For the insertion of accessory or compensator, a slot is usually provided immediately above the objective lenses. The usual accessory plate of a petrographic microscope includes quartz wedge, gypsum plate, and mica plate. Each of these accessories consists of a metal or plastic frame with the optical element mounted in a hole or slot in the center.

 

            The optical element gypsum plate is also known as a one-wavelength or first-order red plate. It is usually a piece of quartz or gypsum that is ground to a precise thickness and mounted between two pieces of glass.

 

            The mica plate on the other hand is also known as the quarter-wavelength plate. This plate is consists of a piece of muscovite or quartz that is mounted between two pieces of glass. It is actually mounted in a manner just like a gypsum plate.

 

            The simplest form of quartz wedge is actually a piece of quartz ground into a wedge shape and then cemented between two pieces of glass. The thickest end of the frame of the wedge is usually located nearest to the handle.