Phase contrast, Zeiss IM35 microscope.
x20 and x32 objectives, with Optovar magnifier x1.0 – x2.0
See also associated post about H&E stained example from same sample of pond water:
Phase Contrast Microscopy of live sample of pond water from Beacon Park:
Most of what looks like bits of debris in the slide on still photographs turns out to be alive once you look at it with phase microscopy! Cork-screwing Spirochete bacteria can be seen plus a whole range of other organisms – many are bacteria, others ciliated organisms.