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Micrographia by Robert Hooke

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This will be more plain, if in the seventh _Figure_ of the 37. _Scheme_ we
suppose AB to represent the sensible Horizon; CDEF, the body of the Sun
really below it; GHIK, the same appearing above it, elevated by the
inflection of the _Atmosphere_: For if, according to the best observation,
we make the visible Diameter of the Sun to be about three or four and
thirty minutes, and the Horizontal refraction according to _Ticho_ be
thereabout, or somewhat more, the lower limb of the Sun E, will be elevated
to I; but because, by his account, the point C will be elevated but 29.
minutes, as having not so great an inclination upon the inequality of the
Air, therefore IG, which will be the apparent refracted perpendicular
Diameter of the Sun, will be less then CG, which is but 29. minutes, and
consequently six or seven minutes shorter then the unrefracted apparent
Diameter. The parts, D and F, will be likewise elevated to H and K, whose
refraction, by reason of its inclination, will be bigger then that of the
point C, though less then that of E; therefore will the semidiameter IL, be
shorter then LG, and consequently the under side of the appearing Sun more
flat then the upper.

Now, because the Rays from the right and left sides of the Sun, &c. have
been observ'd by _Ricciolo_ and _Grimaldus_, to appear more distant one
from another then really they are, though (by very many Observations that I
have made for that purpose, with a very good _Telescope_, fitted with a
divided Ruler) I could never perceive any great alteration, yet there being
really some, it will not be amiss, to shew that this also proceeds from the
refraction or inflection of the _Atmosphere_; and this will be manifest, if
we consider the _Atmosphere_ as a transparent Globe, or at least a
transparent shell, encompassing an opacous Globe, which, being more dense
then the _medium_ encompassing it, refracts or inflects all the entring
parallel Rays into a point or focus, so that wheresoever the Observator is
plac'd within the _Atmosphere_, between the focus and the luminous body,
the _lateral_ Rays must necessarily be more converg'd towards his eye by
the refraction or inflection, then they would have been without it; and
therefore the Horizontal Diameter of the luminous body must necessarily be
augmented.

This might be more plainly manifest to the eye by the sixth _Figure_; but
because it would be somwhat tedious, and the thing being obvious enough to
be imagin'd by any one that attentively considers it, I shall rather omit
it, and proceed to shew, that the mass of Air neer the surface of the
Earth, consists, or is made up, of parcels, which do very much differ from
one another in point of density and rarity; and consequently the Rays of
light that pass through them will be variously inflected, here one way, and
there another, according as they pass so or so through those differing
parts; and those parts being always in motion, either upwards or downwards,
or to the right or left, or in some way compounded of these, they do by
this their motion inflect the Rays, now this way, and presently that way.

This irregular, unequal and unconstant inflection of the Rays of light, is
the reason why the limb of the _Sun_, _Moon_, _Jupiter_, _Saturn_, _Mars_,
and _Venus_, appear to wave or dance; and why the body of the Starrs appear
to tremulate or twinkle, their bodies, by this means, being sometimes
magnify'd, and sometimes diminished; sometimes elevated, otherwhiles
depress'd; now thrown to the right hand, and then to the left.

And that there is such a property or unequal distribution of parts, is
manifest from the various degrees of heat and cold that are found in the
Air; from whence will follow a differing density and rarity, both as to
quantity and refraction; and likewise from the vapours that are interpos'd,
(which, by the way, I imagine, as to refraction or inflection, to do the
same thing, as if they were rarify'd Air; and that those vapours that
ascend, are both lighter, and less dense, then the ambient Air which boys
them up; and that those which descend, are heavier and more dense) The
first of these may be found true, if you take a good thick piece of Glass,
and heating it pretty hot in the fire, lay it upon such another piece of
Glass, or hang it in the open Air by a piece of Wire, then looking upon
some far distant Object (such as a Steeple or Tree) so as the Rays from
that Object pass directly over the Glass before they enter your eye, you
shall find such a tremulation and wavering of the remote Object, as will
very much offend your eye: The like tremulous motion you may observe to be
caus'd by the ascending steams of Water, and the like. Now, from the first
of these it is manifest, that from the rarifaction of the parts of the Air,
by heat, there is caus'd a differing refraction, and from the ascension of
the more rarify'd parts of the Air, which are thrust up by the colder, and
therefore more condens'd and heavie, is caus'd an undulation or wavering of
the Object; for I think, that there are very few will grant, that Glass, by
as gentle a heat as may be endur'd by ones hand, should send forth any of
its parts in steams or vapours, which does not seem to be much wasted by
that violent fire of the green Glass-house; but, if yet it be doubted, let
Experiment be further made with that body that is accounted, by Chymists
and others, the most ponderous and fix'd in the world; for by heating of a
piece of Gold, and proceeding in the same manner, you may find the same
effects.

This trembling and shaking of the Rays, is more sensibly caus'd by an
actual flame, or quick fire, or anything else heated glowing hot; as by a
Candle, live Coal, red-hot Iron, or a piece of Silver, and the like: the
same also appears very conspicuous, if you look at an Object betwixt which
and your eye, the rising smoak of some Chimney is interpos'd; which brings
into my mind what I had once the opportunity to observe, which was, the Sun
rising to my eye just over a Chimney that sent forth a copious steam of
smoak; and taking a short _Telescope_, which I had then by me, I observ'd
the body of the Sun, though it was but just peep'd above the Horizon, to
have its underside, not onely flatted, and press'd inward, as it usually is
when neer the Earth; but to appear more protuberant downwards then if it
had suffered no refraction at all; and besides all this, the whole body of
the Sun appear'd to tremble or dance, and the edges or limb to be very
ragged or indented, undulating or waving, much in the manner of a flag in
the Wind.

This I have likewise often observ'd in a hot Sunshiny Summer's day, that
looking on an Object over a hot stone, or dry hot earth, I have found the
Object to be undulated or shaken, much after the same manner. And if you
look upon any remote Object through a _Telescope_ (in a hot Summer's day
especially) you shall find it likewise to appear tremulous. And further, if
there chance to blow any wind, or that the air between you and the Object
be in a motion or current, whereby the parts of it, both rarify'd and
condens'd, are swiftly remov'd towards the right or left, if then you
observe the Horizontal ridge of a Hill far distant, through a very good
_Telescope_, you shall find it to wave much like the Sea, and those waves
will appear to pass the same way with the wind.

From which, and many other Experiments, 'tis cleer that the lower Region of
the Air, especially that part of it which lieth neerest to the Earth, has,
for the most part, its constituent parcels variously agitated, either by
heat or winds, by the first of which, some of them are made more rare, and
so suffer a less refraction; others are interwoven, either with ascending
or descending vapours; the former of which being more light, and so more
rarify'd, have likewise a less refraction; the latter being more heavie,
and consequently more dense, have a greater.

Now, because that heat and cold are equally diffus'd every way; and that
the further it is spread, the weaker it grows; hence it will follow, that
the most part of the under Region of the Air will be made up of several
kinds of _lentes_, some whereof will have the properties of _Convex_,
others of _Concave_ _glasses_, which, that I may the more intelligibly make
out, we will suppose in the eighth _Figure_ of the 37. _Scheme_, that A
represents an ascending vapour, which, by reason of its being somewhat
_Heterogeneous_ to the ambient Air, is thereby thrust into a kind of
Globular form, not any where terminated, but gradually finished, that is,
it is most rarify'd in the middle about A. somewhat more condens'd about
BB, more then that about CC; yet further, about DD, almost of the same
density with the ambient Air about EE;, and lastly, inclosed with the more
dense Air FF, so that from A, to FF, there is a continual increase of
density. The reason of which will be manifest, if we consider the rising
vapour to be much warmer then the ambient heavie Air; for by the coldness
of the ambient Air, the shell EE will be more refrigerated then DD, and
that then CC, which will be yet more then BB, and that more then A; so that
from F to A, there is a continual increase of heat, and consequently of
rarity; from whence it will necessarily follow, that the Rays of light will
be inflected or refracted in it, in the same manner as they would be in a
_Concave-glase_; for the Rays _GKI_, _GKI_ will be inflected by _GKH_,
_GKH_, which will easily follow from what I before explained concerning the
inflection of the _Atmosphere_.

On the other side, a descending vapour, or any part of the air included by
an ascending vapour, will exhibit the same effects with a _Convex lens_;
for, if we suppose, in the former Figure, the quite contrary constitution
to that last describ'd; that is, the ambient Air FF being hotter then any
part of that matter within any circle, therefore the coldest part must
necessarily be A, as being farthest remov'd from the heat, all the
intermediate spaces will be gradually discriminated by the continuall
mixture of heat and cold, so that it will be hotter at EE, then DD, in DD
then CC, in CC then BB, and in BB then A. From which, a like refraction and
condensation will follow, and consequently a lesser or greater refraction,
so that every included part will refract more then the including, by which
means the Rays, GKI, GKI, coming from a Starr, or some remote Object, are
so inflected, that they will again concurr and meet, in the point M. By the
interposition therefore of this desending vapour the visible body of the
Star, or other Object, is very much augmented, as by the former it was
diminished.

From the quick consecutions of these two, one after another, between the
Object and your eye, caused by their motion upwards or downwards,
proceeding from their levity or gravity, or to the right or left,
proceeding from the wind, a Starr may appear, now bigger, now less, then
really it would otherwise without them; and this is that property of a
Starr, which is commonly call'd twinkling, or scintillation.

The reason why a Star will now appear of one colour, now of another, which
for the most part happens when 'tis neer the Horizon, may very easily be
deduc'd from its appearing now in the middle of the vapour, other whiles
neer the edge; for if you look against the body of a Starr with a
_Telescope_ that has a pretty deep _Convex_ Eye-glass, and so order it,
that the Star may appear sometimes in one place, and sometimes in another
of it; you may perceive this or that particular colour to be predominant in
the apparent Figure of the Starr, according as it is more or less remote
from the middle of the _Lens_. This I had here further explain'd, but that
it does more properly belong to another place.

I shall therefore onely add some few Queries, which the consideration of
these particulars hinted, and so finish this Section.

And the first I shall propound is, Whether there may not be made an
artificial transparent body of an exact Globular Figure that shall so
inflect or refract all the Rays, that, coming from one point, fall upon any
_Hemisphere_ of it; that every one of them may meet on the opposite side,
and cross one another exactly in a point; and that it may do the like also
with all the Rays that, coming from a _lateral_ point, fall upon any other
_Hemisphere_; for if so, there were to be hoped a perfection of
_Dioptricks_, and a transmigration into heaven, even whil'st we remain here
upon earth in the flesh, and a descending or penetrating into the center
and innermost recesses of the earth, and all earthly bodies; nay, it would
open not onely a cranney, but a large window (as I may so speak) into the
Shop of Nature, whereby we might be enabled to see both the tools and
operators, and the very manner of the operation it self of Nature; this,
could it be effected, would as farr surpass all other kind of perspectives
as the vast extent of Heaven does the small point of the Earth, which
distance it would immediately remove, and unite them, as 'twere, into one,
at least, that there should appear no more distance between them then the
length of the Tube, into the ends of which these Glasses should be
inserted: Now, whether this may not be effected with parcels of Glass of
several densities, I have sometimes proceeded so farr as to doubt (though
in truth, as to the general, I have wholly despair'd of it) for I have
often observ'd in Optical Glasses a very great variety of the parts, which
are commonly called Veins; nay, some of them round enough (for they are for
the most part, drawn out into firings) to constitute a kind of _lens_.

This I should further proceed to hope, had any one been so inquisitive as
to have found out the way of making any transparent body, either more dense
or more rare, for then it might be possible to compose a Globule that
should be more dense in the middle of it, then in any other part, and to
compose the whole bulk, so as that there should be a continual gradual
transition from one degree of density to another; such as should be found
requisite for the desired inflection of the _transmigrating_ Rays; but of
this enough at present, because I may say more of it when I set down my own
Trials concerning the melioration of _Dioptricks_, where I shall enumerate
with how many several substances I have made both _Microscopes_, and
_Telescopes_, and by what and how many, ways: Let such as have leisure and
opportunity farther consider it.

The next Quaery shall be, whether by the same collection of a more dense
body then the other, or at least, of the denser part of the other, there
might not be imagin'd a reason of the apparition of some new fix'd Stars,
as those in the Swan, _Cassiope's Charr_, _Serpentarius_, _Piscis_,
_Cetus_, &c.

Thirdly, Whether it be possible to define the height of the _Atmosphere_
from this inflection of the Rays, or from the Quicksilver Experiment of the
rarifaction or extension of the Air.

Fourthly, Whether the disparity between the upper and under Air be not
sometimes so great, as to make a reflecting superficies; I have had several
Observations which seem to have proceeded from some such cause, but it
would be too long to relate and examine them. An Experiment, also somewhat
analogous to this, I have made with Salt-water and Fresh, which two
liquors, in most Positions, seem'd the same, and not to be separated by any
determinate superficies, which separating surface yet in some other
Positions did plainly appear.

And if so, Whether the reason of the equal bounding or _terminus_ of the
under parts of the clouds may not proceed from this cause; whether,
secondly, the Reason of the apparition of many Suns may not be found out,
by considering how the Rays of the Sun may so be reflected, as to describe
a pretty true Image of the body, as we find them from any regular
Superficies. Whether also this may not be found to cause the apparition of
some of those _Parelii_, of counterfeit Suns, which appear coloured, by
refracting the Rays so, as to make the body of the Sun appear in quite
another place then really it is. But of this more elsewhere.

5. Whether the _Phaenomena_ of the Clouds may not be made out by this
diversity of density in the upper and under parts of the Air, by supposing
the Air above them to be much lighter then they themselves are, and they
themselves to be yet lighter then that which is subjacent to them, many of
them seeming to be the same substance with the Cobwebs that fly in the Air
after a Fog.

Now that such a constitution of the Air and Clouds, if such there be, may
be sufficient to perform this effect, may be confirm'd by this Experiment.

Make as strong a Solution of Salt as you are able, then filling a Glass of
some depth half full with it, fill the other half with fresh Water, and
poyse a little Glass-bubble, so as that it may sink pretty quick in fresh
Water, which take and put into the aforesaid Glass, and you shall find it
to sink till it comes towards the middle, where it will remain fixt,
without moving either upwards or downwards. And by a second Experiment, of
poising such a bubble in water, whose upper part is warmer, and
consequently lighter, then the under, which is colder and heavier; the
manner of which follows in this next Quaery, which is,

6. Whether the rarifaction and condensation of Water be not made after the
same manner, as those effects are produc'd in the Air by heat; for I once
pois'd a seal'd up Glass-bubble so exactly, that never so small an addition
would make it sink, and as small a detraction make it swim, which suffering
to rest in that Vessel of Water for some time, I alwayes found it about
noon to be at the bottom of the Water, and at night, and in the morning, at
the top: Imagining this to proceed from the Rarifaction of the Water,
caus'd by the heat, I made tryal, and found most true; for I was able at
any time, either to depress, or raise it, by heat and cold; for if I let
the Pipe stand for some time in cold water, I could easily raise the Bubble
from the bottom, whither I had a little afore detruded it, by putting the
same Pipe into warm Water. And this way I have been able, for a very
considerable time, to keep a Bubble so poys'd in the Water, as that it
should remain in the middle, and neither sink, nor swim: For gently heating
the upper part of the Pipe with a Candle, Coal, or hot Iron, till I
perceived the Bubble begin to descend, then forbearing, I have observed it
to descend to such or such a station, and there to remain suspended for
some hours, till the heat by degrees were quite vanished, when it would
again ascend to its former place. This I have also often observed naturally
performed by the heat of the Air, which being able to rarifie the upper
parts of the Water sooner then the lower, by reason of its immediate
contact, the heat of the Air has sometimes so slowly increased, that I have
observed the Bubble to be some hours in passing between the top and bottom.

7. Whether the appearance of the _Pike_ of _Tenerif_, and several other
high Mountains, at so much greater a distance then seems to agree with
their respective heights, be not to be attributed to the _Curvature_ of the
visual Ray, that is made by its passing obliquely through so differingly
_Dense_ a Medium from the top to the eye very far distant in the Horizon:
For since we have already, I hope, made it very probable, that there is
such an _inflection_ of the Rays by the differing density of the parts of
the Air; and since I have found, by several Experiments made on places
comparatively not very high, and have yet found the pressure sustain'd by
those parts of the Air at the top and bottom, and also their differing
Expansions very considerable: Insomuch that I have found the pressure of
the _Atmosphere_ lighter at the top of St. _Paul_'s Steeple in _London_
(which is about two hundred foot high) then at the bottom by a sixtieth or
fiftieth part, and the expansion at the top greater then that at the bottom
by neer about so much also; for the _Mercurial Cylinder_ at the bottom was
about 39. inches, and at the top half an inch lower; the Air also included
in the Weather-glass, that at the bottom fill'd only 155. spaces, at the
top fill'd 158. though the heat at the top and bottom was found exactly the
same with a scal'd _Thermometer_: I think it very rational to suppose, that
the greatest Curvature of the Rays is made nearest the Earth, and that the
inflection of the Rays, above 3. or 4. miles upwards, is very
inconsiderable, and therefore that by this means such calculations of the
height of Mountains, as are made from the distance they are visible in the
Horizon, from the supposal that that Ray is a straight Line (that from the
top of the Mountain is, as 'twere, a Tangent to the Horizon whence it is
seen) which really is a _Curve_, is very erroneous. Whence, I suppose,
proceeds the reason of the exceedingly differing Opinions and Assertions of
several Authors, about the height of several very high Hills.

8. Whether this Inflection of the Air will not very much alter the supposed
distances of the Planets, which seem to have a very great dependence upon
the Hypothetical refraction or inflection of the Air, and that refraction
upon the hypothetical height and density of the Air: For since (as I hope)
I have here shewn the Air to be quite otherwise then has been hitherto
suppos'd, by manifesting it to be, both of a vast, at least an uncertain,
height, and of an unconstant and irregular density; It must necessarily
follow, that its inflection must be varied accordingly: And therefore we
may hence learn, upon what sure grounds all the Astronomers hitherto have
built, who have calculated the distance of the Planets from their
Horizontal _Parallax_; for since the Refraction and _Parallax_ are so
nearly ally'd, that the one cannot be known without the other, especially
by any wayes that have been yet attempted, how uncertain must the
_Parallax_ be, when the Refraction is unknown? And how easie is it for
Astronomers to assign what distance they please to the Planets, and defend
them, when they have such a curious _subterfuge_ as that of Refraction,
wherein a very little variation will allow them liberty enough to place the
Celestial Bodies at what distance they please.

If therefore we would come to any certainty in this point, we must go other
wayes to work; and as I have here examined the height and refractive
property of the Air by other wayes then are usual, so must we find the
Parallax of the Planets by wayes not yet practiced; and to this end, I
cannot imagine any better way, then the Observations of them by two persons
at very far distant parts of the Earth, that lye as neer as may be under
the same Meridian, or Degree of longitude, but differing as much in
latitude, as there can be places conveniently found: These two persons, at
certain appointed times, should (as near as could be) both at the same
time, observe the way of the _Moon_, _Mars_, _Venus_, _Jupiter_, and
_Saturn_, amongst the fixt Stars, with a good large _Telescope_, and making
little Iconismes, or pictures, of the small fixed Stars, that appear to
each of them to lye in or near the way of the Center of the Planet, and the
exact measure of the apparent Diameter; from the comparing of such
Observations together, we might certainly know the true distance, or
Parallax, of the Planet. And having any one true Parallax of these Planets,
we might very easily have the other by their apparent Diameters, which the
_Telescope_ likewise affords us very accurately. And thence their motions
might be much better known, and their Theories more exactly regulated. And
for this purpose I know not any one place more convenient for such an
Observation to be made in, then in the Island of St. _Helena_, upon the
Coast of _Africk_, which lyes about sixteen degrees to the Southwards of
the Line, and is very near, according to the latest Geographical Maps, in
the same Meridian with _London_; for though they may not perhaps lye
exactly in the same, yet their Observations, being ordered according to
what I shall anon shew, it will not be difficult to find the true distance
of the Planet. But were they both under the same Meridian, it would be much
better.

And because Observations may be much easier, and more accurately made with
good _Telescopes_, then with any other Instruments, it will not, I suppose,
seem impertinent to explain a little what wayes I judge most fit and
convenient for that particular. Such therefore as shall be the Observators
for this purpose, should be furnished with the best _Telescopes_ that can
be had, the longer the better and more exact will their Observations be,
though they are somewhat the more difficultly manag'd. These should be
fitted with a _Rete_, or divided Scale, plac'd at such a distance within
the Eye-glass, that they may be distinctly seen, which should be the
measures of minutes and seconds; by this Instrument each Observator should,
at certain prefixt times, observe the Moon, or other Planet, in, or very
near, the Meridian; and because it may be very difficult to find two
convenient stations that will happen to be just under the same Meridian,
they shall, each of them, observe the way of the Planet, both for an hour
before, and an hour after, it arrive at the Meridian; and by a line, or
stroke, amongst the small fixed Stars, they shall denote out the way that
each of them observ'd the Center of the Planet to be mov'd in for those two
hours: These Observations each of them shall repeat for many dayes
together, that both it may happen, that both of them may sometimes make
their Observations together, and that from divers Experiments we may be the
better assured of what certainty and exactness such kind of Observations
are like to prove. And because many of the Stars which may happen to come
within the compass of such an _Iconism_, or Map, may be such as are only
visible through a good _Telescope_, whose Positions perhaps have not been
noted, nor their longitudes, or latitudes, any where remarked; therefore
each Observator should indeavour to insert some fixt Star, whose longitude,
and latitude, is known; or with his _Telescope_ he shall find the Position
of some notable _telescopical_ Star, inserted in his Map, to some known
fixt Star, whose place in the _Zodiack_ is well defin'd.

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