ABSTRACT
It is possible to exploit any difference (or lack of it) between the refractive index (RI) of the mountant and a transparent specimen to help in the visualization of that object. The closer the refractive indices of the object and the medium surrounding it become, the less will be the contrast, and if the two indices match, the object may even become invisible unless the working aperture of the system is dramatically reduced. This variation in contrast due to RI differences and aperture has been appreciated for some time; for example, it is referred to, with respect to sand grains mounted in Canada balsam, by Sorby (1877) “If the aperture of both the object glass and condenser is large, and the grains are mounted in Canada balsam, little or no trace of them may be visible, but by reducing the aperture their outline becomes more and more distinct, and the shading greater and greeater, until in certain cases it may become so dark as to obscure certain characters.”
