Once the specimen is in focus on a microscope, the amount of light reaching the specimen can be changed by using the:

diaphragm

fine adjustment

rotating nosepiece

eyepiece

The diaphragm controls the amount of light reaching the specimen

diaphragm

Explanation:

The diaphragm of a microscope is the part of a microscope that controls the amount of light that reaches the stage where the the slide containing the specimen is placed.

The rotating disc-like structure is usually located under the stage and it has varying sized holes that can be manipulated to increase or decrease the amount and intensity of light that gets to the specimen on stage.

The correct answer is diaphragm.