Measuring with a microscope

It had never occurred to me before to try to work out a scale of what I am looking at through a microscope. I've only just heard of graticules and don't yet know if we have any. So I measured the size of the illuminated hole in the stage. It was 12mm wide.

Then I viewed the ruler through the microscope. At 4x magnification, that field of view becomes 3mm

At 10x magnification it is just 1mm wide

Also note how wide the scale divisions themselves become!