Number 11 on the figure of the microscope is the ________, which is a diaphragm that regulates the amount of light that illuminates the specimen.

Diaphragm

A dome-shaped muscular partition separating the thorax from the abdomen in mammals, playing a major role in breathing by contracting and flattening to enlarge the thoracic cavity.

Illuminates

Shines light on or brightens, often used metaphorically to clarify or enlighten a subject.

Microscope

An instrument used to view very small objects by magnifying them.