This is apparently a measuring microscope; you can clamp a widget to the table and position any feature
on the widget with microscopic precision (then read out the table position). The eyepiece should have
a crosshair or bullseye reticle. There's lots of designs out there, google images doesn't seem to locate a pic
of this one, though. They don't have an image of mine, either.
Typical use was for astronomical plate images, getting accurate positions of each speck. The tables
for those, though, were backlight plates.