Progress in Practical Microscopy*

SYNOPSIS. Progress in microscopy is largely influenced by the requirements of the biologist. The small size and transparency of living cells to visible light have led to a search for (1) increased resolving power and (2) new methods of obtaining contrast. Little improvement in lateral resolving power is to be expected from the optical microscope, but the electron microscope is at present capable of resolution of the order of 50 Å. or better. Gabor's wave‐front reconstruction method suggests a way of avoiding limitations due to electron lens aberrations. X‐ray microscopes with resolving power intermediate between that of the optical and electron microscope are envisaged. Microradiography, using very small X‐ray sources, has proved useful for the microchemical analysis of certain elements and for the determination of the mass of cellular constituents. Contrast in optical microscopy can be obtained either by means of special illuminating systems used with visible light or else by means of ultra‐violet or infra‐red radiation to which the cell is not necessarily transparent. The development of phase‐contrast and interference‐contrast methods has provided a powerful tool for the examination of transparent materials. Ultra‐violet microscopy has been used both to increase resolving power and to obtain chemical information about the cell. The quartz monochromat objectives of high N.A. are still the best available for photomicrography, but the achromatism of reflecting and semi‐reflecting objectives makes them more suitable for spectromicrography. Such objectives can also be used in the infra‐red region in order to obtain precise chemical information. Another advantage of reflecting systems is their long working‐distance.