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 (To realize that such readings fail to show the air effect it may be simplest to write down ideal readings that would show it exactly, viz. :—

if they had been obtained.

Or the following would do equally well, though less obviously :—

94.4 94.6 94.4 95.2 94.4

where there is an irreversible shift of half a division ($$\tfrac{1}{120}\lambda$$) superposed upon the reversible effect.)

After the overhauling of the foundations, in September, the bands were beautifully distinct, and there was no tremor. There was now no clearly perceptible shift of the middle band, but still there was a concertina action, shown by a broadening of the bands during spin, thus altering the scale of wave-length. The following extract will serve to illustrate this :—

Here the constancy of the number 7 means that no change could be seen, but the readings are not averages, nor was the wire reset, and nothing less than a whole division shift would have been observed. The air effect would require $1⁄2$ a division shift. The existence of the concertina effect was held to render useless an attempt to take a serious set of averages; and by no means could it be wholly got rid of.

Ultimately we decided to do away with the optical frame so close to the disks altogether, and to arrange the mirrors at a distance, out of the blast, on brackets