, an optical instrument, composed of lenses or mirrors, by means of which small objects are made to appear larger than they do to the naked eye.

Microscopes are distinguished into simple and compound, or single and double.

Simple, or Single Microscopes, are such as consist of a single lens, or a single spherule. And a

Compound Microscope consists of several lenses duly combined.—As optics have been improved, other varieties have been contrived in this instrument: Hence reflecting Microscopes, water Microscopes, &c.

It is not certainly known when, or by whom, Microscopes were first invented; although it is probable they would soon follow upon the use of telescopes, since a Microscope is like a telescope inverted. We are iuformed by Huygens, that one Drebell, a Dutchman, had the first Microscope, in the year 1621, and that he was reputed the inventor of it: though F. Fontana, a Neapolitan, in 1646, claims the invention to himself, and dates it from the year 1618. Be this as' it may, it seems they were first used in Germany about 1621. According to Borelli, they were invented by Zacharias Jansen and his son, who presented the sirst Microscope<*> they had constructed to prince Maurice, and Albert arch-duke of Austria. William Borelli, who gives this account in a letter to his brother Peter, says, that when he was ambassador in England, in 1619, Cornelius Drebell shewed him a Microscope, which he said was the same that the arch-duke had given him, and had been made by Jansen himself. Borelli De vero Telescopii inventore, pa. 35. See Lens.

Theory and Foundation of Microscopes.

If an object be placed in the focus of the convex lens of a single Microscope, and the eye be very near on the other side, the object will appear distinct in an erect situation, and magnified in the ratio of the focal distance of the lens, to the ordinary distance of distinct vision, viz, about 8 inches.

So, if the object AB be placed in the focus F, of a small glass sphere, and the eye behind it, as in the focus G, the object will appear distinct, and in an erect posture, increased as to diameter in the ratio of 3/4 of the diameter EI to 8 inches. If, ex. gr. the diameter EI of the small sphere be 1/10 of an inch; then , and , so that ; then as 3/40 : 8, or as 3 : 320, or as 1 : 106 2/3 : : the natural size to the magnified appearance; that is, the object is magnified about 107 times.

Hence the smaller the spherule or the lens is, so much the more is the object magnified. But then, so| much the less part is comprehended at one view, and so much the less distinct is the appearance of the object.

Equal appearances of the same object, formed by different combinations, become obscure in proportion as the number of rays constituting each pencil decreases, that is, in proportion to the smallness of the object-glass.

Wherefore, if the diameter of the object-glass exceeds the diameter of the pupil, as many times as the diameter of the appearance exceeds the diameter of the object; the appearance shall be as clear and bright as the object itself.

The diameter of the object-glass cannot be so much increased, without increasing at the same time the focal distances of all the glasses, and consequently the length of the instrument: Otherwise the rays would fall too obliquely upon the eye-glass, and the appearance become confused and irregular.

There are several kinds of single Microscopes; of which the following is the most simple.

AB (Plate xviii, fig. 1) is a little tube, to one end of which BC, is fitted a plain glass; to which any object, as a gnat, the wing of an insect, or the like, is applied; to the other end AD, at a proper distance from the object, is applied a lens, convex on both sides, of about an inch in diameter: the plane glass is turned to the sun, or the light of a candle, and the object is seen magnified. And if the tube be made to draw out, lenses or segments of different spheres may be used.

Again, a lens, convex on both sides, is inclosed in a cell AC (fig. 2), and held there by the screw H. Through the stem or pedestal CD passes a long screw EF, carrying a stile or needle EG. In E is a small tube; on which, and on the point G, the various objects are to be disposed. Thus, lenses of various spheres may be applied.

A good simple instrument of this kind is Mr. Wilson's pocket Microscope, which has 9 different magnifying glasses, 8 of which may be used with two different instruments, for the better applying them to various objects. One of these instruments is represented at AABB (fig. 3), which is made either of brass or ivory. There are three thin brass plates at E, and a spiral spring H of steel wire within it: to one of the thin plates of brass is fixed a piece of leather F, with a small furrow G, both in the leather, and brass to which it is fixed: in one end of this instrument there is a long screw D, with a convex glass C, placed in the end of it: in the other end of the instrument there is a hollow screw oo, in which any of the magnifying glasses, M, are screwed, when they are to be made use of. The 9 different magnifying glasses are all set in ivory, 8 of which are set in the manner expressed at M. The greatest magnifier is marked upon the ivory, in which it is set, number 1, the next number 2, and so on to number 8; the 9th glass is not marked, but is set in the manner of a little barrel box of ivory, as at b. At ee is a flat piece of ivory, of which there are 8 belonging to this sort of Microscopes (though any one who has a mind to keep a register of objects may have as many of them as he pleases); in each of them there are 3 holes fff, in which 3 or more objects are placed between two thin glasses, or talcs, when they are to be used with the greater magnifiers.

The use of this instrument AABB is this. A handle W, from fig. 4, being screwed upon the button S, take one of the flat pieces of ivory or sliders ee, and slide it between the two thin plates of brass at E, through the body of the Microscope, so that the object to be viewed be just in the middle; remarking to put that side of the plate ee, where the brass rings are, farthest from the end AA: then screw into the hollow screw, oo, the 3d, 4th, 5th, 6th, or 7th magnifying glass M; which being done, put the end AA close to your eye, and while looking at the object through the magnifying glass, screw in or out the long screw D, which moving round upon the leather F, held tight to it by the spiral wire H, will bring your object to the true distance; which may be known by seeing it clearly and distinctly.

Thus may be viewed all transparent objects, dusts, liquids, crystals of salts, small insects, such as fleas, mites, &c. If they be insects that will creep away, or such objects as are to be kept, they may be placed between the two register glasses ff. For, by taking out the ring that keeps in the glasses ff, where the object lies, they will fall out of themselves; so the object may be laid between the two hollow sides of them, and the ring put in again as before; but if the objects be dusts or liquids, a small drop of the liquid, or a little of the dust laid on the outside of the glass ff, and applied as before, will be seen very easily.

As to the 1st, 2d, and 3d magnifying glasses, being marked with a + upon the ivory in which they are set, they are only to be used with those plates or sliders that are also marked with a +, in which the objects are placed between two thin talcs; because the thickness of the glasses in the other plates or sliders, hinders the object from approaching to the true distance from these greater magnifiers. But the manner of using them is the same with the former.

For viewing the circulation of the blood at the extremities of the arteries and vcins, in the transparent parts of fishes tails, &c, there are two glass tubes, a larger and a smaller, as expressed at gg, into which the animal is put. When these tubes are to be used, unscrew the end screw D in the body of the Microscope, until the tube gg can be easily received into that little cavity G of the brass plate fastened to the leather F under the other two thin plates of brass at E. When the tail of the fish lies flat on the glass tube, set it opposite to the magnifying glass, and bringing it to the proper distance by screwing in or out the end screw D, when the blood will be seen clearly circulating.

To view the blood circulating in the foot of a frog; choose such a frog as will just go into the tube; then with a little stick expand its hinder foot, which apply close to the side of the tube, observing that no part of the frog hinders the light from coming on its foot; and when it is brought to the proper distance, by means of the screw D, the rapid motion of the blood will be seen in its vessels, which are very numerous, in the transparent thin membrane or web between the toes. For this object, the 4th and 5th magnifiers will do very | well; but the circulation may be seen in the tails of water-newts in the 6th and 7th glasses, because the globules of the blood of those newts are as large again as the globules of the blood of frogs or small fish, as has been remarked in number 280 of the Philos. Trans. pa. 1184.

The circulation cannot so well be seen by the 1st, 2d, and 3d magnifiers, because the thickness of the glass tube, containing the fish, hinders the approach of the object to the focus of the magnifying glass. Fig. 4 is another instrument for this purpose.

In viewing objects, one ought to be careful not to hinder the light from falling upon them by the hat, hair, or any other thing, especially in looking at opaque objects; for nothing can be seen with the best of glasses, unless the object be at a due distance, with a sufficient light. The best lights for the plates or sliders, when the object lies between the two glasses, is a clear skylight, or where the sun shines on something white, or the reflection of the light from a looking-glass. The light of a candle is also good for viewing very small objects, though it be a little uneasy to those who are not practised in the use of Microscopes.

To cast small Glass Spherules for Microscopes.— There are several methods for this purpose. Hartsoeker first improved single Microscopes by using small globules of glass, melted in the flame of a candle; by which he discovered the animalculæ in semine masculino, and thereby laid the foundation of a new system of generation. Wolfius describes the following method of making such globules: A small piece of very sine glass, sticking to the wet point of a steel needle, is to be applied to the extreme bluish part of the flame of a lamp, or rather of spirits of wine, which will not black it; being there melted, and run into a small round drop, it is to be removed from the flame, on which it instantly ceases to be fluid. Then folding a thin plate of brass, and making very small smooth perforations, so as not to leave any roughness on the surfaces, and also smoothing them over to prevent any glaring, fit the spherule between the plates against the apertures, and put the whole in a frame, with objects convenient for observation.

Mr. Adams gives another method, thus: Take a piece of fine window-glass, and rase it, with a diamond, into as many lengths as you think needful, not more than 1-8th of an inch in breadth; then holding one of those lengths between the fore finger and thumb of each hand, over a very sine flame, till the glass begins to soften, draw it out till it be as fine as a hair, and break; then applying each of the ends into the purest part of the flame, you presently have two spheres, which may be made greater or less at pleasure: if they remain long in the flame, they will have spots; so they must be drawn out immediately after they are turned round. Break the stem off as near the globule as possible; and, lodging the remainder of the stem between the plates, by drilling the hole exactly round, all the protuberances are buried between the plates; and the Microscope performs to admiration.

Mr. Butterfield gave another manner of making these globules, in number 141 Philos. Trans.

In any of these ways may the spherules be made much smaller than any lens; so that the best single Mi- croscopes, or such as magnify the most, are made of them. Leeuwenhoeck and Musschenbroek have suc ceeded very well in spherical Microscopes, and their greatest magnifiers enlarged the diameter of an object about 160 times; Philos. Trans. vol. 7, pa. 129, and vol. 8, pa. 121. But the smallest globules, and consequently the highest magnifiers for Microscopes, were made by F. de Torre of Naples, who, in 1765, sent four of them to the Royal Society. The largest of them was only two Paris points in diameter, and magnified a line 640 times; the second was the size of one Paris point, and magnified 1280 times; and the 3d no more than half a Paris point, or the 144th part of an inch in diameter, and magnified 2560 times. But since the focus of a glass globule is at the distance of one-4th of its diameter, and therefore that of the 3d globule of de Torre, above mentioned, only the 576th part of an inch distant from the object, it must be with the utmost difficulty that globules so minute as those can be employed to any purpose; and Mr. Baker, to whose examination they were referred, considers them as matters of curiosity rather than of real use. Philos. Trans. vol. 55, pa. 246, vol. 56, pa. 67.

Water Microscope. Mr. S. Gray, and, after him, Wolfius and others, have contrived water Microscopes, consisting of spherules or lenses of water, instead of glass. But since the distance of the focus of a lens or sphere of water is greater than that in one of glass, the spheres of which they are segments being the same, consequently water Microscopes magnify less than those of glass, and therefore are less esteemed. Mr. Gray first observed, that a small drop or spherule of water, held to the eye by candle light or moon light, without any other apparatus, magnified the animalcules contained in it, vastly more than any other Microscope. The reason is, that the rays coming from the interior surface of the first hemisphere, are reflected so as to fall under the same angle on the surface of the hinder hemisphere, to which the eye is applied, as if they came from the focus of the spherule; whence they are propagated to the eye in the same manner as if the objects were placed without the spherule in its focus.

Hollow glass spheres of about half an inch diameter, filled with spirit of wine, are often used for Microscopes; but they do not magnify near so much.

Theory of Compound or Double Microscopes.— Suppose an object-glass ED, the segment of a very small sphere, and the object AB placed without the focus F. Suppose an eye-glass GH, convex on both sides, and the segment of a sphere greater than that of DE, though not too great; and, the focus being at K, let it be so disposed behind the object, that . Lastly suppose .| If then O be the place where an object is seen distinct with the naked eye; the eye in this case, being placed in I, will see the object AB distinctly, in an inverted position, and magnisied in the compound ratio of MK × LC to LK × CO; as is proved by the laws of dioptrics; that is, the image is larger than the object, and we are able to view it distinctly at a less distance. For Examp.—If the image be 20 times larger than the object, and by the help of the eye-glass we are able to view it 5 times nearer than we could have done with the naked eye, it will, on both these accounts, be magnified 5 times 20, or 100 times.

Laws of Double Microscopes.

1. The more an object is magnisied by the Micro scope, the less is its field, i. e. the less of it is taken in at one view.

2. To the same eye-glass may be successively applied object-glasses of various spheres, so as that both the entire objects, but less magnified, and their several parts, much more magnified, may be viewed through the same Microscope. In which case, on account of the different distance of the image, the tube in which the lenses are sitted, should be made to draw out.

3. Since it is proved, that the distance of the image LK, from the object-glass DE, will be greater, if another lens, concave on both sides, be placed before its focus; it follows, that the object will be magnified the more, if such a lens be here placed between the objectglass DE, and the eye-glass GH. Such a Microscope is much commended by Conradi, who used an objectlens, convex on both sides, whose radius was 2 digits, its aperture equal to a mustard seed; a lens, concave on both sides, from 12 to 16 digits; and an eye-glass, convex on both sides, of 6 digits.

4. Since the image is projected to the greater distance, the nearer another lens, of a segment of a larger sphere, is brought to the object-glass; a Microscope may be composed of three lenses, which will magnisy prodigiously.

5. From these considerations it follows, that the object will be magnified the more, as the eye-glass is the segment of a smaller sphere; but the field of vision will be the greater, as the same is a segment of a larger sphere. Therefore if two eye-glasses, the one a segment of a larger sphere, the other of a smaller one, be so combined, as that the object appearing very near through them, i. e. not farther distant than the focus of the first, be yet distinct; the object, at the same time, will be vastly magnified, and the field of vision much greater than if only one lens was used; and the object will be still more magnified, and the field enlarged, if both the object-glass and eye-glass be double. But because an object appears dim when viewed through so many glasses, part of the rays being reflected in passing through each, it is not adviseable greatly to multiply glasses; so that, among compound Microscopes, the best are those which consist of one objectglass, and two eye glasses.

Dr. Hook, in the preface to his Micrography, tells us, that in most of his observations he used a Microscope of this kind, with a middle eye-glass of a considerable diameter, when he wanted to see much of the object at one view, and took it out when he would ex- amine the small parts of an object more accurately: for the fewer refractions there are, the more light and clear the object appears.

For a Microscope of three lenses De Chales recommends an object glass of 1/3 or 1/4 of a digit; and the first eye-glass he makes 2 or 2 1/2 digits; and the distance between the object-glass and eye-glass about 20 lines. Conradi had an excellent Microscope, whose object-glass was half a digit, and the two eye-glasses (which were placed very near) 4 digits; but it answered best when, instead of the object-glass, he used two glasses, convex on both sides, their sphere about a digit and a half, and at most 2, and their convexities touching each other within the space of half a line. Eustachius de Divinis, instead of an object-glass convex on both sides, used two plano convex lenses, whose convexities touched. Grindelius did the same; only that the convexities did not quite touch. Zahnius made a binocular Microscope, with which both eyes were used. But the most commodious double Microscope, it is said, is that of our countryman Mr. Marshal; though some improvement was made in it by Mr. Culpepper and Mr. Scarlet. These are exhibited in sigures 5 and 6.

It is observed, that compound Microscopes sometimes exhibit a fallacious appearance, by representing convex objects concave, and vice versa. Philos. Trans. numb. 476, pa. 387.

To fit Microscopes, as well as Telescopes, to shortsighted eyes, the object-glass and the eye-glass must be placed a little nearer together, so that the rays of each pencil may not emerge parallel, but may fall diverging upon the eye.

Reflecting Microscope, is that which magnifies by reflection, as the foregoing ones do by refraction. The inventor of this Microscope was Sir Isaac Newton.

The structure of such a Microscope may be con ceived thus: near the focus of a concave speculum AB, place a minute object C, that its image may be formed larger than itself in D; to the speculum join a lens, convex on both sides, EF, so as the image D may be in its focus.

The eye will here see the image inverted, but distinct, and enlarged; consequently the object will be larger than if viewed through the lens alone.

Any telescope is changed into a Microscope, by removing the objectglass to a greater distance from the eye-glass. And since the distance of the image is various, according to the distance of the object from the focus; and it is magnified the more, as its distance from the object-glass is greater; the same telescope may be successively changed into Microscopes which magnify the object in different degrees. See some instruments of this sort described in Smith's Optics, Remarks, pa. 94.

Solar Microscope, called also the Camera Obscura Microscope, was invented by Mr. Lieberkuhn in 1738 or 1739, and consists of a tube, a looking-glass, a convex lens, and a Wilson's Microscope. The tube (fig. 7) is brass, near 2 inches in diameter, fixed in a circular collar of mahogany, with a groove on the out-| side of its periphery, denoted by 2, 3, and connected by a cat-gut to the pulley 4 on the upper part; which turning round at pleasure, by the pin 5 within, in a square frame, may be easily adjusted to a hole in the shutter of a window, by the screws 1, 1, so closely that no light can enter the room but through the tube of the instrument. The mirror G is fastened to the frame by hinges, on the side that goes without the window: this glass, by means of a jointed brass wire, 6, 7, and the screw H 8, coming through the frame, may be moved either vertically or horizontally, to throw the sun's rays through the brass tube into the darkened room. The end of the brass tube without the shutter has a convex lens, 5, to collect the rays thrown on it by the glass G, and bring them to a focus in the other part, where D is a tube sliding in and out, to adjust the object to a due distance from the focus. And to the end G of another tube F, is screwed one of Wilson's simple pocket Microlcopes, containing the object to be magnified in a slider; and by tube F, sliding on the small end E, of the other tube D, it is brought to a true focal distance.

The Solar Microscope has been introduced into the small and portable Camera Obscura, as well as the large one: and if the image be received upon a piece of half-ground glass, shaded from the light of the sun, it will be sufficiently visible. Mr. Lieberkuhn made considerable improvements in his Solar Microscope, particularly in adapting it to the viewing of opaque objects; and M. Aepinus, Nov. Com. Petrop. vol. 9, pa. 326, has contrived, by throwing the light upon the foreside of any object, before it is transmitted through the object lens, to represent all kinds of objects by it with equal advantage. In this improvement, the body of the common Solar Microscope is retained, and only an addition made of two brass plates, AB, AC, (fig 8), joined by a hinge, and held at a proper distance by a screw. A section of these plates, and of all the necessary parts of the instrument, may be seen in fig. 9, where a c represent rays of the sun converging from the illuminating lens, and falling upon the mirror bd, which is fixed to the nearer of the brass plates. From this they are thrown upon the object at ef, and are thence transmitted through the object lens at K, and a perforation in the farther plate, upon a sereen, as usual. The use of the screen n is to vary the distance of the two plates, and thereby to adjust the mirror to the object with the greatest exactness. M. Euler also contrived a method of introducing vision by reslected light into this Microscope.

The Microscope for Opaque Objects was also invented by M. Lieberkuhn, about the same time with the former, and remedies the inconvenience of having the dark side of an object next the eye; for by means of a concave speculum of silver, highly polished, having a magnifying lens placed in its centre, the object is so strongly illuminated, that it may be examined with ease. A convenient apparatus of this kind, with 4 different sp culums and magnifiers of different powers, was broug<*> to perfection by Mr. Cuff. Philos. Trans. number 45<*> § 9.

Micros<*>pic Objects. All things too minute to be viewed <*>stinctly by the naked eye, are proper objects for the Microscope. Dr. Hook has distinguished them into these three general kinds; viz, exceeding small bodies, exceeeding small pores, or exceeding small motions. The small bodies may be seeds, insects, animalcules, sands, salts, &c: the pores may be the interstices between the solid parts of bodies, as in stones, minerals, shells, &c. or the mouths of minute vessels in vegetables, or the pores of the skin, bones, and other parts of animals: the small motions, may be the movements of the several parts or members of minute animals, or the motion of the fluids, contained either in animal or vegetable bodies. Under one or other of these three general heads, almost every thing about us affords matter of observation, and may conduce both to our amusement and instruction.

Great caution is to be used in forming a judgment on what is seen by the Microscope, if the objects are extended or contracted by force or dryness.

Nothing can be determined about them, without making the proper allowances; and different lights and positions will often shew the same object as very different from itself. There is no advantage in any greater magnifier than such as is capable of shewing the object in view distinctly; and the less the glass magnisies, the more pleasantly the object is always seen.

The colours of objects are very little to be depended on, as seen by the Microscope; for their several component particles, being thus removed to great distances from one another, may give reflections very different from what they would, if seen by the naked eye.

The motions of living creatures too, or of the fluids contained in their bodies, are by no means to be hastily judged of, from what we see by the Microscope, without duc consideration; for as the moving body, and the space in which it moves, are magnified, the motion must also be magnified; and therefore that rapidity with which the blood seems to pass through the vessels of small animals, must be judged of accordingly. Baker on the Microscope, pa. 52, 62, &c. See also an elegant work on this subject, lately published by that ingenious optician Mr. George Adams.

MIDDLE Latitude, is half the sum of two given latitudes; or the arithmetical mean, or the middle between two parallels of latitudc. Therefore,

If the latitudes be of the same name, either both north or both south, add the one number to the other, and divide the sum by 2; the quotient is the middle latitudes, which is of the same name with the two given latitudes. But

If the latitudes be of different names, the one north and the other south; subtract the less from the greater, and divide the remainder by 2, so shall the quotient be the middle latitude, of the same name with the greater of the two.

Ex. 1.Ex. 2.
One lat.35°27′ N.35°27′ S.
the other2113   N.2113   N.
       2 )56402 )1414
Mid. lat.2820 N.Mid. lat.  7  7   S.

Middle Latitude Sailing, is a method of resolving the cases of globular sailing, by means of the Middle Latitude, on the principles of plane and parallel sailing jointly.|

This method is not quite accurate, yet often agrees pretty nearly with Mercator's Sailing, and is founded on the following principle, viz, That the departure is accounted a meridional distance in the middle latitude between the latitude sailed from and the latitude arrived at.

This artifice seems to have been invented, on account of the easy manner in which the several cases may be resolved by the Traverse Table, and to serve where a table of meridional parts is wanting. It is sufficiently near the truth either when the two parallels are near the equator, or not far distant from one another, in any latitude. It is performed by these two rules:

1.As the cosine of the middle latitude :
Is to radius : :
So is the departure :
To the difference of longitude .
2.As the cosine of the middle latitude:
Is to the tangent of the course: :
So is the difference of latitude:
To the difference of longitude.

Ex. A ship sails from latitude 37° north, steering constantly N. 33° 19′ east, for 8 days, when she was found in latitude 51° 18′ north; required her difference of longitude.

51°18′51° 18′
370037 00
        2 )8818Diff. lat. 14 18 = 858 m.
As cos. mid. l. 44090.14417
To tang. cour. 33199.81776
So diff. lat.8582.93349
To diff. long.7862.89542
or 13° 6′ diff. of long. sought.

Middle Region. See Region.

MID-Heaven, Medium Cœli, is that point of the ecliptic which culminates, or is highest, or is in the meridian at any time.

MIDSUMMER-Day, is held on the 24th of June, the same day as the Nativity of St. John the Baptist is held.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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