This is an adapter used with various types of video cameras.  Usually, you unscrew the lens from the camera and screw in the adapter.  The adapter then connects to the trinocular port on the microscope.

A screw mount type of lens, commonly used on smaller 16mm cameras, like the Bolex.  

An industry standard for lens mounting. C-Mount is 1-inch diameter with 32 threads per inch. A C-Mount lens needs C-ring when it is mounted at a CS-Mount camera.

This is a specific type of camera, as well as its corresponding lens mount.  The C-mount lens is found in older versions of security cameras, and has a flange back distance of 17.5mm.  In order to achieve a focused image, a 5mm ring must be inserted between the camera and lens.

A lens mount for screw in lenses. more

An industry standard for mounting a lens to a camera where a 1? x 32 thread is employed and the distance from the image plane is 17.52mm from the shoulder of the lens. A C-mount lens may be used CS-mo [..]

An industry standard for mounting a lens to a camera where a 1" x 32 thread is employed and the distance from the image plane is 17.52mm from the shoulder of the lens. A C-mount lens may be used [..]

A standard screw-in lens mount common to many scientific instruments. (Thread of lens and lens mount: 1-inch diameter, 32 threads/inch. Back focal length: 17.52 mm.)

The standard screw mounting for 2/3″ and 1″ camera lenses. The distance from the flange surface to the focal point is 17.526 mm. A C-mount lens can be used on a camera with a CS- mount by adding an ad [..]

Type of lens mount. C-mount lenses provide a male thread which mates with a female thread on the camera. The thread is nominally 1 inch (25 mm) in diameter, with 32 threads per inch. The flange focal [..]

A type of camera lens mount that enables different lenses to be swapped out and used on a security camera.

A camera and lens standard with a 17.526mm flange distance and mounting threads of 1" x 32 TPI.

Standard lens mount found on many industrial and scientific cameras. The thread of the lens and lens mount is 1 inch in diameter with 32 threads per inch pitch and with a back focal length of 17.52 mm or 0.684 inch.