Once you have turned on the light source for the compound microscope you can adjust the amount of light seen through the oculars by adjusting the .

Option A

Explanation:

Complete question

Once you have turned on the light source for the compound microscope you can adjust the amount of light seen through the oculars by adjusting the .

A:iris diaphragm

B:condenser

C:focus knob

D:objective lens

Solution -

Iris Diaphragm is the part of the microscope which is used for controlling the light that reaches the specimen to be observes. This diaphragm basically controls the size of the opening through which light passes. In highly modified microscope, an Abbe condenser (as a part of an iris diaphragm) is used to regulate both the focus and amount of light entering and reaching the specimen.

Hence, option A is correct