ABSTRACT
Phase-contrast microscopy has for many years been the preferred method for enhancing contrast due to differences in refractive index between the object and its mounting medium. However, the phase differences between the light diffracted by the object and that which is unaffected may also be converted into amplitude differences by the use of some form of interference microscope. Interference is, of course, an important factor in the formation of all microscope images: in bright field and phase contrast, interference occurs amongst the beams diffracted by the object, and also between these beams and the light that passes through the specimen without deviation. In these cases the beams which interfere, as well as acquiring their differences in phase from their interactions with the object, have also been separated by the object and thus caused to take their different paths through the microscope. Instruments generally known as ‘interference microscopes’ are different in that the separation of light into beams which will interfere is actually carried out by the optical system of the microscope, before the light encounters the specimen. In these instruments, each illuminating ray is split into two beams which are coherent one with the other, and thus subsequently capable of interference when recombined.
