Page:Cyclopaedia, Chambers - Volume 2.djvu/194

 MIC

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MIC

Screw P moves four times round, in the time that Wheel is moving once round. Further, iince there is a Pinion of two which takes into the Teeth of a Wheel, whofe Number is 50; therefore this Wheel with 50 Teeth, will move once round in the time that the Wheel of 20 Teeth moves 25 times round ; and confequently rhe Screw P, or Hand A, mult move a hundred times round, in the fame time as the Wheel of fifty Teeth on the Hand b, has moved once round.

Hence it follows, that if the circular Plate W, which is faften'd at right Angles to the other circular Plate, be di- vided into two hundred equal Parts, the Index x, to which the Handle is faiten'd, will move five of thofe Parts in the fame time, in which the Hand a moves one of the hundred Divilions round the Limb of the other circular Plate. Thus by means of an Index, and Plate W, every fifth part of each of the Divilions round the other Plate, may be known.

Farther, iince each of the Screws P and Q_, have exact- ly forty Threads in an Inch 5 therefore the upper Plate G D E F, will move one Inch, while the Hand a moves forty times round j the four thoufandth Part of an Inch, while the Hand moves over one of the Divilions round the Limb; and the twenty thoufandth Part of an Inch, while the Index x moves one Part of the two hundred round the Limb of the circular Plate W : And the under Plate KNMi will move half an Inch, the two thoufandth Part of an Inch, and the ten thoufandth Part of an Inch, the fame way, in the faid refpecf :ve Times.

Hence, if the under Plate, having a large round Hole therein, be fixed to a Telefcope, fo that the Frame is moveable, together with the whole Infirumenr, except the faid lower Plate ; and the llrait fmooth Edge H I, of the fixed Plate A B lH,as hkewife theftrait fmooth Edge DEofthe moveable Plate GDE 1, be perceivable thro' the round Hole in the under Plate, in the Focus of the Object -Glafs j then, when the Flandle of the Micrometer i3 turned, the Edge HI of the narrow Plate A B I H fixed to the Frame, and' D E of the moveable Plate, will appear thro' the Te- lefcope equally to approach to, or recede from each other. By thele Edges we fhall be able to meafure the appa- rent Diameters of the Sun, Moon, ££c. the Manner of do- ing which take as follows :

Suppofe in looking at the Moon thro' the Telefcope, you have turned the Handle till the two Edges D E and H I, ate open'd, fo as juit to touch or clafp the Moon's Edges 5 and tnat there was twenty one Revolutions of the Hand a, to compleat that opening. F'irft fay, As the focal Length of the Objeft-Glafs, which fuppofe ten Feet, is to Radius, fo is 1 Inch to the Tangent of an Angle fubtended by 1 Inch in the Focus of the Object Glafs ; which will be found aSMin. 30 Sec. Again, becaufe there are exactly 40 Threads of the Screws in I Inch ; fay, if 40 Revolutions of the Hind a, give an Angle of 2S' 30", what Angle will 21 Revolutions give ? The Anfwer will be, 15 Min. 8 Sec. And fuch was the Moon's apparent Diameter ; and fo may the apparent Diameters of any other Objects be taken.

It muff be here obferved, that the Divilions on the Top of the Plate G D E F, are diagonal Divilions of the Revo- lutions of the Screws, with diagonal Divilions of Inches againft them. Thus as the faid Plate Hides along, thefe Diagonals are cut by Divifions made on the Edge of the nar- row Plate K L, fixed to the oppofite fides of the Frame by means of two Screws. Thefe diagonal ^Divifions ferve for a Regilter to count the Revolutions of the Screws, and to (hew how many there are in an Inch, or the parts of an Inch. ., „

Mr. Derham tells us, that his Micrometer is not, as ulually,_ to be put into a Tube, hut to meafure the Spectres of the Sun on Paper, (of any Radius) or to meafure any part of it. By this means he can eafily, and very exa&ly,with the help of a fine Thread, take the Declination of a folar Spor at any time of the Day ; arid by his half-Seconds Watch, meafure the diltance of the Spot from the Sun's Fall em or Wellern Limb.

MICROSCOPE, or Encyscope, a Dioptrical Inftru- ment, by means whereof very minute Objects are repre- fented exceedingly large, and view'd very diftinflly; ac- cording to the Laws of Refraaion. See Refraction.

Micro/cafes are properly riiitinguifhed into fimple, or finale-, and compound, or double.

The Simple are thofe which confifl of a fingle Lens, or a fingle Spherule : The Compound confift offeveral duly combined. See Lens.

As Optics have been improved, other Varieties have been contrived, in rhe forts of Microfcofei : Hence Re- feSlmg-Mcrofcop'es, Water-Mi crofcofes, &c. See Reflec- ting, &c.

When, and by whom Micro/copes were firft invented, is not certainly known. Buygens tells us, that one Drelbel, a Dutchman, had the firft Mem/cope, in the Tear rtfu j and that he was reputed the Inventor of it : tho* F. Fontatta a

Neapolitan, claims the Invention to himfelf, but dates it from rhe lame Year. As a Telefcope inverred is a Mkro- Jcope; theDifcovery might eafily enough have arofe from thence. See Telescope.

foundation and Theory of Single Microscopes.

If an Objea A B (Tab. Optics, Fig. 21.) be placed in the Focusof a fmall convex Lens, or a fimple Mcrojape D E, and the Eye be applied clofe to the other fide of the Microfcopc, the Objed will be Jeen dftinB, in an erett Sttua- ation, and magnified in the Ratio of the diftance of the Focus to the diftance wherein Objetti are to be placed to be Jeen di- ftinftly with the naked Eye.

Dem- For rhe Objea A B being placed in the Focus of the convex Lens D E, the Rays' ilfuing from the feveral Points thereof, after Refraaion, will be parallel ro each other. See Lens and Refraction. Confequently the Eye will fee it diftinBly, by virtue of what is proved under the Word Telescope.

Further, finceoneof the Rays A F proceeding from the Point A, after Refraaion, becomes parallel to the incident Ray; and therefore, fctting afide the thickmfs of the Lens, is found directly againll it ; and the fame holds of all the other Rays carried to the Eye : the Rays A F, and BF, to which the reft coming from A and B are parallel, will enter the Eye in the fame manner as if they enter'd without pairing through the Lens ; and will therefore ap- pear ereB 5 as if the Lens were away. See Vision.

Laftly, it is manifeft that the Onj=a A B will he feen under the fame Angle as if viewM by the naked Eye: But fince it appears very diftinct, whereas to the naked Eye, at the fame diftance, it would appear extremely confufed, 'tis the fame thing as if the Objea fhould feem removed to the diltance F H, wherein it is viewed with equal di- flinanefs, andunder the fame Angle. The Diameter of the Objea A B, therefore, will be to the apparent Diameter IK, as FC to F H, i.e. as the diftance of the F icus of the Lens to the diltance wherein an Objea is to be placed in order to view it diltinctly. See Magnitude and Angle.

Huygens takes it for granted, that an Obj e a feen with the naked Eye, is then in its utmofl diftinanefs, when feen at the diftance of 8 Digits, or tenths of a Foot ; which agrees pretty near with the Obfervations of others.

Law; of Single Microscopes.

1. Simple Microfcofes magnify the Diameter of the Objea A B in the Ratio of the diltance of the Focus F C to an interval of 8 Digits, v. g. If the Ssmi diameter of a Lens convex on both fides be half a Digit A B : I K = 5-: 8 = 1 : itf, that is, the Diameter of the Objea will be increafed in a fedecuple Proportion, or as fixreen to one.

2. Since the diltance F H is conftant, via. 8 Digits, by how much the diftance of the Focus, F C is fmall r, fo much the fmaller Ratio will it have to F H ; confequently the Diameter of the Objea will be ^o much the more mag- nified.

3. Since in Plano-convex Lens's, the diftance of the Fo- cus is equal to the Diameter ; and in Lens's convex on both fides, to the Semi-diameter ; fimple Microfcopes will en- large the Diameter fo much the more, as they are Seo- ments of fmaller Spheres.

4. If the Diameter of the Convexities of a Plano-convex Lens, and a Lens convex on both fides, be the fame, viz. = 1 ; the Diftance of the Focus of the firit will be 1 j of the fecond |. Confequently, theSemi-diameter of the Ob- ject A B will be to the apparent one in the firit Cafe as 1 to 8, in the latter as i to 8, i. e. as itoitf. A Lens, therefore, convex on both fides, magnifies twice as much as a Plano- convex.

As the whole depends on the juft and fteady fituation of Objeas with regard to the Lens, various Methods have been contrived to that end : Whence we have feveral different kinds of fingle Microjcopes. The moil fimple is as follows.

1. AB,Fig.iz. is a little Tube, to one of whofe Bafes BC, is fitted a plain Glafs, to which an Object, viz. a Gnat, Wing of an Infect, Down, or the like, is applied: To the other Bafe, AD, at a ptopcr diftance from the Objea is applied a Lens convex on both fides, whofe Semi-diameter is about half an Inch. The plain Glafs is tutn'd to the Sun, or the Light of a Candle, and the Obiea is feen mag- nified. And if the Tube be made to draw out, Lens's of different Spheres may be ufed.

Again, a Lens, convex on both fides, is inclofed in a Cell' A C, and by a Screw H there fattened a-crofs ; thro' the Pedeftal C D paffes a long Screw, by means whereof, and the female Screw I, a Style or Needle fix'd per- pendicular to its extreme, is kept firm at any diftance from the Lens. In E is a little Tube, on which, and on the Point G, the various Objefls are to be difpofed:

Thu.