Note on image formation by rotating mirrors
Timothy John Smith · Proceedings of the Physical Society · 1943
When we view a scene through a system of mirrors, some or all of which are moving with uniform angular velocities about fixed axes, the motion of the image we see may have components of several periods. If the image moves with uniform angular velocity about a fixed axis, this velocity is twice the sum or difference of some or all of the angular velocities of the mirrors; odd multiples of these angular velocities are excluded. It follows that if we wish to bring a moving scene to rest - for instance, to project on a screen a picture recorded on a cinematograph film - by means of mirrors, it is essential to use a half-speed reflector.