAR PRESSURE. 
455. Whilst experimenting with the otheoscope it was found that, for a given 
exhaustion, the nearer the reacting surfaces were together the greater was the speed 
obtained. This was in accordance with previous results (437). In a note on the 
theory of the radiometer which I had the honour of communicating to the Royal 
Society in November, 1876,'" I briefly described a piece of apparatus by which I was 
able to measure the thickness of the layer of molecular pressure generated when radia¬ 
tion impinged on a blackened surface, enclosed in an atmosphere the rarefaction of 
which could be varied at will. 
The apparatus which is represented in fig. 27 (plan and elevation), consists of a 
torsion balance, fitted with a glass suspending fibre and reflecting mirror, as already 
O T f n A 
ft 
<pi 
Plan 
Fig. 27. 
described in previous papers (102, 186, 198, 221, 259). To one end of the beam 
is attached a disk, a, of silver-flake mica (238), lampblacked on the face. The portion 
of the tube in front of this disk is opened out, and a short piece of wider tube is sealed 
on at right angles to the beam, as shown in the plan. The outer end of this additional 
tube is ground flat, and closed with a piece of clear plate glass, d, cemented on. In 
front of the blacked disk is a thin and perfectly clear disk -of mica, twice the diameter 
of the black disk. This is connected by a rigid arm with a lead plate, c, curved to 
fit the inside of the tube, and capable of sliding freely to and fro. The clear mica 
screen, b, is easily and accurately adjusted any desired distance from the blacked disk, 
a, by gently tapping the under part of the glass tube on one side or the other of the lead 
weight. The space between is measured with a millimetre scale. The torsion beam 
is controlled by an outside magnet acting on a small magnetised needle attached to the 
reflecting mirror of the beam. 
456. The friction of the lead plate when sliding to and fro in the glass tube, especially 
* Proc. Roy. Soc., No. 175, 1876, toI. xxv. p. 310. 
