The compressor is an optical accessory for microscopy used to compress the specimen to be observed (often a biological sample) between two glass slides. Typically, the slides are held in frames linked mechanically by a hinged, lever-type, or parallelogram mount. The mount enables the observer to bring the slides together and exert a controlled pressure on the specimen. The compressor is placed on the stage, where it is lit by the illumination system, allowing the specimen to be observed under the microscope.