Cardioid DF condenser

[apochronaut]

This has no relation to me, I just happened to see it on ebay. It's an AO 214F cardioid DF condenser, which has had it's AO dovetail mount removed( two small screws). This makes the condenser useful for a microscope that has a sleeve type condenser mount of 37mm. In the old days, this made these cardioids useful on Bausch & Lomb scopes as well. Perhaps that is why you hardly ever see older B & L cardioids; 95% are paraboloids and AO cardioids are quite common. The AO condensers were cheaper. It will illuminate a field of about 900 um, so should work at 20X with eyepieces of an 18mm f.n. and anything above. As with all oil DF condensers, the higher N.A. objectives will need to have their N.A. reduced somehow. My experience with these condensers is that they need the N.A. to be .85 or lower in order to obtain a nice dark background, so funnel stops or iris diaphragms are necessary for higher resolution imaging.
The price is right. These were 150.00 in 1967!