Correction collar not working

Dear Group,

Today I have a question regarding a mechanical-optical malfunction of
a microscope objective made by a major manufacturer.

The objective is a 40x, Plan Apo with correction collar.

Focussing on a slide one obtains a relatively good image, not as good
as one would expect, but acceptably good. What is curious is that ro-
tating the correction collar throughout its range produces no change in
focus.

An explanation on what might be going on?

Thank you
G.R.


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