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.”
The microscope, nearly at the same period, gave rise to M. Buffon’s
famous system of organic molecules, and M. Needham’s incomprehensible
ideas concerning a vegetable force and the vitality of matter. M. Buffon
has dressed up his system with all the charms of eloquence, presenting
it to the mind in the most agreeable and lively colours, exerting the
depths of erudition in the most interesting and seducing manner to
establish his hypothesis, making us almost ready to adopt it against
the dictates of reason, and the evidence of facts. But whether this
great man was misled by the warmth of his imagination, his attachment to
a favourite system, or the use of imperfect instruments, it appears but
too evident, that he was not acquainted with the objects whose nature he
attempted to investigate; and it is probable, that he never saw[18]
those which he supposed he was describing, continually confounding the
animalculæ produced from the putrifying decomposition of animal
substances, with the spermatic animalculæ, although they are two kinds
of beings, differing in form and nature; so that the beautiful fabric
attempted to be raised on his hypothesis, vanishes before the light of
truth and well conducted experiments.
[18] Porro Buffonius, ut cum illustris viri venia dicam, omnino non
videtur vermiculos seminales vidisse. Diuturnitas enim vitæ quam suis
corpusculis tribuit, ostendit non esse nostra animalcula (id est,
spermatica) quibus brevis et paucarum horarum vita est. Haller
Physiol. tom. 7.
After this period, the mind, either satisfied with the discoveries
already made, which will be particularly described hereafter, or tired
by its own exertions, sought for repose in other pursuits; so that for
several years this instrument was again, in some measure, laid aside. In
1770, Dr. Hill[19] published a treatise, in which he endeavoured to
explain the construction of timber by the microscope, and shew the
number, the nature, and office of its several parts, their various
arrangements and proportions in the different kinds; and point out a way
of judging, from the structure of trees, the uses they will best serve
in the affairs of life. So important a subject soon revived the ardor
for microscopic pursuits, which seems to have been increasing ever
since. About the same time, my father contrived an instrument for
cutting the transverse sections of wood, in order that the texture
thereof might be rendered more visible in the microscope, and
consequently be better understood; this instrument was afterwards
improved by Mr. Cumming. Another instrument for the same purpose, more
certain in its effects, and more easily managed, is represented in Fig.
1. Plate IX; and will be described in one of the following chapters. Dr.
Hill and Mr. Custance now endeavoured to bring back the microscope
nearer to the old standard, to increase the field by the multiplication
of the eye glasses, and to augment the light on the object, by
condensing lenses; and in this they happily succeeded: Mr. Custance was
unrivalled in his dexterity in preparing, and accuracy in cutting thin
transverse sections of wood.
[19] Dr. Hill on the Construction of Timber.
In 1771, my father published a fourth edition of his Micrographia, in
which he described the principal inventions then in use; particularly a
contrivance of his own, for applying the solar microscope to the camera
obscura, and illuminating it at night by a lamp, by which means a
picture of microscopic objects might be exhibited in winter evenings.
It appears[20] from the testimony of M. Æpinus, that Dr. Lieberkühn had
considerably improved the solar microscope, by adapting it to view opake
objects. This contrivance was by some means lost. The knowledge,
however, that such an effect had been produced, led Æpinus to attend to
the subject himself, in which he in some measure succeeded, and would,
no doubt, have brought it to perfection, if he had increased the size of
his illuminating mirror. Some further improvements were made on this
instrument by M. Ziehr; but the most perfect instrument of the kind, is
that of Mr. B. Martin, who published an account of it in the year
1774.[21] The common solar microscope does not shew the surface of any
object, whereas the opake solar microscope not only magnifies the
object, but exhibits on a screen an expanded picture of its surface,
with all its colours, in a most beautiful manner.
[20] Priestley’s Hist. of Optics, p. 743.
[21] Martin’s Description and Use of an Opake Solar Microscope. The
merits and ingenuity in constructing and improving microscopes by this
learned optician, seem to be unnoticed by our late author. The
following pamphlets by Mr. B. Martin are, among others of his valuable
publications, instances of his indefatigable industry. Description and
Use of a Pocket Reflecting Microscope, with a Micrometer; 1739.
Micrographia Nova, or a New Treatise on the Microscope; 1742.
Description of a New Universal Microscope; a Postscript to his New
Elements of Optics; 1759. Description of several Sorts of Microscopes,
and the Use of the Reflecting Telescope, as an universal Perspective
for viewing every Sort of Objects. Optical Essays; 1770. A Description
and Use of a Proportional Camera Obscura, with a Solar Microscope
adapted thereto, annexed to his Description of the Opake Solar
Microscope above-mentioned. Description of a New Universal Microscope;
1776. Description and Use of a Graphical Perspective and Microscope;
1771. Microscopium Polydynamicum, or a New Construction of a
Microscope; 1771. An Essay on the genuine Construction of a standard
Microscope and Telescope; 1776. Microscopium Pantometricum, or a new
Construction of a Micrometer adapted to the Microscope. The most
essential articles in the above works will be hereafter described.
EDIT.
About the year 1774, I invented the improved lucernal microscope; this
instrument does not in the least fatigue the eye: it shews all opake
objects in a most beautiful manner; and transparent objects may be
examined by it in various ways, so that no part of an object is left
unexplored; and the outlines of all may be taken with ease, even by
those who are most unskilled in drawing.
M. L. F. Dellebarre published an account of his microscope in the year
1777. It does not appear from this, that it was superior in any respect
to those that were made in England, but was inferior in others; for
those published by my father in 1771 possessed all the advantages of
Dellebarre’s in a higher degree, except that of changing the eye
glasses.