The microscope comes with nine sample slides made by Joseph Bourgogne. The circular device in the center is a Full Wave Red retardation plate that is used in Polarized Light Microscopy.
This device is called a "Compressorium". It is used to compress objects for microscope imaging. This image shows the compressor "open". 11.5cm
This excerpt from Carpenter, 1856 describes the use of the Compressorium.
The microscope stage has a moveable carrier for either two apertures (A, B) or the Polarizer (C).
This is a Nicol Prism inserted into one of the objective nosepieces.
Compressor "closed", and compressing a sample.