They are made from washers and the hole in the center is filled with the plugs from a paper punch. from left to right they are 15.6mm, 12.7mm and 9.4mm.
This is a 5x shot of a hydra:
This a 10x shot with an Olympus Dplan 10x 0.25 160/.17 objective:
This is a 20x shot of the hydra's tentacles taken with a generic 20x 0.40 160/.17 employing a false DIC technique. The darkstop in the filter holder is swung sideways from the center position under the condenser so that half of the condenser is darkened and half is passing unretarded light.
