The part that connects the arm and base of a microscope is called the stage.

Here's why:

* Arm: The arm is the vertical structure that supports the microscope's head, including the objective lenses and eyepiece.

* Base: The base is the bottom structure that provides stability for the entire microscope.

* Stage: The stage is a flat platform located above the base and below the objective lenses. It holds the specimen being viewed. The stage is attached to the arm, connecting these two important components.

Let me know if you have any other questions about microscopes!