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improper for the former purpose. But Dr. advantage to observers who know how Maskelyne has found that this may be ap- much easier it is to ascertain the contact of plied with very little trouble to that pur. the external edges of two images than their pose also ; and he has furnished the direc- perfect coincidence. tions necessary to be followed when it is The other micrometer invented and de. used in this manner. The addition requisite scribed by Mr. Ramsden, is suited to the for this purpose is a cell, containing two principle of refraction. This micrometer is wires, intersecting each other at right an- applied to the erect eye-tube of a refracting gles, placed in the focus of the eye-glass of telescope, and is placed in the conjugate the telescope, and moveable ronnd about focus of the first eye-glass, as the image is conby the turning of a button. For the desiderably magnified before it comes to the scription of this apparatus, with the method micrometer, any imperfection in its glass of applying and using it, see Dr. Maske- will be magnified only by the remaining Jyoe's paper on the subject, in the Philos. eye-glasses, which in any telescope seldom Trans. Vol. Ixi.

exceeds 5 or 6 times; and besides, the size After all, the use of the object-glass mi- of the micrometer glass will not be the crometer is attended with difficulties, aris- 100th part of the area which would be ing from the alterations in the focus of the required, if it were placed at the object. eye, which are apt to cause it to give dif- glass; and yet the same extent of scale is ferent measures of the same angle at diffe- preserved, and the images are uniformly rent times. To obviate these difficulties, bright in every part of the field of the Dr. Maskelyne, in 1776, contrived a pris- telescope. See Philos. Trans. Vol. Ixix. matic micrometer, or a micrometer consist- In the Philos. Trans. for the year 1782, ing of two achromatic prisms, or wedges, Dr. Herschel, after explaining the defects applied between the object-glass and eye- and imperfections of the parallel-wire microglass of an achromatic telescope, by moving meter, especially for measuring the appaof which wedges nearer to or further from rent diameter of stars, and the distances the object-glass, the two images of an object between double and multiple stars, deproduced by them appeared to approach scribes one for these purposes, which he to, or recede from, each other, so that the calls a lamp micrometer; one that is free focal length of the object-glass becomes a from such defects, and has the advantage of scale for measuring the angular distance of a very enlarged scale. In speaking of the the two images. The rationale and nse of application of this instrument, he says, “It this micrometer are explained in the Philos. is well known to opticians, and others, who Trans. vol. Ixvii.

have been in the habit of using optical inMr. Ramsden has described two new mi- struments, that we can with one eye look crometers, which he has contrived for reme. into a mieroscope, or telescope, and see an dying the defects of the object-glass micro- object much magnified, while the naked meter. One of these is a catoptric microme. eye may see a scale upon which the magniter, which, besides the advantage it derives fied picture is thrown. In this manner I from the principle of reflection, of not be. have generally determined the power of my ing disturbed by the heterogeneity of light, telescopes; and any one who has acquired avoids every defect of other micrometers, a facility of taking such observations, will and can have no aberration, nor any defect very seldom mistake so much as one iu fifty arising from the imperfection of materials, in determining the power of an instrument, or of execution; as the great simplicity of and that degree of exactness is fully suffiits construction requires no additional mir- cient for the purpose. rors or glasses, to those required for the “ The Newtonian form is admirably telescope ; and the separation of the image adapted to the use of this micrometer, for being effected by the inclination of the two the observer stands always erect, and looks specula, and not depending on the focus of in a horizontal direction, notwithstanding lens or mirror, any alteration in the eye of the telescope should be elevated to the an observer cannot affect the angle measur. zenith. The scale of the micrometer at the ed. It has peculiar to itself the advantages convenient distance of 10 feet from the of an adjastment, to make the images coin. eye, with the power of 460, is above a quar. cide in a direction perpendicular to that of ter of an inch to a second ; and by putting their motion; and also of measuring the on my power of 932 I obtain a scale of diameter of a planet on both sides of the more than half an inch to a second, without zero ; which will appear no inconsiderable increasing the distance of the micrometer ; whereas the most perfect of my former wires can pass each other without touching, micrometers, with the same instrument, bad but as near as possible; when they are a scale of less than the 2000th part of an placed by their screws over each other, and inch to a second.

viewed through the eye-tube, they appear “ The measures of this micrometer are but as one wire: the divided circle, t, on not confined to donble stars only, but may the nuts of screws are then slipped round, be applied to any other objects that require without the screw, to bring the first division the utmost accuracy, such as the diameters on them to the index 1; the instrument is of the planets or their satellites, the monu- now adjusted for observing any angle, it is tains of the moon, the diameters of the screwed to the telescope, and by the end. fixed stars, &c."

less screw, h, (fig. 6.) the micrometer is We shall now give an account of a micro- turned round so as to bring a fixed wire, w, meter by Mr. Troughton, which is applied which is perpendicular to the others, to to the eye-piece of a telescope to measure cover the two objects; the two wires are exceedingly small angles, as the diameter of then separated by turning either of the nuts, the heavenly bodies, &c. Plate Microscope, F, until the wires include the angle to be &c. fig. 6, is an orthography projecting end- measured: the whole box (fig. 7.) of the ways; fig. 7, a section of the box containing micrometer slides through the tube, in the the wires; and, fig. 8, a section lengthways : direction of its length, to follow any moving the same letters, as far as they can, are used object. When the observation is completin all the figures. Figs. 6 and 8, A is an eye- ed it is read off by a scale of notches in the tube containing convex lens at each end, box, (fig. 7.) determining the number of rethis slides in another tube, dd, so as to ad- volutions the screw has made, and the divijust the glass to distinct vision of the wires; sions pointed out on the circles, x; by the the tube, dd, is screwed into another, bb, indexes, ll, the number of aliquot parts is which is much larger, through this a thin denoted; the circular plate, s, is divided long box, DD D, containing the wire slides. into degrees as shewn in fig. 6, and it is by The micrometer is screwed to the telescope this that the angle line measured makes by a male screw, ee, (fig. 8.) in the same with the horizon is registered. piece with which is a circular plate, fs, cut The circles are divided in 100 parts, and all round with fine teeth, this plate fits have no determinate value in angular meaagainst the flat bottom of the box, b, and surement, but their value is determined exturus round concentrically with it by means perimentally by observing through the teleof a ring, k, fitting into a conical hole in the scope, it is applied to the diameter of the centre of the plate, ff, and screwed to the sun, or any other body whose angular meabox; a small endless screw, h, (fig. 6.) turn. sure has been previously and accurately ing in two brass collars screwed to the box, b, determined by some other divided instruworks in the teeth cut round the plate, f, and ment, and from this the angle given by each by that means when the milled head on the observation is calculated. arbor of the endless screw is turned, it turns

The micrometer has not only been applied the eye-tube and box, D D, round, to bring to telescopes, and employed for astronomiit to any convenient position for measuring cal purposes; but there have been various the angles required; the box containing the contrivances for adapting it to microscopical wires is shewn open in fig. 7, it containing observations. M. Leeuwenhoek's method two frames, hhh, and illi, one sliding of estimating the size of small objects, was within another, which moves in the box, by comparing them with grains of sand, without lateral shake, yet fitted so as to

of which 100 in a line took np an inch. slide easily backwards and forwards in the These grains he laid upon the same plate box, by the screws, m and n, in the same with his objects, and viewed them at the manner as the microscope in the upper part same time. Dr. Jurin's method was similar of the same plate; o and p are springs to to this ; for he found the diameter of a counteract the screws and make the motion piece of fine silver wire, by wrapping it very pleasant. A wire is stretched across the close upon a pin, and observing how many frame, hh, at right angles to its sides, and rings made an inch : and he used this wire another of the same size is fixed across the in the same manner as Leeuwenhoek used slider, 1111, exactly parallel to the former; his sand. Dr. Hook used to look upon the a small quantity of the underside of the lat- magnified object with one eye, while, at ter is cut away, and its wire is fixed in an. the same time, he viewed other objects, other plane to the wire of hhh, so that the placed at the same distance, with the other

eye. In this manner he was able, by the one eye on the edge of the field of light, and lielp of a ruler, divided into inches and the other eye on the end of the rule, which small parts, and laid on the pedestal of the move till the edge of the field of light and microscope, as it were to cast the magnified the end of the rule correspond; then the disappearance of the object upon the ruler, tance from the end of the rule to the middle and thus exactly to measure the diameter of the stage will be half the diameter of the which it appeared to have through the glass ; field. Ex. gr. If the distance be 10 inches which being compared with the diameter as the whole diameter will be 20, and the it appeared to the naked eye, easily shewed number of the divisious of the micrometer the degree in which it was magnified. A contained in the diameter of the field is the little practice, says Mr. Baker, will ren. magnifying power of the microscope. der this method exceedingly easy and plea

Mr. Adams has applied a micrometer sant.

that instantly sliews the magnifying power Mr. Martin, in his Optics, recommends of any telescope. such a micrometer for a microscope as had

In the Philos. Trans, for 1791, a very been applied to telescopes ; for he advises simple scale micrometer, for measuring to draw a number of parallel lines on a small angles with the telescope, is described piece of glass, with the fine point of a dia. by Mr. Cavallo. This micrometer consists mond, at the distance of one fortieth of an of a thin and narrow slip of mother-of-pearl inch from one another, and to place it in finely divided, and placed in the focus of the focus of the eye-glass. By this method the eye-glass of a telescope, just where the Dr. Smith contrived to take the exact image of the object is formed; whether draught of objects viewed by a double mi- the telescope is a reflector or a refractor, croscope ; for he advises to get a lattice, provided the eye-glass be a convex lens. made with small silver wires or squares, This substance, Mr. Cavallo, after many drawn upon a plain glass by the strokes of a trials, found mnch more convenient than diamond, and to put it into the place of the either glass, ivory, horn, or wood, as it is a image formed by the object-glass. Then, very steady substance, the divisions very by transferring the parts of the object, seen easy marked upon it, and when made as in the squares of the glass or lattice, upon thin as common writing paper it has a very similar corresponding squares drawn on useful degree of transparency. paper, the picture may be exactly taken. MICROPUS, in botany, a genus of the Mr. Martin also introduced into compound Syngenesia Polygamia Necessaria class and microscopes another micrometer, consisting order. Natural order of Compositæ Nuof a screw.

camentaceæ. Corymbiferæ, Jussieu. EssenA very accurate division of a scale is per- tial character: calyx calicled; ray of the formed by Mr. Coventry, of Southwark. corolla none: female, forets wrapped up The micrometers of his construction are in the calycine scales; down none; recepparallel lines drawn on glass, ivory, or tacle chaffy. There are (wo species, ciz. metal, from the 10th to the 10,000th part of' M. supinus, trailing micropns ; and M. an inch. These may be applied to micro. erectus, natives of Spain and the Levant. scopes for measuring the size of minute ob. MICROSCOPE, in optics. By microjects, and the magnifying power of the scopes are understood instruments of whatglasses ; and to telescopes for measuring ever structure or contrivance, that can the size and distance of objects, and the make small objects appear larger than they magnifying power of the instrument. To do by the naked eye. This is effected by measure the size of an object in a single means of convex glasses. When only one microscope, lay it on a micrometer whose convex glass or lens is used, the instrument lines are seen magnified in the same propor. is called a single microscope ; but if two tion with it, and they give, at one view, the or more are employed conjointly to magreal size of the object. For measuring the nify objects, it is then called a double or magnifying power of the compouvd micro compound microscope. When objects are scope, the best and readiest method is the seen through a perfectly flat glass, the rays following : On the stage, in the focus of the of light pass through it from them to the object-glass, lay a micrometer, consisting of eye, in a straight direction, and parallel to an inch divided into 100 equal parts, count each other, and consequently the objects how many divisions of the micrometer are appear very little either diminished or en. taken into the field of view; then lay a two- larged, or nearer or further off than to the foot rule parallel to the micrometer; fix naked eye. But if the glass through whicle they are seen has any degree of convexity, hole in the centre of the pedestal, enables it the rays of light are directed from the cir- to turo verticaliy or horizontally, and reflect cumference towards the centre, in an angle the light, either of a candle or the sky, proportional to the convexity of the glass, directly upwards on the object to be views and meet in a point, at a greater distance ed. V, is a plano-convex lens, which, by from the glass, as it is less or more convex. turning on two screws when the pin at the This point, where the rays meet, is called bottom of it is placed in the hole made for the focus, which is nearer or further off, ac- its reception in the circular plate, L, will cording to the convexity of the glass ; for a transmit the light of a candle to illuminate small degree of convexity throws it to a any opaque object that is put on the round considerable distance ; when the convexity piece of ivory, or ou ebony, for examinais much, the focus is near. The magnifying tion, and it may be moved higher or lower power is in proportion to its convexity; for, as the light requires: this glass is useful to as a flat glass magnifies scarcely at all, the point the sunshine, or the light of a candle, less a glass departs from fatness, the less it upon any opaque object; but in plain daymagnifies ; and the more it approaches to light is of po great use. The brass fish wards a globular figure, the nearer its focus pan, fig. 3, is to fasten a smelt, gudgeon, or is, and the more its magnifying power. We any such small fish upon, to see the blood shall refer to Optics for a more particular circulate in its tail; for which purpose, the account of the principles, and proceed to tail of a fish must be spread across the obdescribe some instruments of which we have long hole, or the end of the pan, by slipmade drawings. Plate Microscope, &c. ping the batton, on the backside of the

The body of the microscope, A A, being pan, into a slit, throngh the circular plate, a large tube, is supported by brass pillars, L; a spring that is beneath the plate presses bb, rising from a wooden pedestal, c, in the button, and will make it steady, and which pedestal is a drawer to hold the ob- present it well; but if it be a frog, a newt, ject-glasses, and other parts of the appara- or eel, in which the circulation is desired to tus ; a lesser tube, e e, slides into the greater, be shewn, a glass tube, fig. 6, is fittest for and is moved up and down therein by a the purpose. The tail of a newt, or eel, rack and pinion turned by a milled-headed or, in a frog, the web between the toe of nut, t; into this tube is pushed the body of the hind-feet, are the parts where it may be the microscope, as shewn in the section seen best. When the object is well exfig. 1, and moving up and down with it, it panded on the inside of the tube, slide the bas at its bottom another tube, much tube along under the circular brass plate, smaller than itself, s, with a male screw, &, L, where there are two springs and a cavity at the end thereof, whereon to screw the ob- made in the shank to hold it, and bring the ject-glasses, or magnifiers: there are five of object directly under the magnifier. these magnifiers, numbered 1, 2, 3, 4, 5, There are three of these glass tubes, which numbers are also marked on the inner smaller one than another, and the size of tube, ee, to direct whereabout to place it, the object must direct which to use; but, according to the magnifier made use of; in general, the less room the creature has but if it does not fit the eye exactly, move to move about in, the easier it may be mag, the inner tube gently higher or lower, by nified, and the quieter it will lie to be exathe not t, till the object appears distinct. mined. Three loose glasses, riz. one plain, The greatest magnifiers have the smallest and two concave, belong also to the micro apertures, and the lower pumbers. L, is a scope, and are designed to confine objects, circular plate of brass, fixed horizontally or place them upon occasionally. between the three brass pillars, bb, and in The long steel wire, fig. 7, (with a pair of the centre thereof a round hole. M, is plyers at one end, and a nob of ivory at the adapted to receive a proper contrivance, other, to hold fast, or lay objects on), slips fig. 4, for holding ivory sliders, wherein ob backwards or forwards in a short brass jects are placed, which contrivance consists tube, whereto a button is fastened, which fits of a spiral steel wire, confined between into a little hole near the edge of the brass three brass circles, one whereof is movea- plate L, and then the object may be readily ble for the admission of a slider. Q, is a brongiit to a right position, and a light be concave mirror set in a box of brass, and cast upon it, either by the looking-glass unturning in an arch, R, upon two small derneath, or, if it be opaque, by the planoscrews. From the bottom of the arch

convex lens, N. somes a pin, which being let down into a L, fig. 4, is a flat piece of ivory, called a

slider, with four round holes through it, and E, also contains a thin brass plate, fig. 4, objects placed in them, between Muscovy which slides in it beneath the frame, fig. 5, talcs, or isinglass, kept in by brass wires. and is moved when necessary by a small

It is proper to have a number of these screw, a, going through the bottom of the sliders filled with curious objects, always box, whose action is counteracted by two ready, as well as some empty opez, for any thin slips of watch-spring, (seen in fig. 3); new thing that offers. When made use of this plate has an oblong hole through it to thrust them between the brass rings of the see through, and on one side it is cut into .contrivance on purpose for them, as shewn fine notches, at such a distance apart that in fig. 4, which shoots into the round hole one turn of the nut, F, when viewed througla M, in the centre of the brass plate, L; this the eye-tube, moves the cross wires in the keeps them steady, and, at the same time, frame, fig. 5, exactly one of these notches, permits them to be moved to and fro for a and by that means the notches register the thorough examination.

number of revolutions the nut has made. The upper part of Plate Microscope, &c. In adjusting and adapting this microscope to describes the construction of a microscope, any instrument, the sliding eye-tube, B, is used for observing the divisions on mathema. to be sliá in or out of the tube, D, till the tical instruments, and subdividing them into cross wire in the frame, fig. 5, are seen persmaller portions. The drawing was made fectly distinct. The microscope is then to from one of those used by Mr. Troughton in be placed in such a position on the instruhis instruments; and the position of four of ment it is applied to, that the line of divithem are shewn in the drawing of one of his sions on the arc shall be parallel to the moastronomical circles, see Circular Instru- tion of the cross wires and frame, fig. 5, and ment, fig. 1, an elevation sideways of the screwed into its holder by the nuts, a a, (as microscope; fig. 2, à section in the same shewn in the drawing of the circle) and thie direction ; fig. 3, a section endways, and focus is adjusted so as to see the divisions four and five parts shewn separately. on the arc distinctly, by turning these

The microscope is fixed to the instru. screws backwards or forwards, and moving ment it is applied to, by two nuts, au, figs. the whole microscope nearer to, or further 1 and 2, which work upon a male screw, cut from the arc, until it is adjusted to distinct on the external tube; these nuts have a vision. The operator then looks through smaller part turned upon them, which ex- the microscope, and observes whether one actly fit into a circular hole in a piece of, 'division on the divided arc of the instrument brass fixed to the instrument, and by screw- answers to twenty of the notches of the ing the puts tight, the microscope is fixed, scale, tig. 4, (which will each be then eqnal as shewn in the drawing of the astronoć to one minute) if not the conical tube, 'G, mical circle before referred to. B, fig. containing the object-glass at its end, must 1 and 2, is the sliding tube containing two be screwed in or out of the body of the miconvex lenses, bd, fig. 2, whose combined croscope, until the image of the division or foci fall upon the wires to be hereafter de. degree is lengthened or shortened, till it scribed; this tube slides in another, D, does, and a loose out on the tube holds it screwed into a thin, square box, E, which at any place required; if this adjustmerit contains the wires and screw, and it is deranges the other before made for distinct shewn opened in fig. 3; it has a square vision, it must be rectified by the nuts, a a, frame, fig. 5, sliding in it; to this is affixed and if this alters the space measured by the a very fine screw, which comes through the scale, fig. 4, the object-glass must be alter. top of the box, and has a nut, F, screwed ed, and then again the nuts, a a, first one on it; at e, within the box, is a spring, form- and then the other, until both adjustments ed of steel wire, acting upon the frame, fig. of measure and distinct vision are perfect. 5, so as to draw it into the box; by this the The small screw which gives motion to the shoulder of the nut, F, is forced down upon scale, fig. 4, is used to adjust it to the point the top of the box, and all shake or loose- of commencement on the circle or divided ness in the motion prevented.

arc; or when two opposite microscopes are The frame has two exceedingly fine wires applied to the same circle, to adjust them stretched across it, as in the figure, and it so that they shall both read alike, that is, is by these the divisions on the instrument so that a line between them shall pass are observed : G is a conical tube, screwed through the centre of the circle, and for the into the principal tube of the instrument, same purpose the small divided circle, x, of with the object-glass at its end: the box, the nut, F, will slip round upon the nut VOL. IV.

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