Page:Cyclopaedia, Chambers - Volume 2.djvu/196

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4. Since the Image is projected to the greater diftance, the nearer another Lens of a Segment of a larger Sphere, is brought to the Object-Giafs ; aMon/oyt may be com- pofed of three Lens's, which will magnify prodigioufly.

5. From thefe Confederations it follows, that the Object will be magnified the more, as the Eye-Glafs is the Segment of a fmaller Sphere ; but the Field of Vilion will be the greater, as the fame is a Segment of a larger Sphere : If then two Eye-Giaffes, the one a Segment of a larger, the other of a fmaller Sphere, be fo combin'd, as that the Object appearing very near thro' them, i. e. not farther diftant than the Focus of the firfl, be yer diftinct ; the Object at the fame time will be exceedingly magnified, and the Field of Vilion much greater than if only one Lens were ufed : And the Object will be flill more magnified, and the Field enlarged, if both the Object and Eye-Glafs be double. But in regard an Object appears dim, when view'd through fo many Glaffes, part of the Rays being reflected in pafling through each ; the multiplying of Lens's is not advifeable : And the belt among compound Micro.

fcofes, are thofe which confiit of one Obiefl-Glafs and two Eye-Glaffes.

For a Microfcope of three Lens's, De-Cbales commends an Object-Glafs of j or | of a Digit j and the firli Eye- Glafs he makes 2, or 2 i Digits ; the diftance between the Object-Glafs and Eye-Glafs about in Lines. ComWihad an excellent Microfcope, the Obje£t-Glafs whereof was half a Digit, and the two Eye-Glaffes (which were placed very near) 4 Digits : But it anfwer'd beft when in lieu of the Object-Glafs, he ufed two Glaffes, convex on both fides, their Sphere about a Digit and half, or at moll two, and their Convexities touching each other within the Space of half a Line. Bufitwhius de Dimms, inflead of an Object- Glafs, convex on both fides, ufed two Piano Convex Lens's, whofe Convexities touched. Grindelius did the fame ; only that the Convexities did not quite touch. Zabnius made a a Binocular Micro/cope, wherein both Eyes were ufed.

StruBure or Mecbanifm of a. Double Mickoscope.

The Induflry and Addrefs of our Country-man, Mr. Mar- fball, here deferves to be remember'd : The molt com- modious Double Microfcope is of his Contrivance. In this, the Eye-Glaffes are placed in the Tube at A and B, Fig. 25. and the Objefl-Glal's at C. The little Pillar D E is turned by means of a Ball E, moveable in the SocketF ; and thus the Microfcope is accommodated to any Situation. The fame Pillar is divided into as many parts, 1, 2, 3, 4, 5, £$c. as there are Lens's of different Spheres to be ufed in view- ing different Objefts ; fo that the diftance of the Object from the Object-Glafs may be found without any trouble. But as it is fcarce exactly enough determined this way, the Tube may be brought nearer the Object at difcretion, by means or the Screw G H.

The Objects are either laid on the Circle I ; or fitted to proper Inttrumcnts, having their Points or Stiles paffing through the little Tube L M.

Laltly, to illumine the Object, a Lens convex on both fides, N O, is difpofed in a convenient Situation. The reft appears from the Figure.

Rejkiling Microscope, is that which magnifies by Re- flection, as the above-mentioned ones do by Refraction. See Reflection.

The Structure of fuch a Microscope may be conceiv'd thus : Near the Focusof a concave Speculum A B, place a minute Object C, that its Image may be form'd larger than it felf in D. To the Speculum join a Lens convex on both (ides E F, fo as the Image D may be in its Focus.

The Eye will here fee the Image inverted, but diftincf, and enlarged ; confequently the Object will be larger than it viewed through the Lens alone. See Mirrour.

The Inventor of this Micro/cope is the great Sir I. Newton ; but it is fomewhat to be feared left the Objects, appear dim.

Any Telefcope is converted into a Microfcope, by re- moving the Object Glafs to agreater diftance from the Eye- Glafs. And fince the diftance of the Image is various, ac- cording to the diftance of the Object from the Focus ; and it is magnified the more, as its diftance from the Object- Glafs is greater ; the imc Telefcope may be fucceffively converted into Microf copes vihicn magnify the Object in dif- ferent degrees. See Telescope.

MID, or MIDDLE, in Philofophy, and Mathematics. See Mean and Medium.

MIDDLE Latitude, in Navigation, is half the Sum of two given Latitudes. SeeLATi tithe.

MIDDLE Latitude is al fo ufi d for a Method of work- ing the feveral Cafes in Sailing, nearly agreeing with Mer- cuor's, way, bur without the help of Meridional Parts. See Sailing. See Meridional V.arti. Mercator, &c.

MIDHAVEN, Medium Ctrl,, in Aftronomy, is that Point of the Ecliptic which culminates, or is in the Meridian. See Culmination, c>c.

( S"4* )

MIL

MIDRIFF, in Anatomy. SeeDiAPiiRAcM. , MIDSH1P-Afe„, are Officers aboard a Ship, whofe Sta- tion, when they are on Duty, is, fome on the Quarter-Deck, others on the Poops, &c. Their Bufinefs is to mind the Btaces, to look out, and to give about the Word of Com- n " nd * r ? m the Captain, and other fupcrior OiEcers. They alio amitor, all occafions both in failing the Ship, and in flowing and rummaging the Hold.

They are ufually Gentlemen, who, having ferved their Um, £ , a 4 Volunteers, and are now upon their preferment

MIDSUMMER DAY, is the Ltival of St. 3ohn the Baptift, held on the 24th Day of June. See Quarter- Day. ^~

MIGRATION, orT™»/»;. raI «, the Paffage or Re- moval of any thing out of one State, or Place into ano,h -r ; particularly of Colonies of People, Birds, &*. into other Countries. See Transmigration.

The Migration of the Souls of Men into other Ani- mals after Death, is the great Doctrine of the Tytba- goreans, call'd the Metempfycbofu. See Metempsychosis.

The Mioration of Birds, as the Swallow, Quail Stork, Crane, Fieldfare, Woodcock, Nightingale, and 0- ther Birds of Paffage, is a very curious Article in Natural Hiltory, and furnilhes a notable Inftance of the powerful Inftinct imprefs'd by the Creator. See Instinct.

Mr. Derham obferves two things very remarkable therein ; the firjl, That thefe untaught, unthinking Creatures flwuld know the proper Times for their Paffage, when to come, and when to go ; as alfo, that fome 'mould come when others go. No doubt, the Temperature of the Air as to heat and cold, and their natural Propenfity to breed their Young, are the great Incentives to thofe Creatures to change their Habitation : But it is an odd Inftinct they ftjould at all ftift their Habitation ; that fome certain J. lace is not to be found in all the Terraqueous-Globe af- fording 'em convenient Food and Habitation all the Year round.

The fecond, That they Iliould know what way to fleer their Courfe, and whither to go. What Inftinct is it, that moves a poor fooliiri Bird to venture over valt Traits of Lands and Sea ? If it be faid, that by their high Afcents up into the Air, they can fee crofs the Seas, yet what fliould teach or perfuade them that that Land is more proper for the purpofe than this ? That Britain, for .inftance, fliould afford them better Accommodation than E;ypf> than the Canaries* than Spain ? or any other of the intermediate Countries? Tbyfico-Tbeol. p. 545.

Lud.de Beaufort remarks, that Birds in their Paffage ob- ferve a wonderful Order and Polity : They fly in Troops, and fleer their Courfe through huge unknown Rcgions.with- out the Compafs. Cofmol. Divin. It is to be added.that the Birds of Paffage are all peculiarly accommodated by the Struflure of their Parts for long flights. See Pass age.

Naturahfls are divided as to the Places whither Birds of Paffage retire when they leave us. Mr. WiHtmgbby thinks the Swallows fly into Egypt and, Mtbjtpia. Ornitb. Lib. 2. c. 3. Olaus Magnus fays, they lurk in Holes, or under Wa- ter ; which is confirmed by Etmulkr, who affures us, that he faw a Bufliel of them taken out of a frozen Fifh-Pond all hanging together head to head, feet to feet, £rfc. in one Clutter Difert. ;. c. 10. Olaus adds, that this is a com- mon thing in the Northern Countries ; and that fuch a Clufter being carried accidentally by fome Boys into a Stove, the Swallows, after thawing, began to fly about but weakly and for a very little time. A funher Confirma- tion ot this Account was given by Dr. Colas, a Perfon very curious in fuch things, to the Royal Society. Speaking of the way of Fi/hing in the Northern Parts, by breaking Holes, and drawing their Nets under the Ice, he related that he faw fixteen Swallows fo drawn out of the Lake of Samrodt, and about thirty out of the King's great Pond in Rofineilen ; and that at Scblebhteti, near a Houfe of the Earl oiDobna, he faw two Swallows juft come out of the Waters that could fcarce ftand ; being very wet and weak with their Wings hanging on the Ground. He added' that he had often obferved the Swallows to be weak for fome days aher their Appearance.

MILDEW. SeeMiLLDEw.

MILE, in Geography, a long Meafure, whereby we ufe toexprefs the diftance between Places. See Measure Distance, £■?<:.

The Mile is of different extent in different Countries. The Geographical or Italian Mile contains a thoufand Geo- metrical Paces, Mile Faffi:s, whence the Term Mile is de- rived. See Pace, League, &c.

Cafimir has made a curious Reduction of the Miles, or Leagues, of the feveral Countries in Europe into Roman Feet which are equal to xheRbinland Feet generally ufed through- out the North. See Foot.

_ . Feet.

The Mile of Italy r. 5coo

Of England. 5, -.

7 A " The