It's utterly amazing what is patentable.  Kodak patented gravity
in their Carrousel slide projector, the one with the round tray that holds 80
or 130 slides.

        The tray's bottom has a slot that is normally in the 0 position where
no slide is stored.  Mounting the tray atop the projector releases a latch
on the bottom of the tray such that the bottom rotates freely and moves the
slot under each position.  In operation, the bottom is clamped firmly such
that the slot is always over the slide gate mechanism.  The slide-change
mechanism moves the tray around one position at a time and the slide which
is over the slot is allowed to fall down in to the gate where it is held
still for showing.  When it is time for the slide to change, the gate opens,
a lifter rises and pushes the slide back up in to the tray and stays there
to make sure that gravity doesn't let the slide drop back down until the
tray turns and the slot is under a new slide at which time the lifter drops
back down and gravity takes over.
I have never read this patent, but I imagine that what was actually
patented was the use of gravity as the force to move the slide in to the
gate.  I saw many other brands of slide projectors that used other mechanisms
for doing this.  about all one could say is that they would be the ones of
choice on the Space Shuttle if you could get them to work long enough.
About the only thing that ever happened to Kodak's mechanism was that it
sometimes got out of time due to dirt mixing with the grease on the cam
stack and the levers would then operate to late or their springs couldn't
fully return them to their proper rest position causing slides to get chewed
or bent when the tray tried to turn with a slide stuck halfway out of the
slot, but it was a generally very reliable mechanism.  The autofocus servo
used in those projectors is a brilliantly simple but effective bit of analog
electronics which probably got some engineer a good promotion, I hope.

        The slide change mechanism would be the perfect sort of device to
modernize with a PIC and a few solenoid levers since the key to its operation
is mechanical timing and not actual complexity.

Martin McCormick