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[zzffnn]

This 20x objective works on any upright 160 mm tube scope.

You can use it without 0.17mm cover slip. You pour pond water sample into a petri dish, keep water layer to about 1.2-1.4mm thick (slightly thicker than a microscope side), then directly observe the petri dish on scope under that objective.

By using it this way, sample volume is much larger and chance of seeing interesting organisms is much higher (than picking sample to observe under cover slips).

Working distance is good enough. Kids at science fair should be able use it that way.

Exact water thickness is not important, at this low NA of 0.40. I don't know the exact optical calculations but I don't see obvious optical issues with 1.0-1.6mm of water layer above my subject of interest (such as diatoms).

Photos of the objective can be found on my eBay page:
http://www.ebay.com/itm/152685757847

This is a Bausch & Lomb Plan 20x 0.40 160/1.2 Ph3 phase contrast DIN objective, in very good optical condition.

By design, this objective is an inverted phase contrast objective corrected for 1.2mm cover glass. But its low NA of 0.4 and phase contrast makes it very tolerate of optical errors caused by cover thickness variations.

So you don't have to use it on an inverted scope, and you can trade 1.2 mm of glass (RI=1.52) with around 1.37 mm of water (RI=133). At low NA of 0.4, such a trade does not produce perceivable image difference. Here is a discussion and some optical calculations relevant to this objective: http://www.photomacrography.net/forum/v ... 64&start=0

I can sell this objective for $47 shipped (I bought it for around $60), outside off eBay. I am selling it, because I have and prefer to use a short barrel objective for parfocal convenience. This objective is a 45mm DIN objective.

[zzffnn]

Still available.

[zzffnn]

Still available, outside of Bay, or through eBay. If purchased directly from me, I will refund you my seller fees (which is about 10% or $5, I think).

[zzffnn]

Still for sale.

[PeteM]

Just to add to Fan's listing -- we used a set of these B&L phase lenses on an inverted Olympus scope. The 20x was as about as clear and sharp as the pricey Olympus 20x on another scope -- it's a nice lens.