What Is a Rheostat on a Microscope?

When studying a specimen with a light microscope, the eye peers into the top of the microscope. The specimen rests on a rectangular glass slide below. Lenses in the microscope magnify the specimen. Light is vitally important for microscopic examination, and most microscopes provide and regulate this vital element.

A rheostat determines how much light illuminates the specimen.

Light and Condenser

In a light microscope the light source lies beneath the glass slide containing the specimen to be studied. A condenser occupies the area between the light source and the slide. The condenser focuses the light on the specimen.


Most laboratory microscopes have a rheostat that regulates light intensity. A little disk--the dimmer switch--controls the rheostat and is rotated to the right or to the left until a satisfactory amount of light illuminates the specimen.

Rheostat Principle

In an electrical circuit, electrical current will decrease if resistance to current flow increases. The rheostat is a device that varies the resistance in a circuit without interrupting the flow of the current in the circuit. When the rheostat of the microscope increases the resistance, less current flows, and a dimmer light will strike the specimen. If the rheostat decreases the resistance, more current flows, and more light hits the specimen.