MECHANICAL CONNEXION BETWEEN ETHER AND MATTER. 
1G3 
fixed to opposite walls of the room, sending the light round a large oblong instead of 
a square, and letting two sides of this oblong pass through the channel between the 
disks. (The arrangement of this experiment is shown in fig. 8 .) Meanwhile, we 
dismantled the machine and sent the disks back to Mather & Platt to be fitted 
with a third one for electrification. (26th Oct. 1893.) 
If there were good reason to push the experiment still further (and for tlie 
present I see no such good reason), I should be disposed to attempt placing the disks 
in an air-tight chamber, kept exhausted by a mechanical oil pump, so as to do away 
with the greatest part of the troublesome air phenomena. 
A possible reason for the concertina effect, and for the slight residual irreversible 
shift sometimes observed, suggests itself in the gradation of density in the air 
between the disks, due to centrifugal force. To estimate its magnitude under any 
circumstances, we may consider the equilibrium of an element dm of air at radius r 
and write :— 
Toi^ dm — dr . rdO, 
dr 
or 
prdr = dp = kdp, 
whence the density at any radius is 
p = p^e 
Hence, for disks a yard in diameter making 3000 revolutions a minute, the density 
at centre is about fths of that at circumference; and the change of density per 
centimetre breadth of beam, at a radius of 1 foot, is 
= -425 X 10“^; 
dr 
which, if P — 1 be taken as proportional to p, gives dp about equal to '23dp ; or say 
lO"*^ as the difference of refractive index, on either side of a beam of light 1 centi¬ 
metre broad, in the region of the mean light path. This is equivalent to the effect 
of a difference of temperature, in the air on either side of the beam, of a -^th of a 
de gree centigrade. 
The gradation of density could therefore cause a distinct effect if the beam of light 
had an odd number of paths between the disks ; but since there are in our case an odd 
number of reflexions, and therefore an even number of paths, with the beam laterally 
inverted at each reflexion, the effects must very nearly compensate each other. 
If by reason of some want of symmetry there was on the whole a centimetre length 
of path uncompensated by a laterally inverted portion elsewhere, the corresponding 
retardation due to gradation of density would be a millionth of a centimetre, causing 
an irreversible shift of - 5 - 0 -th of a band. This cause may therefore account for part 
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