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Micrographia by Robert Hooke

R >> Robert Hooke >> Micrographia




But because we are certain, from the _Laws of refraction_ (which I I have
experimentally found to be so, by an Instrument I shall presently describe)
that _the lines of the angles of Incidence are proportionate to the lines
of the angles of Refraction_, therefore if Glasses could be made of those
kind of Figures, or some other, such as the most incomparable _Des Cartes_
has invented, and demonstrated in his Philosophical and Mathematical Works,
we might hope for a much greater perfection of Opticks then can be
rationally expected from spherical ones; for though, _caeteris paribus_, we
find, that the larger the _Telescope_ Object Glasses are, and the shorter
those of the _Microscope_, the better they magnify, yet both of them,
beside such determinate dimensions, are by certain inconveniences rendred
unuseful; for it will be exceeding _difficult_ to make and _manage_ a Tube
above an _hundred foot long_, and it will be as difficult to _inlighten_ an
Object less then an hundred part of an inch distant from the Object Glass.

I have not as yet made any attempts of that kind, though I know two or
three wayes, which, as far as I have yet considered, seem very probable,
and may invite me to make a tryal as soon as I have an opportunity, of
which I may hereafter perhaps acquaint the world. In the Interim, I shall
describe the Instrument I even now mention'd, by which the _refraction_ of
all kinds of Liquors may be most exactly measur'd, thereby to give the
curious an opportunity of making what further tryals of that kind they
shall think requisite to any of their intended tryals; and to let them see
that the laws of Refraction are not only notional.

The Instrument consisted of five Rulers, or long pieces placed
together, after the manner exprest in the second Figure of the first
_Scheme_, where AB denotes a straight piece of wood about six foot and
two inches long, about three inches over, and an inch and half thick,
on the back side of which was hung a small plummet by a line stretcht
from top to bottom, by which this piece was set exactly upright, and so
very firmly fixt; in the middle of this was made a hole or center, into
which one end of a hollow cylindrical brass Box CC, fashion'd as I
shall by and by describe, was plac'd, and could very easily and truly
be mov'd to and fro; the other end of this Box being put into, and
moving in, a hole made in a small arm DD; into this box was fastned the
long Ruler EF, about three foot and three or four inches long, and at
three foot from the above mention'd Centers PP was a hole E, cut
through, and cross'd with two small threads, and at the end of it was
fixt a small sight G, and on the back side of it was fixt a small Arm
H, with a Screw to fix it in any place on the Ruler LM; this Ruler LM
was mov'd on the Center B (which was exactly three foot distance from
the middle Center P) and a line drawn through the middle of it LM, was
divided by a Line of cords into some sixty degrees, and each degree was
subdivided into minutes, so that putting the cross of the threads in E
upon any part of this divided line, I presently knew what Angle the two
Rules AB and EF made with each other, and by turning the Screw in H, I
could fix them in any position. The other Ruler also RS was made much
after the same manner, only it was not fixt to the hollow cylindrical
Box, but, by means of two small brass Armes or Ears, it mov'd on the
Centers of it; this also, by means of the cross threads in the hole S,
and by a Screw in K, could be fastned on any division of another line
of cords of the same radius drawn on NO. And so by that means, the
Angle made by the two Rulers, AB and RS, was also known. The Brass box
CC in the middle was shap'd very much like the Figure X, that is, it
was a cylindrical Box stopp'd close at either end, off of which a part
both of the sides and bottomes was cut out, so that the Box, when the
Pipe and that was joyned to it, would contain the Water when fill'd
half full, and would likewise, without running over, indure to be
inclin'd to an Angle, equal to that of the greatest refraction of
Water, and no more, without running over. The Ruler EF was fixt very
fast to the Pipe V, so that the Pipe V directed the length of the Ruler
EF, and the Box and Ruler were mov'd on the Pin TT, so as to make any
desirable Angle with the Ruler AB. The bottom of this Pipe V was stop'd
with a small piece of exactly plain Glass, which was plac'd exactly
perpendicular to the Line of direction, or _Axis_ of the Ruler EF. The
Pins also TT were drill'd with small holes through the _Axis_, and
through those holes was stretcht and fastned a small Wire. There was
likewise a small Pipe of Tin loosly put on upon the end of V, and
reaching down to the sight G; the use of which was only to keep any
false Rayes of light from passing through the bottom of V, and only
admitting such to pass as pierced through the sight G: All things being
placed together in the manner describ'd in the Figure; that is, the
Ruler AB being fixt perpendicular, I fill'd the Box CC with Water, or
any other Liquor, whose refraction I intended to try, till the Wire
passing through the middle of it were just covered: then I moved and
fixt the Ruler FE at any assignable Angle, and placed the flame of a
Candle just against the sight G; and looking through the sight I, I
moved the Ruler RS to and fro, till I perceived the light passing
through G to be covered, as 'twere, or divided by the dark Wire passing
through PP: then turning the Screw in K, I fixt it in that posture: And
through the hole S, I observed what degree and part of it was cut by
the cross threads in S. And this gave me the Angle of Inclination, APS
answering to the Angle of Refraction BPE: for the surface of the Liquor
in the Box will be alwayes horizontal, and consequently AB will be a
perpendicular to it; the Angle therefore APS will measure, or be the
Angle of Inclination in the Liquor; next EPB must be the Angle of
Refraction, for the Ray that passes through the sight G, passes also
perpendicularly through the Glass _Diaphragme_ at F, and consequently
also perpendicularly through the lower surface of the Liquor contiguous
to the Glass, and therefore suffers no refraction till it meet with the
horizontal surface of the Liquor in CC, which is determined by the two
Angles.

By means of this Instrument I can with _little trouble_, and a very small
quantity of any _Liquor_, examine, most accurately, the _refraction_ of it
not only for one inclination, but for all; and thereby am inabled to make
very accurate Tables; several of which I have also experimentally made, and
find, that _Oyl of Turpentine_ has a much greater Refraction then _Spirit
of Wine_, though it be _lighter_; and that _Spirit of Wine_ has a greater
Refraction then _Water_, though it be lighter also; but that _salt Water_
also has a greater Refraction then _fresh_, though it be _heavier_: but
_Allum water_ has a less refraction then common _Water_, though heavier
also. So that it seems, as to the _refraction_ made in a Liquor, the
_specifick gravity_ is of no efficacy. By this I have also found that look
what _proportion _the _Sine_ of the Angle of _one Inclination_ has to the
_Sine_ of the Angle of _Refraction_, correspondent to it, the same
_proportion_ have all the _Sines_ of other Inclinations to the _Sines_ of
their appropriate Refractions.

My way for measuring how much a Glass magnifies an Object, plac'd at a
convenient distance from my eye, is this. Having rectifi'd the
_Microscope_, to see the desir'd Object through it very distinctly, at
the same time that I look upon the Object through the Glass with one
eye, I look upon other Objects at the same distance with my other bare
eye; by which means I am able, by the help of a _Ruler_ divided into
inches and small parts, and laid on the _Pedestal_ of the _Microscope_,
to cast, as it were, the magnifi'd appearance of the Object upon the
Ruler, and thereby exactly to measure the Diameter it appears of
through the Glass, which being compar'd with the Diameter it appears of
to the naked eye, will easily afford the quantity of its magnifying.

The _Microscope_, which for the most part I made use of, was shap'd
much like that in the sixth Figure of the first _Scheme_, the Tube
being for the most part not above six or seven inches long, though, by
reason it had four Drawers, it could very much be lengthened, as
occasion required; this was contriv'd with three Glasses; a small
Object Glass at A, a thinner Eye Glass about B, and a very deep one
about C: this I made use of only when I had occasion to see much of an
Object at once; the middle Glass conveying a very great company of
radiating Pencils, which would go another way, and throwing them upon
the deep Eye Glass. But when ever I had occasion to examine the small
parts of a Body more accurately, I took out the middle Glass, and only
made use of one Eye Glass with the Object Glass, for always the fewer
the Refractions are, the more bright and clear the Object appears. And
therefore 'tis not to be doubted, but could we make a_ Microscope _to
have one only refraction, it would, _caeteris paribus_, far excel any
other that had a greater number. And hence it is, that if you take a
very clear piece of a broken _Venice_ Glass, and in a Lamp draw it out
into very small hairs or threads, then holding the ends of these
threads in the flame, till they melt and run into a small round Globul,
or drop, which will hang at the end of the thread; and if further you
stick several of these upon the end of a stick with a little sealing
Wax, so as that the threads stand upwards, and then on a Whetstone
first grind off a good part of them, and afterward on a smooth Metal
plate, with a little Tripoly, rub them till they come to be very
smooth; if one of these be fixt with a little soft Wax against a small
needle hole, prick'd through a thin Plate of Brass, Lead, Pewter, or
any other Metal, and an Object, plac'd very near, be look'd at through
it, it will both magnifie and make some Objects more distinct then any
of the great _Microscopes_. But because these, though exceeding easily
made, are yet very troublesome to be us'd, because of their smallness,
and the nearness of the Object; therefore to prevent both these, and
yet have only two Refractions, I provided me a Tube of Brass, shap'd
much like that in the fourth Figure of the first _Scheme_; into the
smaller end of this I fixt with Wax a good _plano convex_ Object Glass,
with the convex side towards the Object, and into the bigger end I fixt
also with wax a pretty large plano _Convex_ Glass, with the _convex_
side towards my eye, then by means of the small hole by the side, I
fill'd the intermediate space between these two Glasses with very clear
Water, and with a Screw stopp'd it in; then putting on a Cell for the
Eye, I could perceive an Object more bright then I could when the
intermediate space was only fill'd with Air, but this, for other
inconveniences, I made but little use of.

My way for fixing both the Glass and Object to the Pedestal most
conveniently was thus: Upon one side of a round Pedestal AB, in the
sixth Figure of the first _Scheme_, was fixt a small Pillar CC, on this
was fitted a small Iron Arm D, which could be mov'd up and down, and
fixt in any part of the Pillar, by means of a small Screw E; on the end
of this Arm was a small Ball fitted into a kind of socket F, made in
the side of the Brass Ring G, through which the small end of the Tube
was screw'd; by means of which contrivance I could place and fix the
Tube in what posture I desir'd (which for many Observations was
exceeding necessary) and adjusten it most exactly to any Object.

For placing the Object, I made this contrivance; upon the end of a
small brass Link or Staple HH, I so fastned a round Plate II, that it
might be turn'd round upon its Center K, and going pretty stiff, would
stand fixt in any posture it was set; on the side of this was fixt a
small Pillar P, about three quarters of an inch high, and through the
top of this was thrust a small Iron pin M, whose top just stood over
the Center of the Plate; on this top I fixt a small Object, and by
means of these contrivances I was able to turn it into all kind of
positions, both to my Eye and the Light; for by moving round the small
Plate on its center, could move it one way, and by turning the Pin M, I
could move it another way, and this without stirring the Glass at all,
or at least but very little; the Plate likewise I could move to and fro
to any part of the Pedestal (which in many cases was very convenient)
and fix it also in any Position, by means of a Nut N, which was screw'd
on upon the lower part of the Pillar CC. All the other Contrivances are
obvious enough from the draught, and will need no description.

Now though this were the Instrument I made most use of, yet I have made
several other Tryals with other kinds of Microscopes, which both for
_matter_ and _form_ were very different from common spherical Glasses. I
have made a _Microscope_ with one piece of Glass, both whose surfaces were
_plains_. I have made another only with a _plano concave_, without any kind
of reflection, divers also by means of _reflection_. I have made others of
_Waters_, _Gums_, _Resins_, _Salts_, _Arsenick_, _Oyls_, and with divers
other _mixtures of watery_ and _oyly Liquors_. And indeed the subject is
capable of a great variety; but I find generally none more useful then that
which is made with _two Glasses_, such as I have already describ'd.

What the things are I observ'd, the following descriptions will manifest;
in brief, they were either _exceeding small Bodies_, or _exceeding small
Pores_, or _exceeding small Motions_, some of each of which the Reader will
find in the following Notes, and such, as I presume, (many of them at
least) will be _new_, and perhaps not less _strange_: Some _specimen_ of
each of which Heads the Reader will find in the subsequent delineations,
and indeed of some more then I was willing there should be; which was
occasioned by my first Intentions to print a much greater number then I
have since found time to compleat. Of such therefore as I had, I selected
only some few of every Head, which for some particulars seem'd most
observable, rejecting the rest as superfluous to the present Design.

What each of the delineated Subjects are, the following descriptions
annext to each will inform, of which I shall here, only once for all, add,
That in divers of them the Gravers have pretty well follow'd my directions
and draughts; and that in making of them, I indeavoured (as far as I was
able) first to discover the true appearance, and next to make a plain
representation of it. This I mention the rather, because of these kind of
Objects there is much more difficulty to discover the true shape, then of
those visible to the naked eye, the same Object seeming quite differing, in
one position to the Light, from what it really is, and may be discover'd in
another. And therefore I never began to make any draught before by many
examinations in several lights, and in several positions to those lights, I
had discover'd the true form. For it is exceeding difficult in some
Objects, to distinguish between a _prominency_ and a _depression_, between
a _shadow_ and a _black stain_, or a _reflection_ and a _whiteness in the
colour_. Besides, the transparency of most Objects renders them yet much
more difficult then if they were _opacous_. The Eyes of a Fly in one kind
of light appear almost like a Lattice, drill'd through with abundance of
small holes; which probably may be the Reason, why the Ingenious _Dr.
Power_ seems to suppose them such. In the Sunshine they look like a Surface
cover'd with golden Nails; in another posture, like a Surface cover'd with
Pyramids; in another with Cones; and in other postures of quite other
shapes; but that which exhibits the best, is the Light collected on the
Object, by those means I have already describ'd.

And this was undertaken in prosecution of the Design which the _ROYAL
SOCIETY_ has propos'd to it self. For the Members of the Assembly having
before their eys so many _fatal_ Instances of the errors and falshoods, in
which the greatest part of mankind has so long wandred, because they rely'd
upon the strength of humane Reason alone, have begun anew to correct all
_Hypotheses_ by sense, as Seamen do their _dead Reckonings_ by _Coelestial
Observations_; and to this purpose it has been their principal indeavour to
_enlarge & strengthen_ the _Senses_ by _Medicine_, and by such _outward
Instruments_ as are proper for their particular works. By this means they
find some reason to suspect, that those effects of Bodies, which have been
commonly attributed to _Qualities_, and those confess'd to be _occult_, are
perform'd by the small _Machines_ of Nature, which are not to be discern'd
without these helps, seeming the meer products of _Motion_, _Figure_, and
_Magnitude_; and that the _Natural Textures_, which some call the _Plastick
faculty_, may be made in _Looms_, which a greater perfection of Opticks may
make discernable by these Glasses; so as now they are no more puzzled about
them, then the vulgar are to conceive, how _Tapestry_ or _flowred Stuffs_
are woven. And the ends of all these Inquiries they intend to be the
_Pleasure_ of Contemplative minds, but above all, the _ease and dispatch_
of the labours of mens hands. They do indeed neglect no opportunity to
bring all the _rare_ things of Remote Countries within the compass of their
knowledge and practice. But they still acknowledg their _most useful_
Informations to arise from _common_ things, and from _diversifying_ their
most _ordinary_ operations upon them. They do not wholly reject Experiments
of meer _light_ and _theory_; but they principally aim at such, whose
Applications will _improve and facilitate_ the present way of _Manual
Arts_. And though some men, who are perhaps taken up about less honourable
Employments, are pleas'd to censure their proceedings, yet they can shew
more _fruits_ of their first three years, wherein they have assembled, then
any other _Society_ in _Europe_ can for a much larger space of time. 'Tis
true, such undertakings as theirs do commonly meet with small
incouragement, because men are generally rather taken with the _plausible_
and _discursive_, then the _real_ and the solid part of Philosophy; yet by
the good fortune of their institution, in an Age of all others the most
_inquisitive_, they have been assisted by the _contribution_ and _presence_
of very many of the chief _Nobility_ and _Gentry_, and others who are some
of the _most considerable_ in their several Professions. But that that yet
farther convinces me of the _Real esteem_ that the more _serious_ part of
men have of this _Society_, is, that several _Merchants_, men who act in
earnest (whose Object is _meum & tuum_, that great _Rudder_ of humane
affairs) have adventur'd considerable sums of _Money_, to put in practice
what some of our Members have contrived, and have continued _stedfast_ in
their good opinions of such Indeavours, when not one of a hundred of the
vulgar have believed their undertakings feasable. And it it also fit to be
added, that they have one advantage peculiar to themselves, that very many
of their number are _men of Converse and Traffick_; which is a good Omen,
that their attempts will bring Philosophy from _words_ to _action_, seeing
the men of Business have had so great a share in their first foundation.

And of this kind I ought not to conceal one particular _Generosity_, which
more nearly concerns my self. It is the _munificence_ of _Sir John Cutler_,
in endowing a Lecture for the promotion of _Mechanick Arts_, to be governed
and directed by This _Society._This _Bounty_ I mention for the
_Honourableness_ of the thing it self, and for the expectation which I have
of the _efficacy_ of the _Example_; for it cannot now be objected to them,
that their Designs will be esteemed _frivolous_ and _vain_, when they have
such a _real Testimony_ of the _Approbation_ of a _Man_ that is such an
_eminent Ornament_ of this renowned City, and one, who, by the _Variety_,
and the _happy Success_, of his negotiations, has given evident proofs,
that he is not easie to be deceiv'd. This Gentleman has well observ'd, that
the _Arts_ of life have been too long _imprison'd_ in the dark shops of
Mechanicks themselves, & there _hindred from growth_, either by ignorance,
or self-interest: and he has bravely _freed_ them from these
_inconveniences_: He hath not only obliged _Tradesmen_, but _Trade_ it
self: He has done a work that is worthy of _London_, and has taught the
chief City of Commerce in the world the right way how Commerce is to be
improv'd. We have already seen many other great signs of Liberality and a
large mind, from the same hand: For by his _diligence_ about the
_Corporation for the Poor_; by his honorable _Subscriptions_ for the
rebuilding of St. Paul's; by his chearful _Disbursment_ for the replanting
of _Ireland_, and by many other such _publick works_, he has shewn by what
means he indeavours to _establish_ his Memory; and now by this last gift he
has done that, which became one of the _wisest Citizens_ of our Nation to
accomplish, seeing one of the _wisest of our Statesmen, the Lord Verulam_,
first propounded it.

But to return to my Subject, from a digression, which, I hope, my Reader
will pardon me, seeing the Example is so rare that I can make no more such
digressions. If these my first Labours shall be any wayes useful to
inquiring men, I must attribute the incouragement and promotion of them to
a very _Reverend_ and _Learned Person_, of whom this ought in justice to be
said, _That there is scarce any one Invention, which this Nation has
produc'd in our Age, but it has some way or other been set forward by his
assistance_. My Reader, I believe, will quickly ghess, that it is _Dr.
Wilkins_ that I mean. He is indeed a man born for the _good_ of _mankind_,
and for the _honour_ of his _Country_. In the _sweetness_ of whose
_behaviour_, in the _calmness_ of his _mind_, in the _unbounded goodness_
of his _heart_, we have an evident Instance, what the true and the
_primitive unpassionate Religion_ was, before it was _sowred_ by particular
_Factions._ In a word, his _Zeal_ has been so _constant_ and _effectual_ in
advancing all good and profitable _Arts, that_ as one of the Antient
_Romans_ said of _Scipio_, _That he thanked God that he was a _Roman_;
because whereever _Scipio_ had been born, there had been the seat of the
Empire of the world_: So may I thank God, that _Dr. Wilkins_ was an
_Englishman_, for whereever he had lived, there had been the chief Seat of
_generous Knowledge_ and _true Philosophy_. To the truth of this, there are
so many worthy men living that will subscribe, that I am confident, what I
have here said, will not be looked upon, by any ingenious Reader, as a
_Panegyrick_, but only as a _real testimony_.

By the Advice of this _Excellent man_ I first set upon this Enterprise,
yet still came to it with much _Reluctancy_, because I was to follow the
footsteps of so eminent a Person as _Dr. Wren_, who was the first that
attempted any thing of this nature; whose original draughts do now make one
of the Ornaments of that great Collection of Rarities in the _Kings
Closet_. This _Honor_, which his first beginnings of this kind have
receiv'd, to be admitted into the most famous place of the world, did not
so much _incourage_, as the _hazard_ of coming after _Dr. Wren_ did
_affright_ me; for of him I must, affirm, that, since the time of
_Archimedes_, there scarce ever met in one man, in so great a perfection,
such a _Mechanical Hand_, and so _Philosophical_ a _Mind_.

But at last, being assured both by _Dr. Wilkins_, and _Dr. Wren_ himself,
that he had given over his intentions of prosecuting it, and not finding
that there was any else design'd the pursuing of it, I set upon this
undertaking, and was not a little incourag'd to proceed in it, by the
Honour the _Royal Society_ was pleas'd to favour me with, in approving of
those draughts (which from time to time as I had an opportunity of
describing) I presented to them. And particularly by the Incitements of
divers of those Noble and excellent Persons of it, which were my more
especial Friends, who were not less urgent with me for the publishing, then
for the prosecution of them.

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