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MICROSPORANGIUM

1221

MIDAS

or magnifying glasses was vastly improved, and the microscope has ever since been used in research to serious purpose. It has been the means of showing the minute structure of all tissues, the protoplasm upon which their activity depends, their condition in health and disease. It has shown the germs of disease, made analysis of the sense-organs possible, and helped greatly in the comprehension of all nature. Many brilliant and important discoveries have been reached through using it. See BACTERIA, CELL-DOCTRINE, DEVELOPMENT, PROTOPLASM.

The compound microscope consists of mechanical parts and optical parts. The mechanical parts taken together constitute the stand (see illustration). This has a base or foot, from which rises a supporting pillar carrying a stage and an arm. The stage is for holding objects to be examined; it is perforated by a round aperture, through which light is thrown from a mirror located underneath the stage. The size of the aperture is usually regulated by an iris diaphragm, and often there is a glass condenser for the light. The arm carries a tube in which the lenses are inserted. Finally there is a means of bringing the lenses into focus. This is accomplished by a coarse adjustment — usually a rack and pinion movement — to move the tube quickly into position, and a fine adjustment, for slow movements, to get an accurate focus. The

latter is very important in using lenses of high magnifying power. It usually is a micrometer screw and spiral spring. A good stand should be firm, low enough to work upright in order to avoid currents in fluids, which occur when it is (tilted, and have a good, fine adjustment. The lenses or optical parts are the most important parts of a microscope. Those which do [the greatest amount of magnifying j are attached to ! the tube, near j the object, and are called objectives. The oculars or eyepieces fit in the tube near the eye of the observer. The degree of magnifying

MICROSCOPE

power depends on the lenses used. The ordinary working powers range from 75 to 500 diameters. When a higher degree of magnification is desired it is not necessary to purchase a new microscope, but simply to get a new objective. The cost of objectives of high magnifying power is considerable, and the difficulty of working with those magnifying above 600 diameters is considerable, too. Student's microscopes can be obtained from $15 upward; a good one will cost $40 and upward. There are many good makers of microscopes, but those most highly esteemed at present are made by Bausch and Lomb of this country, Leitz of Westlar and Zeiss of Jena, Germany. Microscopic technique has become an art and a science, and on it much of success in discovery depends. See Carpenter's The Microscope and Its Revelations and Beale's How to Work with the Microscope.

Microsporangium (ml'kro-spp-ran'fi-um) (in plants), the sporangium which produces microspores. In pteridophytes they are produced by the water-ferns, selaginella and isoetes; while in all seed-plants (spermato-phytes) the so-called pollen-sacs are micro-sporangia. See HETEROSPORY.

Microspore (ml'kro-spor} (in plants). In cases of heterospory (which see) the small asexual spores are called microspores. In germination a micrpspore produces a male gametophyte, that is, one which bears the sperms. Microspores are found in a few pteridophytes and in all spermatophytes. In the latter group the microspores are better known as pollen-grains. See HETEROSPORY.

Microsporophyll (ml'krd-spo'rd-ftt} (in plants), the sporophyll which bears micro-sporangia. They are chiefly developed in seed-plants, where they are usually called stamens. See HETEROSPORY.

Midas (mi'das), one of the ancient kings of Phrygia, who are always called either Midas or Gordius. According to mythology, Midas, for a kind act, was promised by Dionysus whatever he should ask. He, believing gold to be the best possible thing, asked that everything he touched should turn to gold; but when the request was granted, he found to his sorrow that there are many things more necessary, for even his food became gold, and, in danger of starving, he begged the god to take back the gift. He was sent to wash in the Pac-tolus, the sands of which still yield grains of gold. He decided a contest between Pan and Apollo in favor of Pan, and Apollo in revenge gave him a pair of ass's ears, which he hid under a cap. The secret so oppressed his barber that he dug a hole in the ground and whispered into it: "King Midas has ass's ears." The reed is said to have grown from this hole, and is thought to whisper the secret whenever stirred by the breeze.