What is the little semi-circular thingummy with a slot on old Nikon lenses?

This is a question that I have been meaning to ask someone since the 1970's but never got around to it:-)
I mean the little metal piece screwed onto the aperture ring as seen on this lens (scroll down for the bigger picture)

http://www.ebay.com/...0780407775

Added (1). Ahaa! So it is for metering! Does the body in turn use this lever to move the aperture ring to set auto exposure or is it simply to feed aperture info to the metering system for manual exposure?

It is a linkage to the meter, used to tell the camera what the lens maximum aperture was. It connected with a projection on the camera body. After attaching the lens to the body you had to twist the aperture ring first one way, then the other and all was set. You could focus and meter at full aperture and the camera would automatically stop down to the correct aperture for the shot. I believe this device was the first to allow open aperture metering on Nikon cameras (but I'm open to correction). Before 1such became common you had to focus and meter at full aperture, then manually stop the aperture ring down to that required, before taking the shot. And if you forgot, as we sometimes did, you had a lovely overexposed shot!

Ah, the ol' Nikon gunsight…

It is called an "Aperture Index" or AI, and Nikon AI lenses have them.

They couple to a lever called an "Aperture Feeler" on vintage Nikons so that when you turned the aperture ring, the camera knew what the setting was to properly change the exposure metering. It was a mechanical means of doing so.

This was in the days before electrical sensor lenses where the aperture position is transmitted via electrical circuit.

But, since there are still a lot of high quality AI lenses available, and due to Nikon's commitment to compatibility, Nikon has reintroduced the AI Feeler, beginning with the D7000. Now you can automatically meter those old AI lenses in Aperture Priority on a new Nikon DSLR. The first auto exposure cameras were Aperture Priority only - even those from Canon - so the lens is only capable of Aperture Priority (as well as manual of course).

If you look at item #12 on page #3 of the D7000 manual, you will see that yes indeed, it has a Aperture Feeler (which they refer to as a Meter Coupling Lever).

At this point, I'm not sure what other Nikon DSLRs have this feature, but hope that they will all have them again. The AI lenses were among some of the finest prime lenses ever made, and the ability to reuse them fully again has to be exciting in the eyes of any long-time Nikon owner.

It's the light meter coupling for the early Nikons, such as the F, F2 and Nikkormats. Casually known as "rabbit ears."

Later SLR bodies, which used auto-indexing would have something called the Meter Coupling Ridge, and AF bodies all came with CPU contacts. On these later cameras the "rabbit ears" serve no function at all, however Nikon lenses are always made with one generation of full backward compatibility. I. E.AI lenses work with non-AI bodies, non-G type lenses work with AI bodies, but non-G type lenses won't work fully on non AI cameras.

Confusing? A little until you study the history of Nikon cameras.

Additional Info:

There's a little prong which sticks in between the two ears, the turning of the aperture dial moves the prong to various positions, which tells the body what aperture the lens is at. There's an additional coupling at the back of the lens which then performs the actual stop down.