Wiktionary
ocular micrometer
n. (context mycology biology English) A measuring instrument that is inserted into the eyepiece of a microscope, used to measure microscopic features.
Wikipedia
Ocular micrometer
An ocular micrometer is a glass disk that fits in a microscope eyepiece that has a ruled scale, which is used to measure the size of magnified objects. The physical length of the marks on the scale depends on the degree of magnification.