This is the BR-2A lens reversing ring, you can buy it used from
B & H for $12. It has Nikon F mount on one side and 52mm thread on the other.
This picture was taken with the D70s and Sigma 50mm macro, with this lens you get 1:1 magnification on a 35mm camera, on the D70s you get 1.5:1 magnification because of the sensor size. In reality with the digital camera you also get 1:1 magnification, but when you compare images from a 35mmSLR and a DSLR, it seems that you get closer with the digital.
with this one I reversed the lens, you get close to 2:1 magnification
Then I added this, Nikon PK-13 27.5mm extension tube,
and I got this, this is close to 3:1 magnification
Canon makes the
MP-E 65mm Macro lens, this lens gives you 5:1 magnification, now that's some extreme macro, I'm just waiting for Nikon to come with something like it.