On interference microscopy

Interference microscopy provides a method of increasing the visibility of specimens which are colourless and which differ slightly in refractive index from the surrounding medium by enclosing them between semi-reflecting surfaces, which thus constitute an interferometer. In making observations by this method, it is necessary to use monochromatic light or light of two different wave-lengths, which can be isolated conveniently by the use of a mercury-arc lamp with appropriate filters. This method owes its inception to Frederikse (1933), and has been further developed by the writer in a recent communication (Merton 1947). When monochromatic light falls normally on a pair of semi-reflecting surfaces which are parallel to one another, the intensity of the light which is transmitted is a function of the separation of the plates and, owing to the effect of multiple reflexions, a curve in which intensity is plotted against separation shows a succession of maxima the width of which, in relation to the minima between them, depends on the reflecting power of the semi-reflecting surfaces. The introduction of a body of different refrac­tive index between the plates is optically equivalent to changing their separation with the result that if the plates are at a distance such that the transmission is a maximum, a small change in refractive index is sufficient to cause a great decrease in the transmission. In such a case the body would be seen as a dark object on a bright background. For other separations of the plates the reverse may occur and the body may be seen bright on a dark background. When two radiations (e. g. the green and violet lines of mercury) are used simultaneously it frequently occurs that the object appears in one colour and the background in another.