(Edit: I see that ChrisR out-typed me./Chris S.)
Dalhoff, welcome to the forum!
I would suggest reading the FAQ
How can I hook a microscope objective to my camera?.
If you have questions after reading that, just ask.
In the meantime, perhaps someone can tell you if a Jena planachromat 12.5x/0.25 is a candidate for mounting on a camera without a microscope. I suspect it is not, though have no personal knowledge of this lens.
As you may know, many microscope objectives require additional optics to correct chromatic (and sometimes other) aberrations. This is very impractical for macro photography, so we use microscope objectives that do not require additional corrective optics. Most often, this means certain Nikon, Mitutoyo, or Olympus objectives (although not every objective within these brands qualifies). Certain older American Optical objectives have also been demonstrated to work well.
I would suggest waiting until someone who knows your lens confirms whether or not it is internally corrected.
Cheers,
--Chris