instance, beef tea may be inoculated with a mixture of species, poured out upon a flat surface, and allowed to solidify in such a way that the bacteria are fixed in their separated situations. About each germ a pure colony grows up, which may be isolated before admixture of other species has taken place. Bacteria are often colored for observation under the microscope. Some are so small that more than 3,300,000,-000,000 of them would only amount to the volume of a drop of water. About 40,000,-000,000 area bacteria would weigh one grain. Most species of bacteria are quite harmless, and many are necessary and useful. The knowledge of the nature of disease microbes has been of the greatest importance in medicine and surgery. The process of catching a disease is no longer mysterious, many of its channels are known, and the bacteria of the disease may be combated both indirectly and directly. Infectious disease may be traced to emanations from some person sick with that disease. Microbes breed true; and the destruction of germs and sterilization of all instruments have greatly diminished fevers, plagues etc. The greatest success in bacteriology has been won against diphtheria. Almost as notable is the success of vaccination against smallpox and bubonic plague. Weakened bacteria are injected into the blood, which is henceforth fortified against the more virulent forms of the same disease.

Yeasts (q.v.) develop by spores, not by fission; but these also are microbes or vegetable micro-organisms.

Molds are microbes which send forth shoots at a certain stage of growth. It is these shoots which give the appearance of moldiness. Molds (a. v.) are serious enemies to the farming and silkworm industries.

Micrometer ( m-krŏm'e-r ), an instrument for measuring the dimensions of very small objects. The object measured is nearly always the image produced by a microscope or by a telescope. From the size of the image the angular or linear size of the object may be inferred when the focal lengths of the lenses are known. Practically all micrometers are based upon the principle of the screw. In the focal plane of the instrument is fixed one line, usually a spider-web. On a small metallic frame is mounted another spider-web. This metallic frame is the nut of a screw with a fine thread and a divided head. By moving the instrument or the object, one side of the object is made to coincide with the fixed thread and the other side of the object with the movable thread. The number of revolutions of the screw required to carry the movable thread from this position to one of coincidence with the fixed thread is the size of the object in terms of the screw. The angular distance between two stars can thus be measured with

the utmost accuracy; while with a microscope objects even smaller than ^'jtt °f an inch have been measured.

Microphone (m'kr-fon), an instrument in which sound-waves are employed to produce variations of electrical resistance, and hence transmit electrical effects to a considerable distance. The principle upon which the instrument rests is the fact that the electrical resistance of carbon varies with the pressure to which the carbon and its connections are subjected. One of the earliest forms of microphones was that devised by Hughes in England in 1878. It consisted essentially of a small rod of gas carbon, standing upright with its lower end on a carbon button and with its upper end .held in a carbon collar. These three carbons, resting upon a resonance box, formed a part of an electric circuit, vhich also included a telephone. This instrument is so sensitive that the tread of a fly can be heard at a long distance. The modern telephone transmitter is essentially a microphone, in which the pulsations of the air due to the human voice alternately increase and diminish the pressure at one of the contacts in the telephone circuit. See Telephone.

Micropyle {m'kr-pl) (in plants), the small opening left by the integument or integuments of an ovule, through which the pollen-tube ordinarily passes to the nucellus When the seed ripens, the micropyle is usually left as the weakest spot in the seed-coat, and through it the embryo first protrudes. See Ovule.

Mi'croscope, a magnifying instrument. The simple microscope consists of a single convex lens or set of lenses by which the object is viewed directly. The compound microscope is a combination of lenses. One set — the objective — placed near the object forms a real image, and this is further enlarged by a magnifying eyepiece placed next to the eye of the observer. The date of the invention of the microscope is uncertain. Roger Bacon in 1276 used a lens of rock crystal for magnifying objects, and he is generally regarded as the inventor of the simple microscope. The weight of evidence seems to point to Galileo as the inventor of the compound microscope in 1610. Those ascribed to the Janssens in 1590 were simple microscopes. Simple microscopes were brought into general use by Leeuwenhoek (1632-1723) and Malpighi (1628-94). The former had a collection of more than 400 lenses, magnifying from 40 to 270 diameters, and he was the father of microscopic observation. From that time to the present the microscope has been greatly in use. At first it was the newness and the wonders of the microscopic world that made the attraction, but gradually the microscope came to be used as a tool of study. In 1840 the manufacture of lenses