an optical system in which the space between the object and the first lens is filled with an immersion fluid (see Figure 1). Immersion systems are used in microscopes. Cedar or mineral oil (with an index of refraction of 1.515), a solution of glycerin in water (1.434), water (1.333), monobromonaphthalene (1.656), vaseline oil (1.503), and methylene iodide (1.741) are used as immersion fluids.
The optical characteristics of an immersion fluid (dispersion and index of refraction) are taken into account in the design of the immersion system; therefore, such a system may be used only with the fluid for which it was designed. Otherwise the image quality will deteriorate sharply. The inclusion of lens in an immersion system provides the possibility of increasing its aperture A and consequently the resolution of the microscope. A “dry” system cannot have A > 1, and A is as high as 1.3 in oil immersion systems and to 1.6 in a monobromonaphthalene system. In an immersion system light diffusion is reduced, and the image contrast is thus increased. This is especially important for the investigation of poorly reflecting objects.
Immersion systems make possible the study of objects that lie at different depths in the immersion fluid by immersing the lens in it. For example, microorganisms can be observed in water with a lens designed for immersion in water.
L. A. FEDIN