The is a substage lens that concentrates light on the specimen.

microscope's condenser

Explanation:

The microscope's condenser is a substage lens that concentrates light on the specimen.

The condenser of a microscope is a structure that helps concentrate light rays from the light source to illuminate the specimen on the stage of the microscope. It is made up of a system of lens that converges the ray of light and an aperture diaphragm that can be used to control the amount of light that gets to the specimen on the stage of the microscope.

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