HiJ Jan, there's a very fortunate bonus with the variable filter that will delight you I think, as it did me!
Used to control illumination level of course it's perfect - but, when used for polarisation, yes, with the top polarised (linear) filter in the slide-in-out holder above the Orthoplan's nosepiece not only is the illumination able to be set, but if you turn the whole variable filter, i.e. both of it's rotating layers as one, still set for your preferred illumination level, you will find that polarisation will be able to be used, right to extinction (with a totally dark background and only the polarised material 'shining' through)!!
If you need brighter polarisation in terms of how brightly the polarised material, say crystals within plant tissue for example, 'shines' just turn the filter (one layer only) as for adjusting brightness without polarisation - the whole thing brightens, but as you again turn the whole assembly to bring back your polarisation level of extinction, you'll find that the shining polarised material is brighter/darker!
I use this variable filter exclusively now on my Orthoplan, both for illumination intensity AND for polarisation when I pull-out the holder to move the polarised filter (analyser) over the nosepiece.
This is the piece of linear polarising sheet cut to make an analyser for the Orthoplan's filter-holder - that slides in above the nosepiece.
ws_orthoplan filter analyser holder (2).jpg [ 55.58 KiB | Viewed 109 times ]
Into the slot it goes,
ws_orthoplan filter analyser holder (3).jpg [ 49.26 KiB | Viewed 109 times ]
In normal brightfield use the holder is pushed in,
ws_orthoplan filter analyser holder (5).jpg [ 57.69 KiB | Viewed 109 times ]
For use as a polariser-analyser the holder is pulled out to move the filter into place,
ws_orthoplan filter analyser holder (4).jpg [ 56.66 KiB | Viewed 109 times ]
Here's a quick video of use of the variable ND filter to control light intensity on my Orthoplan, the lamp is left on full and needs no adjustment. Also in the video I show use of the ND filter as the bottom filter (i.e. that over the light-port which is turned for extinction adjustment) for polarisation - also it still works for intensity - turn one of the two filter's layers for intensity, turn the whole thing for degree of extinction.... A very handy thing indeed, costing me about £9 from e-bay a couple of years back now.
Remember - linear
filters must be used.