falls on the other side of the slit, and that both these images would come
into view, and the line midway between them would coincide with the true
position. No such effect was observed, and would be very unlikely to
occur. If the lens was imperfect, the faults would be all over the
surface, and this would produce simply an indistinctness of the image.

Moreover, in the latter part of the observations the mirror was inverted,
thus producing a positive rotation, whereas the rotation in the preceding
sets was negative. This would correct the error mentioned if it existed,
and shows also that no constant errors were introduced by having the
rotation constantly in the same direction, the results in both cases being
almost exactly the same.



Periodic Variations in Friction.


If the speed of rotation varied in the same manner in each revolution of
the mirror, the chances would be that, at the particular time when the
reflection took place, the speed would not be the same as the average
speed found by the calculation. Such a periodic variation could only be
caused by the influence of the frame or the pivots. For instance, the
frame would be closer to the ring which holds the mirror twice in every
revolution than at other times, and it would be more difficult for the
mirror to turn here than at a position 90° from this. Or else there might
be a certain position, due to want of trueness of shape of the sockets,
which would cause a variation of friction at certain parts of the
revolution.