In my own experiments the same reasoning shows that if this possible error
made a difference of 1 per cent. in Foucault's work (and his result is
correct within that amount), then the error would be but .00003 part.



Distortion of the Revolving Mirror.


It, has been suggested that the distortion of the revolving mirror, either
by twisting or by the effect of centrifugal force, might cause an error in
the deflection.

[Illustration: FIG. 15]

The only plane in which the deflection might be affected is the plane of
rotation. Distortions in a vertical plane would have simply the effect of
raising, lowering, or extending the slit.

Again, if the _mean_ surface is plane there will be no effect on the
deflection, but simply a blurring of the image.

Even if there be a distortion of any kind, there would be no effect on the
deflection if the rays returned to the same portion whence they were
reflected.

The only case which remains to be considered, then, is that given in Fig.
15, where the light from the slit _a_, falls upon a distorted mirror, and
the return light upon a different portion of the same.