758 
DE. OLIVEE LODGE ON ABERRATION PROBLEMS. 
them simply cylindrical and unbraced at the centre hole.'"'' The bevelling and the 
clamping ought to afford margin enough to run them up to 6000 (though the makers 
recommend no speed over 4000) ; but hitherto I have not spun them at more than 
3000 revolutions to the minute, and even at this speed, I now have a screen or 
sentry-box of double boiler plate (consisting of a small iron boiler cut in halves 
longitudinally and one half fitted inside the other) for the observer, whose eje is in 
the plane of the disks, to look through. 
The use of steel disks is sufficiently justified by the high speed they will stand, but 
it may be also held that iron is the most probable great constituent of the earth; and 
further, that as there are so many other ways of experimenting on transparent matter, 
opaque matter is appropriate in this experiment. 
Since steadiness of rotation was very essential, I arranged to rotate the disks 
horizontally on a vertical shaft balanced on a steel j^oint in an oil vessel, and with a 
slightly flexible or elastic bearing near the top, so as to get the whole to sleep like a 
tee-totum ; and in order to avoid any lateral strain, as of driving belt, to drive electri¬ 
cally by a dynamo armature on the axle itself (fig. 9). 
Messrs. Mather and Platt were good enough to undertake this part of the work, 
using their smallest size Manchester dynamo as motor, with its axle set up on end, 
the armature being wound with less wire than usual, and being extra strengthened 
against centiifugal force by steel wire. The ordinary bearings of the dynamo remain, 
with oiling wicks inserted, and hence there is a little nicety required to get all three 
bearings in a precise line. It is also needful for the shaft to be vertical, to avoid any 
attenq^t at gyroscopic precession. 
Fearing lest some damage to the disks might occur from sudden application of too 
great moment to the armature, especially by reason of some accidental jamb or other 
sudden stoppage, I imitated a device employed in some milk-whirling machines, and 
introduced a wooden cup or friction coupling betv^een the top of the d3^namo shaft 
and the bottom of the disk spindle. The cup being made of hard wood grips the 
rounded end of the disk spindle, and thus applies to it sufficient rotating moment, 
but permits slip in preference to violent acceleration. This plan is, I think, on the 
whole advisable, and is certainly a safeguard. It may seem to spoil the tee-totum 
idea, but the dynamo axle, which is supported at each end, and constrained to rotate 
about rather a long axis, is to be regarded as a driving machine only; the “top” 
begins from the wood cup upwards. The brass collar of the upper bearing is let into 
an india-rubber cylindrical socket, so as to afford some very slight play ; and just above 
the wooden cup is a safety collar or loose guide not touching the shaft. 
My assistant, Mr. B. Davies, had some trouble in getting and keeping the two 
shafts accurately aligned, especially since any w^ear of the w^ooden cup tends to throw 
* Tlie connexion between tenacity and maximum peripheral speed for a ring is T = pi "; while, for a 
uniform disk with a small hole in it, Ewing adapts Gkossmann to show that it is T = j (3 p) pv 
where p is density of material, and p is Poisson’s ratio. See ‘ Nature,’ vol. 43, pp. 462, 514, 534. 
