The body tube of a microscope is the cylindrical part that connects the objective lens to the eyepiece. It is responsible for holding the lenses in place and maintaining the proper optical alignment between them. The body tube also provides a pathway for light to travel from the objective lens to the eyepiece.

The length of the body tube is crucial for achieving the proper magnification of the microscope. The focal length of the objective lens and the eyepiece must be precisely matched to the length of the body tube in order to produce a sharp, focused image.

Most microscopes have a fixed body tube length, but some models feature an adjustable body tube that allows the user to change the magnification by varying the distance between the objective lens and the eyepiece.