DK. OLIVER LODGE ON ABERRATION PROBLEMS. 
775 
Third spin. Reversed. Direction to help reflected beam. Results :— 
Width of yellow band. 
Shift of middle band. 
stationary. 
65 
* 
8 to right 
1250 revolutions. 
63| 
7 return 
Stationary. 
631 
8| „ 
1250 revolutions. 
63 
Stationary. 
64| 
^ ») 
1250 revolutions. 
61| 
H „ 
Stationary. 
63 
Average 63'4 
Average 7‘7 
The shift was a trifle greater than before, but so was the tremor. Another shift 
taken same day-gave a shift of 9 and a return of 12. The effect of the tremor seemed 
increasing. 
45. Since such shift as is observed is apparently independent of the width of tJre 
bands, it is manifestly well to reduce its apparent significance by having the bands 
very broad. It might be doubtful how far accuracy of setting could be accomplished 
with the spider line in the midst of a very broad band. To test this the following 
observations were made. 
Accuracy of Setting of Micrometer Wire. 
The bands were broadened, by tilting the back mirror a little, until it took two 
revolutions of a micrometer head to carry a wire from one to the next. The light 
was going three times round, and the cover glasses were on. The vertical wire was 
then carefully set in the centre of the middle band (it is always easy to tell the 
middle band, even without the colour of the others, by their concertina-like motion 
to and from it when the corner of the frame is pressed) and the X was set as near as 
possible on the yellow of the first band to the left. The position of this colour was 
not so well defined as when the light only went twice round, especially when the disks 
were spinning; neither was the middle band quite so clear then as when they were 
stationary. It is the setting of the vertical wire in the middle band that is really 
important. Readings of both micrometers being taken, the wires were displaced at 
random and then re-set and re-read. This was done several times. The following 
are the results of successive re-settings on the same bands. 
