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In recent years, the versatility of the light microscope has increased continuously through development of new methods, techniques, and equipment. Illumination devices, now available for microscopy, comprise accessories for brightfield, phase-contrast, and dark-field illumination in transmitted or reflected nonpolarized or polarized light, for fluorescence microscopy, interference microscopy, image formation in ultraviolet, visible or infrared light, and others. Objectives for microscopy fall into five major categories: achromats, fluorite objectives, apochromats, planachromats, and planapochromats. These objectives are available as dry systems, water-, oil-, or glycerin immersions, for regular (nonpolarized) light, or selected for freedom from polarization for use in transmitted or reflected polarized light; corrected for observation of objects covered with cover glass of specified thickness or for uncovered objects; with normal or abnormally long or short working distances. The variety of objectives is so great that, for certain focal lengths—as, for instance, for 4 mm—there are between 20 and 30 objective types, differing from each other in performance characteristics. The variety of eyepiece types is not so great. There are Huygens and Ramsden eyepieces of simple design, compensating or planocompensating eyepieces, projection eyepieces, eyepieces especially corrected to improve the “flatness of the field,” and other types. The selection of the illumination method, the objective, and the eyepiece for any specific task in microscopy should be based on a systematic analysis of the optical properties of the object and its surroundings. The following suggestions are offered in the hope that they will help the microscopist in making the selections. The variety of objects observed and analyzed with the microscope is very great indeed. In order to make observation under the microscope possible, the object or its minute structures must differ sufficiently from their surroundings in regard to one or more of the following optical properties: the refraction of light (including double refraction of anisotropic objects), the reflection of light (including bireflection of anisotropic objects), the absorption of light (including dichroism or pleochroism of anisotropic objects), and the emission of light (fluorescence or phosphorescence).
Zieler, H. W.
Zieler Instrument Co., Boston, Mass.