760 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
A perforated steel disk, pierced with circles of 3, 5, 9, 15, and 25 holes, and fixed 
on the spindle, gives the speed quite accurately by the musical note from an air jet 
through it, or even a card held against it. But the particular octave was occasionally 
checked by a wheel speed-counter; and recently one of the mercury speed-indicators 
of Messrs. Napieii Bros, has been fitted on, and works very well for approximate 
readings. 
The current is supplied usually from about twenty small storage cells, which I 
happened to have. The regulation is effected by one of Rousseau’s very convenient 
carbon rheostats introduced into the armature circuit, the whole voltage of the battery 
being always supplied to the field magnet. 
When the whirling machine arrived it was mounted on a stone pier in the middle 
of my laboratory, a pier built up from the sandstone rock beneath, and it was bolted 
down to timbers embracing the pier. The disks were then put on, a pulley gearing 
from the ceiling being convenient for raising and lowering them; and after some 
preliminary work the battery was applied. The field magnet took only 4 or 5 
amperes from the 20 cells with gas-engine going ; 10 amperes began to drive the 
machine slowly, and 30 amperes gave a speed of 800 revolutions up to 1400 revolu¬ 
tions a minute, though the last figure was not reached before the disks were encased. 
The volts actually on the terminals were from 20 to 30. The bearings, however, were 
not at this time quite easy, and less power now suffices. Thus the numerous spins at 
1250 revolutions per minute taken during March, 1892, required, to maintain full 
speed, 385 watts, viz., 27'5 volts and 14 amperes. 
35. The optical parts were the first to arrive (about June, 1891), and were the sub¬ 
ject of much preliminary experiment. After a few alterations, such as planing down 
the base of the telescope-holders to a better angle, there was no particular difficulty in 
getting the light of an oxyhydrogen lantern to go three times round the frame and 
then enter the telescope. A dark room and strong light were useful for making this 
adjustment, for the course of the ray could then be tracked without difficulty ; but 
after the adjustments were made, a paraffin lamp could be substituted for the lantern 
without too much enfeebling the image. On now inserting in its place the semi¬ 
transparent mirror, a second image made its appearance. Removing the lens of the 
collimator, the two images of the “ slit,” or square aperture usually employed, were 
seen very small and separate. By adjusting the semi-transparent mirror, which only 
moved the reflected image, the two were made to coincide. The collimating lens was 
then re-inserted, and the telescope focussed for infinity. Bands at once appeared. 
They were usually slant, but when best defined became horizontal. In these early 
trials vertical bands were only got with difficulty; they seemed to prefer being 
horizontal. Shaking the wdiole table on which the frame rested did not hurt them, 
but pressing gently on the wooden frame distorted them. It was easier to get them 
with light which had gone only once or twice round the frame, but there was no real 
difficulty with three times round. Introducing the wedge compensator and turning it 
