What does C mean in objective?

[Leitzcycler]

Leitz Wetzlar Mikroskop Objektiv C Pl 100/ 1,32 Oel 170/ 0,17

What does "C" mean?

[MicroBob]

Hi,
this is from the first series of Plan objectives Leitz made, around 1960. "C" means "coated".
The objectives of this series are of different correction state: The Plan 100 is a actually a Planapo 100!
Unfortunately this is also one of the few objectives from Leitz that tend frequently to delaminate. So you should check with a phase telecope whether it is ok or not
I have two: One delaminated, one with loose front lens - a perfect DIY kit for the patient microscope repairer!

Bob

[Rorschach]

Hi,

I recently acquired a Leitz objective with these markings:
C PL 10:1
A 0.25
170/-
3577

The letter 'C' is in red.

Does this mean that this objective is actually PL Apo, like in the case of the 100x?

[apochronaut]

Red was used to signify something special about an objective from way back but only with certain manufacturers. Eventually, as more and more makers adopted that significance, a red ring around an objective came to mean that an objective is special purpose. The purpose is usually revealed in a user manual or catalogue.
Back when your objective was made, red lettering would have had a specific significance but that significance is likely unique to the Leitz stable. In some cases red indicated strain free, or pol. use.
If it is 37mm parfocal in comparison, I had a 37mm parfocal 50X Leitz objective once which had all the characters in red plus the words Meth. Jodi. on it, or Meth. Jod., can't remember exactly but it was a methyl iodide immersion objective.

I have an infinity corrected AO objective from the early 60's. Nothing special , 45X .66 but cat.# 3007, so not apparently in any series. It has the 7 yellow rings typical of the two other 45X .66 achromats but additionally it has a big red ring around the bottom. Haven't found out it's special purpose yet.

[Scarodactyl]

Oh, that makes it go faster.

[apochronaut]

No, they were still putting knurling on the widest part, so it rolls off the desk pretty much at the same speed as the other knurled objectives. Later, when they brought out the plan objectives, they dropped the knurling altogether. You should see how fast those new ones roll! I think they also rotate in the nosepiece a little faster, since there is less wind resistance.

[PeteM]

Leitz used red lettering to distinguish strain-free condenser lens markings (e.g. for polarization) from normal ones. Could be they used the same scheme for objectives?

[apochronaut]

Red was common for strain free or pol.

[Rorschach]

Interesting, at least two possibilities then. I hope the special coating applies here (pun intended) as a PL Apo would be more useful than polarization.