768 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
bein^ usually selected. Then read their positions, and started a spin. When spinning 
at a given speed, I reset the wires and read them afresh. Then stopped the motion, 
reset them, and rea,d once more. 
Lastly, shifted the vertical wire till it coincided with the X> and thus obtained the 
interval between them, or the width of one band corresponding to yellow light. 
Fig. 12. 
Appearance of tLe bands with the two micrometer wires set in position. 
This process was quite satisfactory, but it is not worth while to quote all the 
readings ; for it was noteworthy that up to 800 revolutions no perceptible shift of 
the middle band had occurred, but at that speed the first tremor set in, and although 
there was supposed to be no contact anywhere between machine and frame, yet it was 
transmitted somehow, through the earth at any rate, and sometimes produced an 
effect obviously spurious. Often the first spin of a set had the effect of shaking 
things into place, and subsequent spins were better. This was the case, for instance, 
on March 16, when the following readings were taken. 
March 16. Bands very clear and sharp. The first spin was spoiled by a tremor 
which set in at 800 revolutions and fogged the bands while it lasted ; when they 
re-appeared they had been shaken aside and broadened. No shift had occurred up to 
800. It was afterwards found that the semi-transparent plate was not held quite 
tightly enough, and that it was affected by tremor with just the above effect. I give 
only the results of this first spin ; not the details. 
First Spin. 
The width of a band was 
< 
when originally at rest . 
when in motion at 1250 revs. . 
when at rest again . . . . 
91 divisions. 
170 
144 
3 J 
33 
The middle band 
r shifted 
’ L returned. 
37 divisions, to the right. 
22 
33 
This was very bad, but went on with another attempt. 
Second Spin. —Direction of motion as before, viz., such as to assist the reflected 
half-beam. 
