Microscopic objects are those which need to be studied with microscopes: such are all extremely small bodies, pores, or motions.

Extremely small bodies are either parts of larger bodies, or entire bodies that are very fine, such as small seeds, insects, sands, salts, etc.

Extremely small pores are the interstices between the solid parts of bodies, as in bones, minerals, scales, etc., or like the openings of small vessels, such as the vessels that take in air in plants, the pores of the skin, etc., of animals.

Extremely small motions are those of the different parts or limbs of small animals, or those of fluids contained in the bodies of animals or plants.

Under one or the other of these three headings, everything that surrounds us can furnish a subject to be studied, or to amuse and instruct. Yet many persons are so little aware how far the use of microscopes reaches, and are so challenged to find objects to study, that after considering some of the most common, either alone or with friends, they abandon their microscopes as having rather little use. We shall attempt to undeceive them with a number of facts which we shall place, at the appropriate time, before the reader’s eyes; and perhaps by this means we will engage the curious to put their leisure hours agreeably and usefully to work in the contemplation of the marvels of nature instead of spending them in idleness full of tedium or in the pursuit of some ruinous passion. But before discussing the study of microscopic objects , we must speak of the instrument that magnifies them for our eyes.

We know that there are two types of microscope, single and double: the single microscope has only one lens; the double has at least two combined together. Each of these types has its particular utility: for a single glass makes the object appear closer and more distinct, and the combination of glasses presents a wider field; or to put it differently, it opens up all at once a larger part of the object which it magnifies equally. It is difficult to decide which of the two microscopes one should prefer, because each gives a different sort of pleasure. We can attest great authorities in favor of both: Leeuwenhoeck never used anything but the single microscope, and M. de Hook made all his observations with the double microscope.  [1] The former’s famous microscopes consisted of a single lens placed between two silver plates which were pierced with a little hole, and there was in front a movable needle to hold the object and apply it to the viewer’s eye. It is with these single microscopes that he made those marvelous discoveries which have amazed the world.

Today Mr. Wilson’s pocket microscope is considered the best; and the most esteemed double reflector microscope is a perfected diminutive of the great double microscope of MM. Culpepper, Scarlet, and Marshal.  [2] We have given the description in relation to our figures of these machines.  [3] But it is very important, before going on to the method of examining microscopic objects, to know the power of the lenses of a microscope, and to find out the real size of the objects shown by them.

On the glass surface of the single microscope. Vision is incapable of making out an object held too close to the eyes; but if we consider it through a convex lens, however close the focus of that lens, we will see the object in it very distinctly, and the focus of the lens will be closer as it gets smaller, so that the power of this lens, to magnify an object, will be larger in the same proportion.

We see from these principles why the first and most powerful lens is so small, and we can easily calculate the power of each convex lens of the single microscope: for the power of the lens to magnify is in the same proportion as is its focus with respect to the naked eye. If the focus of a convex lens is, for example, one inch, and the naked eye is clear at eight inches, as is ordinary vision, we will be able to see an object that is one inch from the eye through this lens, and the diameter of that object will appear eight times larger than to the simple naked eye. But as the object is magnified evenly, as much in length as in width, we must square that diameter to learn how much it is magnified, and we will find that this glass magnifies the surface of the object sixty-four times.

Furthermore, supposing a convex lens with a focus far from the center of the lens, by the tenth part of an inch, in eight inches there are eighty tenths of an inch, consequently the object through this lens will appear eighty times nearer than to the naked eye; so consequently we will see it eighty times longer and eighty times wider than it appears to the naked eye; and as eighty multiplied by eighty gives six thousand four hundred, the object will really appear that large.

Let us take another step. If a convex lens is so small that its focus is only the twentieth part of an inch away, we will find that eight inches, the common distance of the naked eye, contains one hundred sixty of these twentieths, and that consequently the length and width of an object seen through that lens will both be magnified one hundred sixty times, which multiplied by one hundred sixty gives the square, which comes to twenty-five thousand six hundred. The result is that this lens will make the object appear twenty-five thousand six hundred times as great in surface as it appears to the naked eye at the distance of eight inches.

Therefore, in order to learn the power of a lens in the single microscope, it only takes approaching it to its true focus, which is easily determined, because the lens is at that distance when the object appears perfectly distinct and well-defined. Then with a small compass we measure carefully the distance between the center of the glass and the object to be examined; and applying the compass on a scale where the inch is divided into tenths and hundredths by diagonal lines, you will easily learn how many parts of an inch this distance contains; this point being known, you look to see how many times these parts are contained in eight inches, which is the ordinary distance of the naked eye, and you will know how many times the diameter is magnified; square this diameter, and you will have the surface; and if you wish to know the thickness of the solidity of your object, you will multiply the surface by the diameter to have the cube or mass. The following table will give you the calculation already made.

Table of the power of convex lenses which are used in single microscopes according to the distance of their foci calculated on a scale of one inch divided into one hundred parts, where we see how many times the diameter, surface, and cube are magnified through these lenses with relation to eyes whose unaided vision is eight inches or eight hundred hundredths of an inch.

The most powerful lens in M. Leeuwenhoeck’s cabinet of microscopes presented to the Royal Society has its focus at the distance of one-twentieth of an inch; consequently, it magnifies the diameter of an object one hundred sixty times, and the surface twenty-five thousand six hundred times. But the most powerful lens of Mr. Wilson’s single microscope, as it is made today, ordinarily has its focus at the distance of only about the fiftieth part of an inch; consequently, it magnifies the diameter of an object four hundred times, and its surface sixty thousand times.

As this table has been calculated in round numbers, it is so easy that anyone who knows how to divide and multiply a small number of figures will easily understand it.

This same table can serve to calculate the power of the glasses of the double microscope, especially since they hardly magnify more than those of Mr. Wilson’s single microscope. The principal advantage we derive from the combination of glasses is to see a wider field or a larger part of the object magnified to the same degree.

On the real size of objects viewed through microscopes . It is not enough to know the power of the lenses of microscopes; we also have to find the true size of the objects we examine when they are excessively small: for although we know they are magnified so many thousand times, we can with that knowledge attain only an imperfect calculation of their true size. In order to conclude something certain, we need some larger object of which we really know the dimensions. Indeed, size itself being merely a comparison, the sole means we have to judge the size of something is to compare it with something else, and to find how many times the smaller body is contained in the larger. To make this comparison in microscopic objects , English scientists have thought up several ingenious methods. It will be helpful to put some of the easy and practicable ones before the reader’s eyes.

M. Leeuwenhoeck’s method for calculating the size of salts in fluids, of small animals in semine masculino [in male semen], in distillate of pepper, etc., was to compare them with the size of a grain of sand, and he made his calculations in the following manner.

He would observe a grain of sea sand with his microscope, such that one hundred of these grains placed end to end form the length of one inch; then, observing a small animal that was nearby, and measuring it carefully by eye, he would conclude that the diameter of this little animal was, for example, less than the twelfth part of the diameter of a grain of sand, and that consequently, following common rules, the surface of the grain of sand was 144 times, and all its solidity 1728 times, greater than that of this little animal. He would make the same proportional calculation according to the smallness of the animals he showed through the microscope.

Here is the method Mr. Hooke used to learn how much an object is magnified by the microscope: “Having adjusted the microscope to see very distinctly the desired object, at the same moment that I look at this object through the glass with one eye, with the other naked eye I look at other objects at the same distance; in this way I am in a position, by means of a ruler divided into inches and into small parts and placed at the foot of the microscope, to see how many parts of this ruler the appearance of the object contains, and to measure exactly the diameter of that appearance, which compared with the diameter it seems to have with the naked eye easily gives me the quantity of its magnification.”

The ingenious Dr. Jurin gives us another most curious method for arriving at the same end in his Physico-mathematical Dissertations :  [4] here it is. Wind a very fine silver wire around a needle or similar body, such that the turns of the wire are exactly touching, and leave no space; to be sure of it, you will examine it very attentively with a microscope. Then measure very exactly with a compass the interval between the two end turns of the silver wire to learn the length of the needle that is covered by this wire, and applying this compass spread to a scale of inches divided into tens and hundreds by diagonal lines, you will know how many parts of an inch it contains. Next you will count the number of turns of the silver wire included in this length, and you will easily know by division the real thickness of the wire in several small pieces. If the object you wish to examine is opaque, you will cast upon it some of these little bits, and if it is transparent you place them underneath, then you compare by eye the parts of the object with the known thickness of these bits of wire.

With this method Dr. Jurin observed that four globules of human blood usually covered the width of one bit of wire, which he had found to be 1/485 inch, and that the diameter of each globule was therefore 1/1920 part of an inch. Which was also confirmed by the observations of Leeuwenhoeck on human blood which he made with a piece of the same wire that Dr. Jurin sent him. See Philosophical Transactions , no. 377. [5]

I pass in silence over other, more complicated methods, but I must not forget to note that the visible area, the field of view, or the portion of an object seen by the microscope is in proportion to the diameter and the area of the lens being used and its power; for if the lens is extremely small, it magnifies considerably, and consequently we can distinguish by means of it only a very small portion of the object; thus we must use the most powerful lens for the smallest objects, and always proportionately. Without giving here the cumbersome rules for the field of objects seen by each lens, it is enough to say that this area differs little from the size of the lens one uses, and that if the total of an object is much greater than that volume, one cannot see it well through that lens.

After combining the power of microscopes and giving the methods of determining the real size of microscopic objects , it remains for us to describe the manner of examining them, preparing them, and applying them to the microscope.

On the examination of microscopic objects . Whatever object we have to examine, we must consider carefully its size, tissue, and nature, to be able to apply the appropriate glasses, and in a manner to know them perfectly. The first step must always be to examine that object through a lens that can show it in its entirety; for by observing the way in which the parts are placed with respect to one another, we will see that it will be simpler to examine afterward each in particular, and to judge it separately if we can. When we have formed a clear idea of the whole, we can divide it as much as we want; and the smaller the parts into which it is divided, the more powerful the lens must be to see them clearly.

We must pay particular attention to the transparency or the opacity of an object, and on this depends the choice of the glasses we must use; for a transparent object can tolerate a much more powerful lens than an opaque object, since the proximity of the glass that magnifies greatly must necessarily obscure an opaque object and prevent us from seeing it, unless we use the microscope for opaque objects. Several objects, however, become transparent when they are divided into extremely thin or tiny parts.

We must also pay attention to the nature of the object, whether it is alive or not, solid or fluid; whether it is an animal, plant, or a mineral substance, and take note of all the circumstances that depend on it, to apply it in the most appropriate manner. If it is a living animal, we must be careful to squeeze, shock, or decompose it as little as possible, the better to learn its true shape, situation, and nature. If it is a fluid, and is too thick, it must be diluted with water; if it is too thin, some of the aqueous parts must be evaporated off. There are substances that are better suited to observation when they are dry, and others on the contrary when they are wet; some when they are fresh, and others when they have been kept a while.

Next we must take great care to obtain the necessary light, for on this depends the truth of all our examinations. A little experience will show how objects appear different in one position and in one kind of light, from what they are in another position; so they should be turned in all directions, and made to pass through all levels of light until we are sure of their true shape. For as Mr. Hooke says, in a great number of objects it is very difficult to distinguish a raised portion from a hollow, a shadow from a black spot, and the color white from simple reflection. The eye of a fly, for example, appears in one kind of light like a trellis with great numbers of holes in it; with sunlight it appears as a surface covered with gilded nails; in a certain position it appears like a surface covered with pyramids; in another it is covered with cones, and in still others it seems covered with entirely different shapes.

The level of light must be proportional to the object: if it is black, it will be better seen in strong light, but if it is transparent, the light should be proportionately weaker. That is why there is an invention in the single and double microscope to deflect the excessive quantity of light rays when we examine these sorts of transparent objects with the most powerful lenses.

The light of a candle, for most objects, and especially for those that are extremely small and transparent, is preferable to daylight, and for others daylight is better: I mean the light of a clear day. As for the sun’s rays, they are reflected so brightly by the object, and give such extraordinary colors, that one can determine nothing by means of them; consequently, this light must be regarded as the worst.

What I am saying about the sun’s rays must not extend, however, to the solar microscope. On the contrary, it can be used only with the brightest sunlight; indeed, with this microscope we do not see the object itself in the place where it is struck by the sun’s rays; we see only its image or its shadow represented on a screen, and consequently no confusion can result between the bright reflection of the sun’s rays, which do not come from the object to the eye as in other microscopes. But also, in the solar microscope we must limit ourselves to knowing the true shape and size of an object, without expecting to learn its colors, because it is not possible for a shadow to have the colors of the body it represents.

On the preparation and application of microscopic objects. There are numerous objects that require many precautions to place them correctly before the lenses. If they are flat and transparent, so they cannot be harmed by being pressed, the best method is to enclose them in the slides between two pieces of talc. By this means the wings of butterflies, the scales of fish, the dust of flowers, etc., the different parts and even the entire bodies of small insects and a thousand other like things can be preserved. One must therefore always have a certain number of these slides on hand for this purpose.

When we make a collection of microscopic objects , we must not fill the slides at random, but take care to sort the objects according to their size and transparency, putting in the same slide only those we can observe with the same lens, and then we will mark on the slide the number that designates the appropriate lens for the objects it contains. The numbers marked on the slides avoid the uncertainty we can be in about which lens should be used.

When placing your objects on slides, it is well to have a convex glass with a focus of about one inch, and to hold it by hand to adjust it properly between the talcs, before enclosing them with copper rings.

Small, living objects, like lice, fleas, crane flies, small bugs, small spiders, mites, etc., can be placed between pieces of talc without being killed or wounded if you take care not to press the copper rings holding the talc; and by this means they will remain alive for entire weeks. But if they are too large to be placed in this manner, they must be placed on a slide with concave glasses intended for this use, or else they can be held by a needle to observe them, or yet again you can hold them with tweezers.

If you have fluids to examine so as to see the small animals they may contain, take a small drop of the fluid with a quill or paintbrush and drop it onto a piece of talc or one of the concave glasses, and put it in this manner under the lens. But should the case present itself, in which (as often happens) you find in making your observation that these little animals swim together and are so prodigiously numerous that, with their rolling continually over each other, you cannot clearly identify their shape and their species, you must take a part of the drop from the glass and substitute for it a little clear water, which will make them appear separate and quite distinct. It is just the opposite when you want to examine a fluid to find what salts it contains, for then you need to evaporate it so that those salts that remain on the glass can be more easily observed.

Dissecting small insects, like fleas, lice, crane flies, mites, etc., requires much patience and dexterity, yet it can be done by means of a fine lancet and a needle, if you place these animals in a drop of water: then it is possible to separate their parts easily and place them under the microscope to observe their stomach and entrails.

Opaque bodies, such as seeds, sands, woods, etc., require other precautions. Here is the best way to study them. Cut some playing cards into small pieces of about one-half inch in length and one-tenth of an inch wide; wet them for half of their length with strong but transparent gummy water, and with that water you will attach your object to it. As the figures on the cards are red and black, if you cut your pieces of cards on these figures you will have a contrast of almost all colors for your objects; and fixing black objects on white, white ones on black, blues or greens on the red or the white, and the other colored objects on the pieces that are the most opposite them in color, you will observe them at greater advantage. These pieces are principally destined for the newly invented microscope for opaque objects, and they must be positioned with the tweezers; but they are also useful for other microscopes that can reveal opaque objects.

You will need a small square box in which to preserve these pieces of cards, with a number of small shallow holes, and you will paste a paper on one side of each card to serve as bottom.

Precautions in studying microscopic objects. In examining objects in all levels of light, you must assert nothing until after repeated experiments and exact observations. Form no judgment about objects that are stretched with too much force, or shrunken by drying out, or which are outside their natural state in any manner whatsoever, without taking appropriate care.

It is highly doubtful whether we can judge the true colors of the objects we see with the most powerful lens; for as the pores or interstices of an object are magnified in proportion to the power of the glass being used, and as the particles that compose its matter must by the same principle appear separated several thousand times more than to the naked eye, the reflection of the rays of light that reach our eyes must be very different and produce different colors; and certainly the variety of the colors of certain objects that are observed there justify this remark.

Nor must we determine without much reflection all the movements of the living creatures or of the fluids that contain them when they are seen under the microscope; for as the body moves, and the space in which it moves is magnified, the movement also must be magnified, and consequently we must judge on these principles the rapidity with which the blood seems to flow in the vessels of tiny animals. Suppose, for example, that a horse and a rat make their limbs move at exactly the same moment of time: if the horse goes a mile while the rat covers fifty perches [6] (although the number of steps be the same for each), you will easily agree, it seems to me, that the horse’s motion is more rapid. The motion of a mite seen by the microscope or by the naked eye is perhaps no less different.

1. Antonie Philips van Leeuwenhoek (1662–1723); Robert Hooke (1635–1703).

2. James Wilson (ca. 1665–1730) introduced his screw-barrel microscope to the Royal Society in 1702. The three other inventors referred to here are: Edmund Culpepper (c. 1670-1738); Edward Scarlett (c. 1677-1743); and John Marshall (1663-1725).

3. This vague reference is perhaps to Plate II in the series on Optics, which shows a single-lens microscope, similar to Culpepper’s.

4. James Jurin (1684–1750), English physician and physicist, F. R. S., whose papers to the Royal Society were published under the title Physico-Mathematical Dissertations , 1732. The method is described on page 45, according to Henry Baker, in The Microscope Made Easy (London, 1743), p. 46.

5. “De globulorum sanguineorum magnitudine, etc. ex. epistola D. Antonii a Leuwenhock ad Jacobum Jurin, M. D. R. S. Secr,” Philosophical Transactionis , Vol. 32, no. 377 (Dec. 1723), article VII. In fact, however, these pages on the magnification powers are taken almost verbatim from the French translation (Le Microscope à la portée de tout le monde, Paris, 1754) of Henry Baker’s The Microscope Made Easy (1743), chap. 10, pp. 47–53.

6. Perches . “In France [the perche ] varies by location: 18, 20, 22, and as much as 27 feet” ( Dictionnaire de Trévoux ).