766 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
shook too much to give good observations. At 1800 revolutions, a shift of half a 
band occurred, Reversing the rotation, the shift was appreciably the same. It was 
probably due to pressure of the blast on the frame itself. 
Hence, ordered a light wooden circular drum to be put inside the frame to catch 
the air pressure, and also to keep hot air from the mirrors if possible. 
Davies took out the old wooden socket from the axle, and made a new boxwood 
one to connect the two shafts. We also now got the large boiler-plate protector for 
the eye end of the observing telescope to protrude through. It just shows at the 
back of fig. 11, 
Resilvered the mirrors, and got the optical arrangements more perfect. Also 
ordered a much heavier mass of metal to whirl in subsequent experiments. For the 
present, however, we go on with the steel disks, 
40. During February, 1892, the various preparations wmre made. The drum was 
a very satisfactory maliogany structure, octagonal outside and circular inside, with 
long slit windows glazed with optical glass to catch the blast. The drum was fitted 
inside the frame, with |-th-inch clear space all round between the tw^o. It was 
secured to the floor of the frame and joined with the lid so as to be fairly air-tight. 
Still observed a shift, often of about fths band. At speeds above 2000 the bands 
usually disappeared, from shaking. Removed the lid of the frame for the air to escape, 
and the shift was smaller. It was specially noticeable that it lagged. It took a 
little time to reach its maximum, and when the disks stopped the bands continued to 
recover for some time afterwards. It was just as if a solid had experienced a strain 
and sub-permanent set. All the effects were irreversible. Gradually made the drum 
more air-tight and supported it by wooden bars from above. A shift of ^th band was 
still got at a speed of 1000, with the light three times round. Strains in the drum 
still seemed to be transmitted somehow to the frame. 
Made a saw-cut all round the floor of the frame, so as to leave the floor on the 
drum Instead of on the frame; also supplied a lid to the drum, and supported it by 
this lid from long wooden girders, as shown in fig. 11, keeping it and its supports 
everywhere out of contact with the frame or its supports, except the upright 
posts from the tables.* Studied the bands more particularly and observed the 
double hyperbolm system. Found also the great advantage of the middle white 
* Quite recently (MarcL, 1892) I liave taken a series of spins witkout tke di-um, and witk no cover 
glasses on except one over tlie front mirror, soiiietliing being there necessary to protect from wind the 
semi-transparent plate, which is of course extremely sensitive. 
By this time, as described above, the bottom of the square frame had been removed, being attached 
to the drum instead, and we now found that this floor must have been the cause of most of the trouble. 
For without it the blast produced no effect on any of the bands, and neither shift or concertina action 
occurred, up to a speed of 800. No motion of the middle band could be detected, the light going 
twice round. 
It follows that the blast did not take effect on the mirrors, nor directly on the frame, but that by 
exhausting the air near the middle of the box it must have sucked and bent the floor sufficiently to strain 
