If we consider the microscope as an instrument consisting of one lens only, it is not at all improbable that it was known to the ancients much sooner than the last century; nay, even in a degree to the Greeks and Romans: for it is certain, that spectacles were in use long before the above-mentioned period: now, as the glasses of these were made of different convexities, and consequently of different magnifying powers, it is natural to suppose, that smaller and more convex lenses were made, and applied to the examination of minute objects. In this sense, there is also some ground for thinking the ancients were not ignorant of the use of lenses, or at least of what approached nearly to, and might in some instances be substituted for them. The two principal reasons which support this opinion are, first, the minuteness of some ancient pieces of workmanship, which are to be met with in the cabinets of the curious: the parts of some of these are so small, that it does not appear at present how they could have been executed without the use of magnifying glasses, or of what use they could have been when executed, unless they were in possession of glasses to examine them with. A remarkable piece of this kind, a seal with very minute work, and which to the naked eye appears very confused and indistinct, but beautiful when examined with a proper lense, is described “Dans l’Histoire de l’Academie des Inscriptions,” tom. 1, p. 333. The second argument is founded on a great variety of passages, that are to be seen in the works of Jamblichus, Pliny, Plutarch, Seneca, Agellius, Pisidias, &c. From these passages it is evident that they were enabled by some instrument, or other means, not only to view distant objects, but also to magnify small ones; for, if this is not admitted, the passages appear absurd, and not capable of having a rational meaning applied to them. I shall only adduce a short passage from Pisidias, a christian writer of the seventh century, Τα μελλοντα ως δια διοπτρου συ βλεπεις: “You see things future by a dioptrum:” now we know of nothing but a perspective glass or small telescope, whereby things at a distance may be seen as if they were near at hand, the circumstance on which the simile was founded. It is also clear, that they were acquainted with, and did make use of that kind of microscope, which is even at this day commonly sold in our streets by the Italian pedlars, namely, a glass bubble filled with water. Seneca plainly affirms it, Literæ, quamvis minutæ et obscuræ, per vitream pilam aqua plenam majores clarioresque cernuntur. Nat. Quæst. lib. 1, cap. 7. “Letters, though minute and obscure, appear larger and clearer through a glass bubble filled with water.” Those who wish to see further evidence concerning the knowledge of the ancients in optics, may consult Smith’s Optics, Dr. Priestley’s History of Light and Colours, the Appendix to an Essay on the first Principles of Natural Philosophy by the Rev. Mr. Jones, Dr. Rogers’s Dissertation on the Knowledge of the Ancients, and the Rev. Mr. Dutens’s Enquiry into the Origin of the Discoveries attributed to the Moderns.[6]
[6] A new edition in French of this learned and valuable work, with many and useful notes, is just published. Edit.
The history of the microscope, like that of nations and arts, has had its brilliant periods, in which it has shone with uncommon splendor, and been cultivated with extraordinary ardour; these have been succeeded by intervals marked with no discovery, and in which the science seemed to fade away, or at least lie dormant, till some favourable circumstance, the discovery of a new object, or some new improvement in the instruments of observation, awakened the attention of the curious, and animated their researches. Thus, soon after the invention of the microscope, the field it presented to observation was cultivated by men of the first rank in science, who enriched almost every branch of natural history by the discoveries they made with this instrument: there is indeed scarce any object so inconsiderable, that has not something to invite the curious eye to examine it; nor is there any, which, when properly examined, will not amply repay the trouble of investigation.
I shall first speak of the SINGLE MICROSCOPE, not only as it is the most simple, but because, as we have already observed, it was invented and used long before the double or compound microscope. When the lenses of the single microscope are very convex, and consequently the magnifying power very great, the field of view is so small, and it is so difficult to adjust with accuracy their focal distance, that it requires some practice to render the use thereof familiar; at the same time, the smallness of the aperture to these lenses has been found injurious to the eyes of some observers: notwithstanding, however, these defects, the great magnifying power, as well as the distinct vision which is obtained by the use of a deep single lens, more than counterbalances every difficulty and disadvantage. It was with this instrument that Leeuwenhoek and Swammerdam, Lyonet and Ellis examined the minima of nature, laid open some of her hidden recesses, and by their example stimulated others to the same pursuit.
The construction of the single microscope is so simple, that it is susceptible of but little improvement, and has therefore undergone but few alterations; and these have been chiefly confined to the mode of mounting it, or the additions to its apparatus. The greatest improvement this instrument has received, was made by Dr. Lieberkühn, about the year 1740; it consisted in placing the small lens in the center of a highly polished concave speculum of silver, by which means he was enabled to reflect a strong light upon the upper surface of an object, and thus examine it with great ease and pleasure. Before this contrivance, it was almost impossible to examine small opake objects with any degree of exactness and satisfaction; for the dark side of the object being next the eye, and also overshadowed by the proximity of the instrument, its appearance was necessarily obscure and indistinct.
Dr. Lieberkühn adapted a microscope to every object; it consisted of a short brass tube, at the eye end of which a concave silver speculum was fixed, and in the center of the speculum a magnifying lens: the object was placed in the middle of the tube, and had a small adjustment to regulate it to the focus; at the other end of the tube there was a plano convex lens, to condense and render more uniform the light which was reflected from the mirror. But all these pains were not bestowed upon trifling objects; his were generally the most curious anatomical preparations, a few of which, with their microscopes, are, I believe, deposited in the British Museum. It will be proper, in this place, to give some account of Mr. Leeuwenhoek’s microscopes, which were rendered famous throughout all Europe, on account of the numerous discoveries he had made with them, as well as from his afterwards bequeathing a part of them to the Royal Society. The microscopes he used were all single, and fitted up in a convenient simple manner; each of them consisted of a very small double convex lens, let into a socket between two plates rivetted together, and pierced with a small hole; the object was placed on a silver point or needle, which, by means of screws adapted for that purpose, might be turned about, raised or depressed at pleasure, and thus be brought nearer to, or be removed farther from the glass, as the eye of the observer, the nature of the object, and the convenient examination of its parts required. Mr. Leeuwenhoek fixed his objects, if they were solid, to the foregoing point with glue; if they were fluid, he fitted them on a little plate of talc, or exceeding thin blown glass, which he afterwards glued to the needle, in the same manner as his other objects. The glasses were all exceeding clear, and of different magnifying powers, which were proportioned to the nature of the object, and the parts designed to be examined. But none of those, which were presented to the Royal Society, magnify so much as the glass globules, which have been used in other microscopes. He had observed, in a letter of his to the Royal Society, that from upwards of forty years experience, he found that the most considerable discoveries were to be made with such glasses, as magnifying but moderately, exhibited the object with the most perfect brightness and distinctness. Each instrument was devoted to one or two objects: hence he had always some hundreds by him.[7] There is some reason for supposing, that Leeuwenhoek was acquainted with a mode of viewing opake objects, similar to that invented by Dr. Lieberkühn.[8]
[7] Philosophical Transactions, No. 980, No. 458.
[8] Priestley’s History of Optics, p. 220.
About the year 1665, small glass globules began to be occasionally applied to the single microscope, instead of convex lenses. By these globules, an immense magnifying power is obtained. The invention of them has been generally attributed to M. Hartsoeker; it appears, however, to me, that we are indebted to the celebrated Dr. Hooke for this discovery; for he described the manner of making them in the preface to his “Micrographia,” which was published in the year 1665. Now the first account we have of any microscopical discovery by M. Hartsoeker, was that of the spermatic animalculæ, made by him when he was eighteen years old; which brings us down to the year 1674, long after Dr. Hooke’s publication.
As these glass globules have been very useful in the hands of experienced observers, I shall lay before my readers the different modes which have been described for making them, that the reader may be enabled thereby to ascertain the reality of the discoveries that have been said to be made with them.