The two images are separated either laterally within the visual field or at different focal planes, as determined by the optical principles employed.
One of the first usable interference microscopes was designed by Dyson[1] and manufactured by Cooke, Troughton & Simms (later Vickers Instruments), York England.
With this design (marketed by E. Leitz) 60 mm beam separation was achieved in microscopy but here the new difficulty has arisen of balancing optical thicknesses of two separate microscope slide preparations (sample and dummy) and maintaining this critical balance during longer observations (e.g. time-lapse studies of living cells maintained at 37 °C), otherwise a gradual change in background interference colour occurs over time.
The main advantage offered by interference microscopy measurements is the possibility of measuring the projected dry mass of living cells, which was first effectively exploited by Andrew Huxley in studies of striated muscle cell structure and function, leading to the sliding filament model of muscle contraction.
[4] The popularity of interference microscopy peaked around 1940–1970s and fell after that because of the complexity of the instrument and difficulties in both its use and in the interpretation of image data.