Part 3

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.

Take a small rod[9] of the clearest and cleanest glass you can procure, free, if possible from blebs, veins, or sandy particles; then by melting it in a lamp with spirit of wine, or the purest and clearest sallad oil, draw it out into exceeding fine and small threads; take a small piece of these threads, and melt the end thereof in the same flame, till you perceive it run into a small drop, or globule, of the desired size; let this globule cool, then fix it upon a thin plate of brass or silver, so that the middle of it may be directly over the center of a very small hole made in this plate, turning it till it is fixed by the before-mentioned thread of glass. When the plate is properly fixed to your microscope, and the object adjusted to the focal distance of the globule, you will perceive the object distinctly and immensely magnified. “By these means,” says Dr. Hooke, “I have been able to distinguish the particles of bodies not only a million times smaller than a visible point, but even to make those visible whereof a million of millions would hardly make up the bulk of the smallest visible grain of sand; so prodigiously do these exceeding small globules enlarge our prospect into the more hidden recesses of nature.”

[9] Lectures and Collections by Dr. Hooke.

Mr. Butterfield, in making of the globules, used a lamp with spirit of wine; but instead of a cotton wick, he used fine silver wire, doubled up and down like a skain of thread.[10] He prepared his glass by beating it to powder, and washing it very clean; he then took a little of this glass upon the sharp point of a silver needle, wetted with spittle, and held it in the flame, turning it about till a glass ball was formed; then taking it from the flame, he afterwards cleaned it with soft leather, and set it in a brass cell.

[10] Philos. Trans. No. 141.

No person has carried the use of these globules so far as Father Di Torre, of Naples, nor been so dexterous in the execution of them; and if others have not been able to follow him in the same line, it may be fairly attributed to a want of that delicacy of touch for adjusting the objects to their focus, and that acuteness of vision which can only be acquired by long practice. Di Torre has also described, more minutely than any other author, the mode of executing these globules, which, as it throws much light upon the preceding description by Dr. Hooke, will not, it is presumed, be unacceptable to the reader.

Three things are necessary for forming of these globules: 1. A lamp and bellows, such as are used by the glass-blowers. 2. A piece of perfect tripoli. 3. A variety of small glass rods. When the flame of the lamp is blown in an horizontal direction, it will be found to consist of two parts; from the base to about two thirds of its length, it is of a white colour; beyond this, it is transparent and colourless. It is this transparent part which is to be used for melting the glass, because by this it will not be in the least sullied; but it will be immediately soiled, if it touch the white part of the flame. The part of the glass which is presented to the flame, ought to be exceeding clean, and great care should be taken that it be not touched by the fingers. If the glass rod has contracted any spots, it must either be thrown away, or the parts that are spotted must be cut off.

The piece of tripoli which is to be used in forming the globules, should be flat on one side, and so large that it may be handled conveniently, and protect the fingers from the flame. A piece four or five inches long, and three or four inches thick, will answer very well. The best tripoli for this purpose is of a white colour, with a fine grain, heavy and compact, and which, after it has been calcined, is of a red colour. This kind resists the fire best, is not apt to break when calcined, and the melted glass does not adhere to it. To calcine this tripoli, cover it well all round with charcoal nearly red hot, leaving it thus till the charcoal is quite cold; it may then be taken out. Let several hemispherical cavities be made on the flat side of the tripoli; they should be of different sizes, nicely polished, and neatly rounded at the edges, in order to facilitate the entrance of the flame. The large globules are to be placed in the large cavities, and the minuter ones, in the small cavities. The holes in the tripoli must never be touched with the finger. If it be necessary to clean them, it should be done with white paper; the larger globules may be cleaned with wash leather. The glass rods should be of various sizes, as of ¹⁄₁₀th, ¹⁄₂₀th, ¹⁄₃₀th of an inch in diameter, as clean and free from specks and bubbles as possible.

TO MAKE SMALL GLASS MICROSCOPIC GLOBULES.

Take two rods of glass, one in each hand, place their extremities close to each other, and in the purest part of the flame; when you perceive the ends to be fused, separate them from each other; the heated glass following each rod, will be finer, in proportion to the length it is drawn to, and the smallness of the rod; in this manner you may procure threads of glass of any degree of fineness. Direct the flame to the middle of the thread, and it will be instantly divided into two parts. When one of the threads is perfectly cool, place it at the extremity of the flame, by which it will be rendered round; and, if the thread of glass be very fine, an exceeding small globule will be formed. This thread may now be broke off from the rod, and a new one may be again drawn out as before, by the assistance of the other glass rod.

The small ball is now to be separated from the thread of glass; this is easily effected by the sharp edge of a piece of flint. The ball should be placed in a groove of paper, and another piece of paper be held over it, to prevent the ball from flying about and being lost. A quantity of globules ought to be prepared in this manner; they are then to be cleaned, and afterwards placed in the cavities of the tripoli, by means of a delicate pair of nippers. The globules are now to be melted a second time, in order to render them completely spherical; for this purpose, bring one of the cavities near the extremity of the flame, directing this towards the tripoli, which must be first heated; the cavity is then to be lowered, so that the flame may touch the glass, which, when it is red hot, will assume a perfect globular form; it must then be removed from the flame, and laid by; when cold, it should be cleaned, by rubbing between two pieces of white paper. Let it now be set in a brass cap, to try whether the figure be perfect. If the object be not well defined, the globule must be thrown away. Though, if it be large, it may be exposed two or three times to the flame. When a large globule is forming, it should be gently agitated by shaking the tripoli, which will prevent its becoming flat on one side. By attending to these directions, the greater part of the globules will be round and fit for use. In damp weather, notwithstanding every precaution, it will often happen, that out of forty globules, four or five only will be fit for use.

Mr. Stephen Gray, of the Charter-House, having observed some irregular particles within a glass globule, and finding that they appeared distinct and prodigiously magnified when held close to his eye, concluded, that if he placed a globule of water, in which there were any particles more opake than the water, near his eye, he should see those particles distinctly and highly magnified. This idea, when realized, far exceeded his expectation. His method was, to take on a pin a small portion of water which he knew had in it some minute animalculæ; this he laid on the end of a small piece of brass wire, till there was formed somewhat more than an hemisphere of water; on applying it then to the eye, he found the animalculæ most enormously magnified; for those which were scarce discernible with his glass globules, with this appeared as large as ordinary sized peas. They cannot be seen in day-time, except the room be darkened, but are seen to the greatest advantage by candle-light. Montucla observes, that when any objects are inclosed within this transparent globule, the hinder part of it acts like a concave mirror, provided they be situated between that surface and the focus; and that by these means they are magnified three times and an half more than they would be in the usual way. An extempore microscope may be formed, by taking up a small drop of water on the point of a pin, and placing it over a fine hole made in a piece of metal; but as the refractive power of water is less than that of glass, these globules do not magnify so much as those of the same size which are made of glass: this was also contrived by Mr. Gray. The same ingenious author invented another water microscope, consisting of two drops of water, separated in part by a thin brass plate, but touching near the center; which were thus rendered equivalent to a double convex lens of unequal convexities.

Dr. Hooke describes a method of using the single microscope, which seems to have a great analogy to the foregoing methods of Mr. Gray. “If you are desirous,” says he, “of obtaining a microscope with one single refraction, and consequently capable of procuring the greatest clearness and brightness any one kind of microscope is susceptible of; spread a little of the fluid you intend to examine, on a glass plate, bring this under one of your microscopic globules, then move it gently upwards, till the fluid touch the globule, to which it will soon adhere, and that so firmly, as to bear being moved a little backwards or forwards. By looking through the globule, you will then have a perfect view of the animalculæ in the drop.”[11]

[11] Hooke’s Lectures and Conjectures, p. 98.

Having laid before the reader the principal improvements that have been suggested, or made in the single microscope, it remains only to point out those instruments of this kind, which, from the mode in which they are fitted up, seem best adapted for general use; the peculiar advantages of which, as well as the manner of using them, will be described in the third chapter of this work.

Fig. 1. Plate VI. A botanical microscope, contrived by Dr. Withering.

Fig. 2. Plate VI. A botanical microscope, by Mr. B. Martin, being the most universal pocket microscope.

Fig. 3. Plate VI, represents that which was used by M. Lyonnet for dissecting the cossus.

Fig. 5. Plate VI. The tooth and pinion microscope, which is now generally substituted in the room of Wilson’s. Fig. 1. Plate II. B.

Fig. 1. Plate VII. B. The aquatic microscope used by Mr. Ellis for investigating the nature of coralline, and recommended to botanists by Mr. Curtis, in his valuable publication, the “Flora Londinensis.”

Fig. 7. Plate VIII. A botanical magnifier, or hand megalascope, which by the different combinations of its three lenses produces seven different magnifying powers; when the three are used together, they are turned in, and the object viewed through the apertures in the sides.

Fig. 8. Plate VIII. A botanical magnifier, having one large lens and two small ones, but not admitting of more than three powers.

A COMPOUND MICROSCOPE, as it consists of two, three, or more glasses, is more easily varied, and is susceptible of greater changes in its construction, than the single microscope. The number of the lenses, of which it is formed, may be increased or diminished, their respective positions may be varied, and the form in which they are mounted be altered almost ad infinitum. But among these varieties, some will be found more deserving of attention than others; we shall here treat of these only.

The three first compound microscopes deserving of notice, are those of Dr. Hooke, Eustachio Divinis, and Philip Bonnani. Dr. Hooke gives an account of his in the preface to his Micrographia, which has been already cited; it was about three inches in diameter, seven long, and furnished with four draw-out tubes, by which it might be lengthened as occasion required: it had three glasses--a small object glass, a middle glass, and a deep eye glass. Dr. Hooke used all the glasses when he wanted to take in a considerable part of an object at once, as by the middle glass a number of radiating pencils were conveyed to the eye, which would otherwise have been lost: but when he wanted to examine with accuracy the small parts of any substance, he took out the middle glass, and only made use of the eye and object lenses; for the fewer the refractions are, the clearer and more bright the object appears.

An account of Eustachio Divinis’s microscope was read at the Royal Society, in 1668.[12] It consisted of an object lens, a middle glass, and two eye glasses, which were plano convex lenses, and were placed so that they touched each other in the center of their convex surfaces; by which means the glass takes in more of an object, the field is larger, the extremities of it less curved, and the magnifying power greater. The tube, in which the glasses were inclosed, was as large as a man’s leg, and the eye glasses as broad as the palm of the hand. It had four several lengths; when shut up, it was sixteen inches long, and magnified the diameter of an object forty-one times; at the second length, ninety times; at the third length, one hundred and eleven times; at the fourth length, one hundred and forty-three times. It does not appear that E. Divinis varied the object lenses.

[12] Philos. Trans. No. 42.

Philip Bonnani published an account of his two microscopes in 1698;[13] both were compound; the first was similar to that which Mr. Martin published as new, in his Micrographia Nova,[14] in 1742. His second was like the former, composed of three glasses, one for the eye, a middle glass, and an object lens; they were mounted in a cylindrical tube, which was placed in an horizontal position; behind the stage was a small tube, with a convex lens at each end; beyond this was a lamp; the whole capable of various adjustments, and regulated by a pinion and rack; the small tube was used to condense the light on the object, and spread it uniformly over it according to its nature, and the magnifying power that was used.

[13] Bonnani Observationes circa Viventia.

[14] Micrographia Nova, by B. Martin, 4to.

If the reader attentively consider the construction of the foregoing microscopes, and compare them with more modern ones, he will be led to think with me, that the compound microscope has received very little improvement since the time of Bonnani. Taken separately, the foregoing constructions are equal to some of the most famed modern microscopes. If their advantages be combined, they are far superior to that of M. Dellebarre, notwithstanding the pompous eulogium affixed thereto by Mess. De L’Academie Royale des Sciences.[15]

[15] Memoires sur les Differences de la Construction et des Effets du Microscope, de M. L. F. Dellebarre, 1777.

From this period, to the year 1736, the microscope appears not to have received any considerable alteration, but the science itself to have been at a stand. The improvements which were making in the reflecting telescope, naturally led those who had considered the subject, to expect a similar advantage would accrue to microscopes on the same principles: accordingly we find two plans of this kind; the first was that of Dr. Robert Barker. This instrument is entirely the same as the reflecting telescope, excepting the distance of the two speculums, which is lengthened, in order to adapt it to those pencils of rays which enter the telescope diverging; whereas, from very distant objects, they come in a direction nearly parallel. But this was soon laid aside, not only as it was more difficult to manage, but also because it was unfit for any but very small or transparent objects: for the object being between the speculum and the image, would, if it were large and opake, prevent a due reflection of light on the object.

The second was contrived by Dr. Smith.[16] In this there were two reflecting mirrors, one concave and the other convex; the image was viewed by a lens. This microscope, though far from being executed in the best manner, performed, says Dr. Smith, very well, so that he did not doubt but that it would have excelled others, if it had been properly finished.

[16] Dr. Smith’s Optics, Remarks, p. 94.

As some years are more favourable to the fruits of the earth, so also some periods are more favourable to particular sciences, being rich in discovery, and cultivated with ardor. Thus, in the year 1738, Dr. Lieberkühn’s invention of the solar microscope was communicated to the public: the vast magnifying power which was obtained by this instrument, the colossal grandeur with which it exhibited the minima of nature, the pleasure which arose from being able to display the same object to a number of observers at the same time, by affording a new source of rational amusement, increased the number of microscopic observers, who were further stimulated to the same pursuits by Mr. Trembley’s famous discovery of the polype: the wonderful properties of this little animal, together with the works of Mr. Trembley, Baker, and my father, revived the reputation of this instrument.[17]

[17] Trembley Memoires sur les Polypes. Baker’s Microscope made Easy; Attempt towards an History of the Polype; Employment for the Microscope. Adams’s Micrographia Illustrata. Joblot’s Observations d’Histoire Naturelle.

Every optician now exercised his talents in improving, as he called it, the microscope; in other words, in varying its construction, and rendering it different from that sold by his neighbour. Their principal object seemed to be, only to subdivide the instrument, and make it lie in as small a compass as possible; by which means, they not only rendered it complex and troublesome in use, but lost sight also of the extensive field, great light, and other excellent properties of the more ancient instruments; and, in some measure, shut themselves out from further improvements on the microscope. Every mechanical instrument is susceptible of almost infinite combinations and changes, which are attended with their relative advantages and disadvantages: thus, what is gained in power, is lost in time; “he that loves to be confined to a small house, must lose the benefit of air and exercise.”