MICROCONODON. 450 MICKOMETER. to liplonjr to a group of vertebrates, intermediate in position lietwoen the higlicst anoniodont rep- tiles, the Theroniorpha, and the lowest poly- protodont mannnals. Consult Osborn, "On the SIrueturc and Classification of the Jlesozoic !Mannnalia," in Journal of the PhihuJrljihid Arnd- cmii of Xitturul Sciences for 18S8 (Philadelphia, ]SSS). MI'CROCOSM (Lat. microcosmus, Gk. ^iiKp6- Ko<Tfjios. iiiil.iiikusiiws. little world, from juixpiis, iiiil.riis^ small + KOa/ioc, kosmos, world) and MACROCOSM (from Gk. fiaKpdr, makros, jrreat + Kdc/toc, Icosiiios, world). The belief, current in ancient times, that the world or cos- mos was animated, or had a soul, led to the notion that the parts and members of organic beings must have their counterparts in the mem- bers of the cosmos. The natural philosophers of the sixteenth centurv took up this notion anew in a sipinewhat modilicd shape, and considered the world as a human organism on the large scale, and man as a world, or cosmos, in miniature; hence they called man a microcosm, and the uni- verse itself the macrocosm. With this was as- sociated the Viclief that the vital movements of the microcosm exactly corresponded to those of the macrocosm, and this led to the further as- .suniption that the movements of the stars ex- ercised an influence on the temperament and fortunes (if men. Ml CROCOS'MIC SALT, or Salt of Pnos- iMioHi s. All aniiiionium-sodium-hvdrogen phos- phate that crystallizes in the nionoelinie system, and is found native as the 7nineral stcrcoritc. It was known to the older chemists, who extracted it from human urine. It may be made by dis- solving ervstallized sodium phosphate and am- monium chloride in water, heating the solution to boiling, then filtering and cooling to crystalli- zation. On heating, the crystals melt readily, giving up water of crj'stallization, and later am- monia, and leaving sodium phos|ihate, which melts and solidifies on cooling to a clear color- less glass. It is used chiefly as a flux in blow- j)ipe analysis. MICRODIS'CUS (Xeo-Lat., from Gk. /iiKp6c, iiiil.roa. small -r ('(iTKor. diskos, disk). A small Cambrian trilobitc with body of oval outline, bead and tail shields alike, and onl.v three or four thoracic segments. See Agnostus; Cam- hkiax System. MICRO-FAR'AD. See Farad. MICROLES'TES (Xeo-Lat.. from Gk. fimpdc, mikros, small + Ayari/;, Irslfs, robber). A small fossil jaw with multitubereulate teeth found in the Triassic rocks of Wiirttcmberg and Kngland. This fossil has figured prominently in discussions on the origin of the mammalia, and it is usuall.v placed among the prolotheriau mam- mals: but. as the skull to which it belongs is entirely unknown, its exact systematic position is undeterminable, and it ma.y prove to be the jaw of an anoniodont reptile (Theromorpha) in- stead of that of a mammal. MI'CROM'ETER (from Gk. ^KfiAf. mikros, small -f u(riii,v, luilroii, measure). .An.v device by means of which it is possilde to make a linear measurement more nceuratelv by using levers, screws, or magnif.ying glasses than by using a simple rule or scale. Fig. 1 shows a simple form of lever micrometer adapted to the meastirement Fia. 1. LEVEK MICROM ETER. of thicknesses, diameters, and the like. The movable lever AB turns on a pivot at C, and since the arm CB is five times as long as the arm CA. the pointer at the end of B will move , over the scale D five times as far as the points are opened at A ; and conse- quentl.y the measurement is aboiit five times as accurate as if a scale were applied directly. Fig. 2 illustrates a form of simple screw mi- crometer. The screw has ten threads to the inch, and consequentl.y one complete revolution will remove the point of the screw. A. from the plate, C, one-tenth of an incli. The head, B, of the screw has its rim divided into one hundred equal parts; hence a rotation of the screw through one of these i)arts means one one-hundredth of a complete revolution, and such a motion would remove the point from the plate by a distance of 1/100 of 1/10, or l/IOOO inch. A very common form of screw mi- crometer, described and il- lustrated under Caupers, has fort.v threads to the inch, and the head is di- vided into twenty-five parts, making the accuracy 1/25 of 1/40, or again 1/1000 of an inch. In working with the tele- scope and the microscope it becomes necessary to make measurements u])on the image formed b.v the ob- .jective. and for this purpose a micrometer ocular is emjiloyed. The simplest form of this device is a fine scale ruled >ipoa glass in hundredths of an inch, or tenths of a millimeter, and so mounted in the draw tube that it will be seen distinctly by means of the eyepiece, and hence will be in the plane of the image formed by the objective. The scale appears to lie upon the object, and it is only nccessar.y to read olV the dimensions. A revolution of the draw tube makes measurements in ililferent directions possible without moving the object. A more accurate and satisfactory micrometer ocular is that devised by Kamsden. and illustrat- ed in Fig. 3. li is the divided head of a mi- crometer screw, S, reading to a hiui<lrcdth of a screw revolution, 1/200 millimeter for example. The screw is so arranged that it will cause a rec- tangular frame, A.. to move backward and for- ward as the screw revolves, .cross the middle of the frame. ... are stretched (wo fine spider- lines, at riglit angles to the axis of the screw, and qiiite close together. The whole device is so attaelied to the draw tube of the microscope or telesco|)e that the spider-lines lie in the focal plane of the objective, and hence are distinctly seen nmgnified liy the ocular. In making meas- urements with this instrument the screw S is turned intil the spider-lines straddle one point, and then a reading is made of the jiosition of the head. 11. Next the screw is again turned un- til the lines stradille the other iioint, another Fia. 2. SIMPLE BENCH MICROMETER.
