I was working with an add-on +3 dioptre magnifying lens, the kind that screw
on like a filter, and dropped it. The lens came out of its mount but to my
surprise neither the mount nor the lens itself were damaged.

I found the retaining spring on the floor and worked out which way the lens
went into the mount, it sits against a flange with the retainer on top in a
slot inside the rim of the mount. I then set about replacing the retainer
on top, it is roughly a circle of thin springy metal that has been cut.

I tried putting one end in first and then working around to the other end
but each time, no matter how hard I pushed it into the slot in the mount,
when I got to the other end about 1mm was left overlapping. I tried pushing
the two ends in opposite directions so that they would slide past each other
down together into the slot. I tried putting the two ends in first and
working the curve into the slot, hoping that it would pop inside the rim.
Nothing worked. Clearly as it came out of the slot there must be a way to
get it in. In the end I gave up and cut 1mm off the spring. The lens is
now fine.

Is there some way a normal tool-using human can get this retaining ring in
or do you require special tools?

David