MICROGRAPH 



Micrograph (Gr. mikros, small ; 

 graphein, to write). Instrument for 

 microscopic writing. The word is also 

 used occasionally as a contraction 

 for a micro-photograph, i.e. a photo- 

 graph of a microscopic subject. 



Microhm. In electricity, the 

 unit of resistance equal to one 

 millionth of an ohm (q.v.). 



Micrometer (Gr. mikros, small ; 

 metron, measure). Instrument for 

 making more accurate linear mea- 



Micrometer. Plain screw micro- 

 meter : a, straight edge ; b, divided 

 head ; c, end of screw ; '/, stop 

 between which and c, the object 

 being measured is held 



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surements than is possible with a 

 simple rule or scale. The most 

 common form is the micrometer 

 screw, which depends upon the fact 

 that if a screw is completely ro- 

 tated, its point will move through 

 a distance equivalent to that 

 between two consecutive threads. 

 By making the pitch small and 

 by attaching to its base a 

 graduated head of comparatively 

 large diameter it is possible to 

 read a fraction of a rotation and 

 thus measure with great accuracy 

 the distance moved over by the 

 point of the screw. For example, 

 if the pitch of the screw be ^th of 

 an inch, the point of the screw 

 advances that distance for a whole 

 turn. If the head attached to the 

 screw be divided into 500 equal 

 parts, then one division on this 

 head corresponds to a movement 

 of the end of the screw equal to the 

 one-ten-thousandth of an inch. 



In a telescope the micrometer is 

 attached to a framework of wires 

 fitted over the eyepiece. The 

 micrometer screw alters the posi- 

 tion of movable wires with regard 

 to a central one and facilitates the 

 measurement of small angular 

 distances among neighbouring 

 stars. It was invented by William 

 Gascoigne in 1638. See Calipers ; 

 Telescope. 



Micronesia (Gr. mikros, small ; 

 nesos, island). Collective name of 

 several groups of small islands in 

 the Pacific Ocean. They are situ- 

 ated between the equator and 

 lat. 20 N. and long. 130 to 180 E. 

 The chief are the Ladrone, Caroline, 

 Marshall, Gilbert, and Pelew Archi- 

 1 pelagoes, all separately described. 

 ', Politically they were apportioned 

 1 among Great Britain, the U.S.A., 

 I and Germany, but since the Great 



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War the islands belonging to the 

 latter have come under mandate 

 to Japan. 



Micronesian. Term denoting 

 the inhabitants of the diminutive 

 islands N. of Melanesia in the W. 

 Pacific. First occupied by a 

 Papuan, then by an Indonesian 

 immigration, this group was after- 

 wards affected by other racial 

 and cultural influences, especially 

 Malayan, Japanese, and Samoan. 

 Micronesians are slenderer, shorter, 

 darker, hairier, and longer-headed 

 than the Polynesians. Local forms 

 of dress and equipment, the semi- 

 divine status of the chiefs, and the 

 veneration of stone pillars, some- 

 times stone-circled, are remini- 

 scent of a megalithic domination. 



Microphone. Instrument for 

 the intensification of sound. The 

 failure of the well-known Bell 

 transmitter to carry sound over 

 long distances led Professor Hughes 

 to invent the microphone in 1878. 

 The instrument depends upon the 

 fact that, if there are loose contacts 

 in an electric circuit, the resistance 



Microphone. Simple carbon micro- 

 phone. B, hollow deal box, open 

 both ends ; C C, carbon blocks ; 

 D, carbon pencil ; A A, wires to 

 telephone and battery. D rests 

 lightly at each end in holes in C C 



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of the circuit varies as sound 

 waves cause the contacts to 

 vibrate. By suitably arranging the 

 loose contacts, an exceedingly 

 sensitive instrument can be con- 

 structed, responding to even the 

 faintest sounds. 



Many forms of microphone are 

 extensively used. In the Hunnings- 

 cum-Deckert microphone a funnel- 

 shaped mouthpiece concentrates 

 the sound waves on to a thin disk 

 of carbon. The surface of a second 

 carbon plate is cut into small pyra- 

 mids, and the space between the two 

 plates is filled with carbon granules 

 which are prevented from settling 

 into a close mass by the shape of 

 the second carbon plate. The 

 vibration of the first disk presses 

 and releases the carbon granules 

 alternately, so causing variation 

 of electrical resistance. See Air 

 Defences ; Telephone. 



Micro-photography. Art of 

 photographing very small objects. 

 Micro - photographs are usually 

 taken for the purpose of subsequent 

 magnification by projecting upon 

 a screen by an optical lantern, or 



MICROSCOPE 



for the purpose of making illus- 

 trations. Such photographs are 

 taken by a camera with a long 

 bellows which fits over the eye- 

 piece of the microscope. Photo- 

 graphs may be taken by sunlight 

 or artificial light. 



Microscope (Gr. mikros, small ; 

 skopein, to look at). Optical in- 

 strument for the examination and 

 magnification of small objects. In 

 its simplest form, that of a single 

 lens (q.v.), it is prehistoric, for the 

 phenomenon of magnification by 

 a curved transparent disk of 

 material must have been noticed 

 in the earliest times. But the 

 high-powered compound micro- 

 scope is a comparatively modern 

 invention. 



Early observers found that the 

 single lens gave a coloured and 

 distorted image, and it was not 

 until the invention of the achro- 

 matic lens (q.v.) by Chester Moor 

 Hall, 1729, and John Dollond, 

 1752, that any great advance was 

 made in the use of the instrument. 

 The theoretical researches from 

 1873-81 of Professor E. Abbe, 

 combined with the practical skill 

 of the German glass-maker Dr. 

 Schott, brought about an enormous 

 increase in the powers of the 

 microscope and laid the foundation 

 of modern microscopy. Sir Almroth 

 Wright and J. J. Lister, the father 

 of Lord Lister, also did much to 

 discover the principles of . the 

 modern scientific microscope. 



The simplest form of microscope 

 consists of a magnifying lens at one 

 end of a tube and another lens at 

 the other end, serving as an eye- 

 piece. All modern microscopes are 

 constructed on this principle, the 

 two simple lenses being replaced 

 by two complicated systems of 

 lenses. The system nearest the 

 object being examined is called the 

 objective, and that nearest the eye, 

 the eyepiece. The objective is the 

 more important part, and may 

 consist of a large number of lenses 

 of varying powers and properties 

 according to the type of micro- 

 scopic work being undertaken. Its 

 function is to collect the rays of 

 light from the object and bring 

 them to the focal image. 



The strain on one eye of 

 examining objects with a micro- 

 scope is very great, and in 1860 

 F. H. Wenham designed a binocular 

 microscope with two tubes and two 

 eyepieces, which has considerably 

 lessened the strain of the work. 

 The light rays from the objective 

 are split up by a prism, and the 

 two images combined to give what 

 is known as stereoscopic vision. 



With the improvement in the 

 composition of the glass used in 

 microscopes there came a very great 



