M I C 



MID 



face, and capable of being split into very 

 thin elastic leaves or scales. It consists 

 principally of flint and clay, with a little 

 magnesia and oxide of iron. It is com- 

 monly called talc, but mineralogists ap- 

 ply this term to a different mineral. The 

 brilliant scales in granite are mica. The 

 varieties which have been examined with 

 reference to their optical properties and 

 chemical constitution, are potassa-mica, 

 the most common variety, which has two 

 axes ; magnesia-mica, which has but one 

 axis ; and lithia-mica, including lepidote 

 and several large-foliated varieties of 

 -what was formerly called common mica. 



MICA-SLATE or SCHIST. Micaceous 

 schistus. One of the metamorphic or 

 crystalline stratified rocks, of the hypo- 

 gene class, which is characterized by 

 being composed of a large proportion of 

 mica united with quartz. This rock is 

 termed laminar, when the mica occurs 

 in continuous laminae, alternating with 

 layers of quartz ; granular laminated, 

 when the plates of mica are formed of 

 scales, and those of quartz of granules or 

 crystals ; porphyritic, when either of the 

 preceding varieties contains crystals of 

 hornblende, felspar, or garnet. 



MICA'CEOUS ROCKS. Rocks of 

 which mica is the chief ingredient : these 

 are mica- slate and clay-slate. Though 

 few in number, they are extensively dis- 

 tributed. 



MI'CROCOSM {niKpoi, small, Kdafxoi, 

 world). A term fancifully applied by the 

 ancient philosophers to man, from an 

 idea of his resemblance in miniature to 

 the macrocosm, or great world. 



MICROCO'SMIC SALT. A triple salt, 

 consisting of the phosphates of soda and 

 of ammonia, employed as a flux in ex- 

 periments with the blow-pipe. 



MICRO'METER (/u^Kpof, small, fie- 

 rpov, a measure). An instrument adapted 

 to a telescope, for the purpose of measur- 

 ing small distances, or the diameters of 

 objects which subtend very small angles, 

 as those of the celestial bodies. 



MICRO'PYLE (MtKpof, small, nvXn, a 

 gate). The botanical designation of the 

 foramen of the ripe seed, comprising the 

 exostome and the endostome of the ovule, 

 which lead to the internal portion of the 

 ovule, or, the nucleus. 



MrCROSCOPE {imtKpo?, small, cKoirea, 

 to view). An instrument for furnishing 

 magnified images of objects so minute 

 that, when held at the distance of dis- 

 tinct vision, viz , from eight to ten inches 

 of the eye, the unassisted eye is inca- 

 219 



pable of distinguishing their form and 

 component parts. This end is obtained 

 by enlarging the angle of vision. 



1. The single microscope consists of a 

 convex lens, with a very short focal dis- 

 tance. If an object be viewed through it 

 at less than its focal distance, but near 

 to the focus, the rays transmitted through 

 the lens will be rendered more conver- 

 gent, and an eye on which they fall will 

 see the object under a greater angle of 

 vision, i. e. it will see the object magnified. 



2. The compound microscope consists 

 of two or more convex lenses, or of 

 a combination of concave specula and 

 lenses. The fonner is termed a dioptric 

 or refracting, the latter a catoptric or 

 reflecting microscope. 1. In its simplest 

 form, the refracting microscope consists 

 of only two convex lenses, that nearest 

 to the object being called the object-glass, 

 the other the eye-glass. 2. In reflecting 

 microscopes, the place of the object-glass 

 is occupied by a concave speculum, which 

 also produces an inverted image. 



3. The solar microscope is nothing more 

 than a magic lantern, the light of the 

 sun being used instead of that of a lamp. 

 It consists of two lenses, one of which is 

 called the condenser, because it is em- 

 ployed to concentrate the rays of the 

 sun, in order to illuminate more strongly 

 the object to be magnified. The other is 

 a double convex lens, of considerable 

 power, by which the image is formed. 

 To these is added a plane mirror, for 

 reflecting the rays of the sun on the 

 condenser. 



4. The oxy-hydrogen microscope is an 

 instrument in which, instead of the light 

 of the sun or of a lamp, an intense light 

 is employed, by means of the combustion 

 of a piece of lime in a stream of oxy- 

 hydrogen gas. A cylinder of the ignited 

 lime, of half an inch in diameter, diffuses 

 a light greater than that yielded by 153 

 wax candles. 



MI'CROSCOTIUM. The Microscope ; 

 a modern southern constellation, consist- 

 ing of ten stars. 



MICROZOA'RIA {^xlKpa ^wa, small 

 animals). The designation given by De 

 Blainville to the infusory animalcules of 

 the earlier writers. They are distin- 

 guished into the heteropoda and the 

 apoda, the former comprising the sec- 

 tions rotiferae and ciliiferae, the latter 

 having no external appendages. 



MID-HEAVEN. An astronomical 

 term for that point of the ecliptic which 

 is on the meridian at any given moment. 

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