All things too minute to be viewed distinctly by the naked eye, are proper objects for the microscope. Whatever object offers itself as the subject of our examination, the size, contexture, and nature of it are first to be considered, in order to apply to it such glasses and in such a manner as may show it best. The first step should always be to view the whole of it together, with such a magnifier as can take it all in at once; and after this the several parts of it may the more fitly be examined, whether remaining on the object or separated from it. The smaller the parts are, the more powerful ought the magnifiers to be which are employed: the transparency or opacity of the object must also be considered, and the glasses employed accordingly suited thereto; for a transparent object will bear a much greater magnifier than one which is opake, since the nearness that a glass must be placed at, unavoidably darkens an object if in its own nature opake, and renders it very difficult to be seen, unless by the help of the apparatus contrived for that purpose, which has a silver spectrum. Most objects, however, become transparent by being divided into extremely thin parts.

The nature of the object also, whether it be alive or dead, a solid or a fluid, an animal, a vegetable, or a mineral substance, must likewise be considered, and all the circumstances of it attended to, that we may apply it in the most advantageous manner. If it be a living object, care must be taken not to squeeze or injure it, that we may see it in its natural state and full perfection. If it be a fluid, and that too thick, it must be diluted with water; and if too thin, we should let some of its watery parts evaporate. Some substances are fitted for observation when dry, others when moistened; some when fresh, and others after they have been kept some time.

Light is the next thing to be taken care of, for on this the truth of all our observations depends; and a very little observation will show how very different objects appear in one degree of it to what they do in another; Microscopic other; so that every new object should be viewed in all degrees of light, from the greatest glare of brightness to perfect obscurity; and that in all positions to each degree, till we hit upon the certain form and figure of it. In many objects it is very difficult to distinguish between a promiscuity and a depression, a black shadow and a black stain; and in colour, between a bright reflection and whiteness. The eye of a fly in one kind of light appears like a lattice drilled full of holes, in the sun-shine like a solid substance covered with golden nails; in another position like a surface covered with pyramids, in another with cones, and in others with still different shapes.

The degree of light must always be suited to the object. If that be dark, it must be seen in a full and strong light; but if transparent, the light should be proportionally weak; for which reason there is a contrivance both in the single and double microscope to cut off abundance of the rays, when such transparent objects are to be examined by the largest magnifiers. The light of a candle for many objects, and especially for such as are very bright and transparent, and very minute, is preferable to day-light; for others, a serene day-light is best; but sun-shine is the worst light of all; for it is reflected from objects with too much glare, and exhibits such gaudy colours, that nothing can be determined from it with any certainty. This, however, is not to be extended to the solar or camera obscura microscope; for in that nothing but sun-shine can do, and the brighter that is, the better; but in that way we do not see the object itself on which the sun-shine is cast, but only the image or shadow of it exhibited on a screen; and therefore no confusion can arise from the glaring reflection of the sun's rays from the object to the eye, which is the case in other microscopes. But then in that solar way we must rely contented with viewing the true form and shape of an object, without expecting to find its natural colour; since no shadow can possibly wear the colour of the body it represents.

Most objects require also some management in order to bring them properly before the glasses. If they are flat and transparent, and such as will not be injured by pressure, the best way is to inclose them in sliders between two mucovoy talcs or ringglasses. This way the feathers of butterflies, the scales of fishes, and the farinae of flowers, may be very conveniently preserved, as also the parts of insects, the whole bodies of minute ones, and a great number of other things. These are to be kept in sliders, each containing three, four, or more holes; and these must not be filled promiscuously; but all the things preserved in one slider should be such as require one and the same magnifying power to view them, that there may not be a necessity of changing the glasses for every object; and the sliders should be marked with the number of the magnifier it is proper to be viewed with. In placing the objects in the sliders, it is always proper to have a small magnifier, of about an inch focus, in your hand, to examine and adjust them by, before they are fixed down with the rings.

Small living objects, such as lice, fleas, bugs, mites, minute spiders, &c. may be placed between these talcs without injuring them, if care be taken to lay on the brass rings without pressing them down, and they will remain alive many weeks in this manner; but if they are too large to be treated thus, they should be either preserved between two concave glasses, or else viewed immediately, by holding them in the pliers, or sticking them on the point at the other end of that instrument.

If fluids come under examination, to discover the animalcules in them, a small drop is to be taken with a hair-pencil, or on the nib of a clean pen, and placed on a plate of glass; and if they are too numerous to be thus seen distinctly, some water, warmed by holding it in the mouth, must be added to the drop, and they will then separate and be seen distinctly. This is particularly necessary in viewing the animalcules in the semen masculinum of all creatures; which, though extremely minute, are always so numerous, that without this caution their true form can seldom be seen. But if we are to see the salts in a fluid, the contrary method must be observed, and the plate of glass must be held gently over the fire, till part of the liquor is evaporated.

The dissection of minute animals, as lice, fleas, &c. requires patience and care: but it may be done very accurately by means of a needle and a fine lancet, placing the creature in a drop of water; for then the parts will readily unfold themselves, and the stomach, guts, &c. be very distinctly seen.

These seem the best ways of preserving transparent objects; but the opake ones, such as seeds, woods, &c. require a very different treatment, and are best preserved and viewed in the following manner.

Cut cards into small slips about half an inch long and a tenth of an inch broad; wet these half-way of their length in gum-water, and with that fasten on several parcels of the object; and as the spots of cards are of different colours, such should be chosen for every object as are the most different from its own colours. These are very convenient for viewing by the microscope made for opake objects with the silvered speculum; but they are proper for any microscope that can view opake bodies.

A small box should be contrived for these slips, with little shallow holes for the reception of each: and this is conveniently done, by cutting pieces of paste-board, such as the covers of books are made of, to the size of the box, so that they will just go into it; and then cutting holes through them with a small chisel, of the shape of the slips of card: these paste-boards having then a paper pasted over their bottom, are cells very proper for the reception of these slips, which may be taken out by means of a pair of pliers, and will be always ready for use.

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 drips. Nothing can be determined about them, without making the proper allowances; and different lights and positions will often show the same object as very different from itself. There is no advantage in any greater magnifier than such as is capable of showing the object in view distinctly; and the less the glass magnifies, 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 by this means removed to great distances from one another, may give reflections