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A survey of developments made during the first half of the current century concerning apparatus for microscopy discloses that two major trends have prevailed: 1. The microscope is used to fulfill its original purpose: to form magnified images of small objects or structures in order to reveal detail which cannot be seen by the unaided eye. The type of microscope which at the beginning of the current century was developed more than any other type is known today as the biological or medical microscope for transmitted light. The use of this microscope with its original facilities is restricted to thin and more or less transparent objects. Developments during the past 50 years have been concerned with not only the creation of illumination apparatus for the revelation of detail of a wider variety of objects with other optical properties, but also with special types of stands to make manipulation more convenient. Representative of this trend are dark-field condensers for transmitted and reflected light, vertical illuminators, metal microscopes, slit ultramicroscopes, and fluorescence microscopes. 2. Magnifying capacity and resolving power of the microscope are incidental. Analysis of the object under the microscope comprises not only the study of object detail, but also qualitative and quantitative analysis of optical properties (that is, anisotropism, refractive index, reflectance, selective absorption, etc.), other physical properties (melting points, hardness), and even chemical properties (for example, molecular structure revealed in infrared microspectro-photometry). At the beginning of the century, the Abbé theory focused attention so emphatically on highest magnification and highest resolving power that the illumination apparatus for transmitted light was designed to perform with maximum efficiency only at highest magnifications. The illumination apparatus for transmitted light was provided with an aperture-controlling iris diaphragm in the lower focal plane of a condenser of short focal length and high numerical aperture with facilities for displacement to create unidirectional, oblique illumination for objectives of highest numerical aperture. These were the conditions under which, according to the Abbé theory in its original restricted form, maximum resolving power prevails. Spherical and chromatic correction of the condenser did not seem to be of primary importance, because a wide cone of illumination (obtainable by opening the iris diaphragm) was considered detrimental to highest resolving power, and light of the shortest possible wavelength was considered preferable to light from the entire visible spectrum. Facilities for illumination of a large field at low magnification (requiring a condenser of longer focal length) with all wavelengths of the visible spectrum (requiring chromatically corrected condenser systems) having a numerical aperture approaching that of the objective (requiring aplanatic correction of the condenser) were not provided at that time and represent later developments. In fact, some of these developments are quite recent and were preceded by similar developments of the illumination systems of other types of microscopes, the metal microscope, for example. Those who have used biological microscopes for photomicrography will remember that their difficulties increased with decreasing magnification. Only recently were biological microscopes provided with illumination apparatus that is equally efficient at low and high magnification, for small and large fields of view, and for low and high numerical apertures of illumination. For further convenience, these microscopes are equipped with built-in light sources of sufficient intensity for photomicrography, another development in which the metal microscope preceded the biological microscope. The devices for oblique illumination with transmitted light are not considered of such great importance as they were in former years.
Zieler, H. W.
Sales Manager, W. H. Kessel and Co., Chicago, Ill.