The other way round

With an up-right microscope one puts the slide the other way round on the stage than when using an inverted microscope. I should know it by now...

But I was becoming a bit desperate while calibrating the 40x PlanApo Oil objective on my up-right Zeiss microscope and I did not get the stage micrometer sharp. The 10x objective showed the lines alright. So why not the 40x objective?? Did the micrometer move with the oil? Should I turn the objective just a little bit closer? Afraid of breaking either the micrometer or - even worse - the objective, I was becoming quite anxious.

And then it struck me. I checked how did I put the micrometer on the stage. Upside down, of course. I am an inverted microscope user indeed. So I turned the slide and calibrated the objective.