764 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
good quality and uniformly thick, it is extremely unlikely for anything like super¬ 
position to occur after so long a joimiey, and often the useless image is not even in the 
field—never with a high power, 
I must say that the satisfactory behaviour of the optical arrangements is due to the 
skill of Mr. Adam Hilger in working glass to true planes. It is a difficult matter, 
for he says they are apt to change after being taken off the tool. He has several 
times tried to improve on the first set of four mirrors he sent me, but without success. 
Those now sent usually have radii of curvature from three to eight hundred metres, 
and are not at all satisfactory, though their curvature is too small to detect with a 
spherometer.* Judging by their behaviour the original set must be very good. I 
expect they are of superior, or older glass. They are, of course, mounted so as not to 
strain them in the least. 
38. To support the optical frame over the whirling machine, with the plane of the 
light between its two disks, a substantial wooden structure was erected, from brick 
piers coming up through the floor, entirely independent of any support from the 
whirling table or its stone pier. To this the frame was fixed, and it was supplied 
v/ith a lid and floor, to box in the disks and make them easier to drive. The lid had 
a domed cavity for the top of the spindle ; the floor had a hole edged with thin india- 
rubber sheeting to permit the spindle bearing to pass through air-tight without 
transmitting vibrations. 
In order that the semi-transparent plate might not be affected by the blast from the 
whirling plates, a couple of optical glass windows were inserted to screen it com¬ 
pletely. I feared lest the blast would have some effect upon the mirrors themselves, 
but they were substantially backed by thick brass plates bearing steadily against 
three accurate screws in a strong frame (see Plate 31), and I hoped it might not, 
39. On the 21st of July, 1891, a first complete spin was taken. The bands being 
vertical, the cross wires were set on one of the dark ones, and the speed increased until 
a shift of three bands might have occurred. The shift actually observed was 1^ band, 
and they recovered their old position very fairly when the motion ceased. Strongly 
suspecting this shift to be spurious, I had the brushes of the dynamo reversed, and 
later in the same day was able to take a reversed spin. Tlie shift was approximately 
the same in amount and in the same direction. The centrifugal force of the blast 
evidently did affect the mii’rors. Pressing their supporting plates by hand, a similar 
shift could be got: the screws did not hold them with absolute firmness, and it 
seemed as if the end held by only one screw yielded more than the end held by two, 
BO as to produce a minute tilt. 
To see if the pressure of the blast distorted the frame as a whole, or only tilted the 
mirrors, the box was made air-tight, like an organ chest, and air was pumped into it 
* I measure it by focussing a telescope on tbe image they give of an object at a considerable known 
distance. 
