DR. OLIVER LODGE OH ABERRATION PROBLEMS. 
767 
band ; all others being readily moved by trivial strains. Always now set the vertical 
spider line in the centre of the middle white band and watched for its shift. 
To see if the irreversible and lagging (very slight) shift still observed could be due 
to the action of hot air on the glass windows, I arranged to throw into the drum 
air heated by passing through a metal pipe in a gas flame. The hot air-stream 
flickered the side bands about, but did not alter the middle one. 
Fig. 11. 
General view of the whirling machine, and independent support of optical apparatus, in action. The 
speed indicator is seen on top of front post, ammeter and voltmeter on framework. The long 
upper girders support the glazed drum which encloses the disks and secures the mirrors from the 
blast. Telescope, &c., are at back, not seen ; boiler-plate screen for observer just visible. Every¬ 
thing independent of floor, and no contact between anything on central altar with anything on 
gallows framework. 
Quantitative Experiments. 
41. I now began a series of actual readings, the plan adopted being to set the 
vertical line of the micrometer in the middle of the white band, and the X of the 
micrometer on some definite colour of the first bright band on the left : the yellow 
the frame and tilt the mirrors about a horizontal axis; this kind of tilt being well calculated to shift the 
vertical bands. 
I tried also the horizontal set of bands without any floor to the frame (both with and without the 
drum), and perceived no shift in the middle one of these either, due to the motion of the disks. 
