_Note_, There are but three Affections of Light in which I have observ'd
its Rays to differ; _viz._ Refrangibility, Reflexibility, and Colour;
and those Rays which agree in Refrangibility, agree also in the other
two, and therefore may well be defined Homogeneal; especially since Men
usually call those things Homogeneal, which are so in all Qualities that
come under their Knowledge, tho' in other Qualities, that their
Knowledge extends not to, there may possibly be some Heterogeneity.
3. Those Colours I call Simple or Homogeneal, which are exhibited by
Homogeneal Light.
4. And those Compound or Heterogeneal, which are exhibited by
Heterogeneal Light.
5. Different Colours, I call, not only the more eminent Species, Red,
Yellow, Green, Blue, Purple, but all other the minutest Gradations; much
after the same manner, that not only the more eminent Degrees in Musick,
but all the lead Gradations, are esteem'd different Sounds.
_PROPOSITIONS._
1. The Sun's Light consists of Rays differing by indefinite Degrees of
Refrangibility.
2. Rays which differ in Refrangibility, when parted from one another, do
proportionally differ in the Colours which they exhibit. These Two
Propositions are Matter of Fact.
3. There are as many Simple or Homogeneal Colours, as Degrees of
Refrangibility. For to every Degree of Refrangibility belongs a
different Colour, by _Prop._ 2. and that Colour is Simple, by _Def._ 1,
and 3.
4. Whiteness, in all respects like that of the Sun's immediate Light,
and of all the usual Objects of our Senses, cannot be compounded of two
Simple Colours alone. For such a Composition must be made by Rays that
have only two Degrees of Refrangibility, by _Def._ 1 and 3. and
therefore it cannot be like that of the Sun's Light. by _Prop._ 1. nor,
for the same Reason, like that of ordinary white Objects.
5. Whiteness, in all respects, like that of the Sun's immediate Light,
cannot be compounded of Simple Colours without an indefinite Variety of
them. For to such a Composition, there are requisite Rays endu'd with
all the indefinite Degrees of Refrangibility, by _Prop._ 1. And those
infer as many Simple Colours, by _Def._ 1 and 3. and _Prop._ 2 and 3.
To make these a little plainer, I have added also the Propositions that
follow.
6. The Rays of Light do not act on one another, in passing through the
same Medium.
7. The Rays of Light suffer not any change of their Qualities from
Refraction.
8. Nor afterwards from the adjacent quiet _Medium_: These two
Propositions are manifest _de Facto_ in Homogeneal Light, whose Colour
and Refrangibility is not at all changeable, either by Refraction, or by
the Contermination of a quiet _Medium_. And as for Heterogeneal Light,
it is but an Aggregate of several sorts of Homogeneal Light, no one sort
of which suffers any more alteration than if it were alone; because the
Rays act not on one another, by _Prop._ 6. and therefore the Aggregate
can suffer none. These two Propositions also might be further proved
apart, by Experiments too long to be here described.
9. There can no Homogeneal Colours be reduced out of Light by
Refraction, which were not commixt in it before. Because by _Prop._ 7.
and 8. Refraction changeth not the Qualities of the Rays, but only
separates those which have divers Qualities, by means of their different
Refrangibility.
10. The Sun's Light is an Aggregate of an indefinite variety of
Homogeneal Colours, by _Prop._ 1, 3, and 9. And hence it is, that I call
Homogeneal Colours also Primitive or Original. And thus much concerning
Colours.
_For a further Illustration of this Doctrine, Mr. _Newton_, in his Book
of Opticks lately published, has by undeniable Experiments explained
most of the Principal Phænomena of Light and Colours: To which we
refer the Reader._
_A Demonstration concerning the Motion of Light, communicated from
_Paris_._
Philosophers have been labouring for many Years to decide by some
Experiment, whether the Action of Light be conveyed in an instant to
distant Places, or whether it requireth time. M. _Romer_, of the _Royal
Academy_ of Sciences, hath devised a way taken from the Observations of
the first Satellit of _Jupiter_, by which he demonstrates, that for the
distance of about 3000 Leagues, such as is very near the bigness of the
Diameter of the Earth, Light needs not one Second of Time.
Let (in _Fig. 3. Plate 3._) _A_ be the _Sun_, _B_ _Jupiter_, _C_ the
first Satellit of _Jupiter_, which enters into the shadow of _Jupiter_,
to come out at _D_, and let _EFGHKL_ be the _Earth_, placed at divers
distances from _Jupiter_.
Now suppose the _Earth_, being in _L_, towards the second Quadrature of
_Jupiter_, hath seen the first Satellit, at the time of its emersion, or
issuing out of the shadow at _D_, and that about 42½ Hours after
(_viz._ after one Revolution of this _Satellit_) the _Earth_ being in
_K_, do see it return'd in _D_: It is manifest, that if the Light
require time to traverse the Interval _LK_, the _Satellit_ will be seen
return'd later in _D_, than it would have been if the _Earth_ had
remained in _L_. So that the Revolution of the _Satellit_ being thus
observ'd by the Emersions, will be retarded by so much time, as the
Light shall have taken in passing from _L_ to _K_; and that on the
contrary, in the other Quadrature _FG_, where the _Earth_ by approaching
goes to meet the Light, the Revolutions of the Emersions will appear to
be shortned, by so much as those of the Emersions had appear'd to be
lengthned. And because 42½ Hours, which this _Satellit_ very near
takes to make one Revolution, the distance between the _Earth_ and
_Jupiter_, in both the Quadratures, varies at least 210 Diameters of the
_Earth_: It follows, that if for the Account of every Diameter of the
_Earth_ there were required a Second of Time, the Light would take 3½
Minutes for each of the Intervals _GF_, _KL_; which would cause near
half a quarter of an Hour between two Revolutions of the first
_Satellit_, one observ'd in _FG_, and the other in _KL_, whereas there
is not observed any sensible difference.
Yet doth it not follow hence, that Light demands no time. For after M.
_Romer_ had examin'd the thing more nearly, he found that what was not
sensible in two Revolutions, became very considerable in many being
taken together; and that, for Example, forty Revolutions observed on the
side _F_, might be sensibly shorter, than forty others observ'd in any
place of the _Zodiack_ where _Jupiter_ may be met with; and that in
proportion of Twenty two for the whole Interval of _HE_, which is the
double of the Interval that is from hence to the Sun.
The necessity of this new Equation of the Retardment of Light, is
establish'd by all the Observations that have been made in the _Royal
Academy_, and in the _Observatory_, for the space of eight Years; and it
hath been lately confirmed by the Emersion of the first _Satellit_
observ'd at _Paris_, the _9th_ of _November_ last, at 5 a-clock 35' 45"
at Night, 10 Minutes later than it was to be expected, by deducting it
from those that had been observ'd in the Month of _August_, when the
_Earth_ was much nearer to _Jupiter_; which M. _Romer_ had predicted to
the said Academy from the beginning of _September_.
But to remove all doubt, that this Inequality is caused by the
Retardment of the Light, he demonstrates, that it cannot come from any
Excentricity, or other Cause of those that are commonly alledged to
explicate the Irregularities of the _Moon_, and the other Planets;
though he be well aware, that the first _Satellit_ of _Jupiter_ was
Excentrick; and that, besides his Revolutions were advanced or retarded,
according as _Jupiter_ did approach to or recede from the Sun; as also,
that the Revolutions of the _Primum Mobile_ were unequal: Yet, saith he,
these three last Causes of Inequality do not hinder the first from being
manifest.
_An introductory _Essay_ to the Doctrine of _Sounds_, containing some
Proposals for the improvement of _Acousticks_; As it was presented to
the _Dublin Society_, Nov. 12. 1683, by the Right Reverend Father in
God _Narcissus_ Lord Bishop of _Ferns_ and _Leighlin_._
Being to treat of the Doctrine of _Sounds_, I hold it convenient to
premise something in the general, concerning this Theory; which may
serve at once to ingage your Attention, and excuse my Pains, when I
shall have recommended them, as bestow'd on a Subject not altogether
useless and unfruitful.
And for this purpose I shall omit to speak any thing of the _Excellency_
of the Matter in Hand; though it might be celebrated by Arguments drawn
from several Topicks, and particularly from this, that new Discoveries
and Improvements may be made, both as to the _Generation_, _Propagation_
and _Reception_ of Sounds into the Sense; which, in a peculiar manner
agrees to this, above the Object of any other Sense whatsoever. I shall,
I say, omit these things, and apply my self wholly to the _Usefulness_
of the Theory, that we are now falling upon, which I think cannot better
be discovered, than by making a comparison 'twixt the Senses of _Seeing_
and _Hearing_, as to their Improvements. I mean, by shewing, that this
latter of _Hearing_ is capable of all those improvements which the Sense
of _Seeing_ has receiv'd from Art, besides many more advantages that the
_Ear_ may enjoy, by the help of our Doctrine, above the _Eye_; all which
moreover will be of as great benefit to Mankind, as any thing that
_Opticks_ have yet discover'd, if not of greater; which, with some other
pre-eminencies that it has upon another Score, will happily render
_Acousticks_ the nobler Science of the two.
In order to the making good what I but now premised of the Comparison of
these two Faculties of _Seeing_ and _Hearing_, as to their Improvements,
I observe;
That _Vision_ is threefold, _Direct_, _Refracted_, and _Reflex'd_;
answerable whereunto we have _Opticks_, _Dioptricks_, and _Catoptricks_.
In like manner _Hearing_ may be divided into _Direct_, _Refracted_ and
_Reflex'd_; whereto answer three parts of our Doctrine of _Acousticks_,
which are yet nameless, unless we call them _Acousticks_, _Diacousticks_,
and _Catacousticks_, or (in another Sense, but to as good Purpose)
_Phonicks_, _Diaphonicks_, and _Cataphonicks_.
I. Direct Vision has been improv'd two ways, _ex parte_ Objecti, and _ex
parte_ Organi _vel_ Medii.
1. _Ex parte Objecti_, Direct Vision has receiv'd advantages by the Arts
of _Producing_, _Conserving_ and _Imitating Light_ and _Colours_, which
are the Objects of Vision.
1. For the Art of _Producing Light_, we have the Frication of all hard
Bodies that beget Fire; especially of the Flint and Steel; and instead
of the Flint, most hard Stones (as well as the Cane) may be us'd to the
same effect, as upon trial I have found. Add hereto the lately invented
_Phosphorus_, which is a new and admirable way of producing a _Lucid
Substance_ by Art, out of a Body in itself not _Lucid_; and therefore
may not unfitly be term'd an _Artificial production_ of _Light_.
And then of the Art of _Conserving Light_, the _Lapis Bononiensis_ is a
notable Instance; and so happily were the _Sepulchral Lamps_ of the
Ancients.
2. As to _Colours_, 'tis the greatest part of the Art of _Dying_ to be
able to make and fix (that is preserve) them; and the _Painters_ and
_Limners_ will own it to be no small part of their Skill to be able well
to _Mix_ (that is, in effect, to _Generate_) _Colours_.
3. For _Imitation of Light_ and _Colours_, 'tis well known how far
_Perspective_ with the Art of _Limning_ and _Shadowing_ have gone
therein, which all tend some way to the Advance or Improvement of
_Direct Vision_.
Add to all these, That _a due Application of Light to the Object_
renders it Visible, if it were not so before; as appears from a dark
Room illuminated; or else makes it better and more truly discernable by
the Sense of _Seeing_, if before it might have been discern'd.
Hence the same _Colour_, in a diverse Light, will appear different, and
no _Picture_ can well be discern'd or judg'd of but by its true Light.
Besides, the _Limner_ will assure you, that he can hardly make true
Work, or hit the Air of a Face exactly, unless he draw by a
_North-Light_, by reason of the steadiness of that, and the uncertainty
of all other Lights whatsoever. Which things shew, that the _Art of duly
applying Light to the Object_ does very much help and improve Vision. So
also does the due placing of the _Object_, as to _Height_ and
_Distance_. But to enumerate all things that help _Direct Vision_, would
be infinite.
2. _Ex parte Organi vel Medii_, Direct Vision has been improv'd by
making use of a _Tube_, without Glasses, or a Man's clos'd Hand, to look
thro'; which admitting into the _Eye_ only the principal Rays, that come
directly from the Object, do very much strengthen and clear the Sight,
by excluding all the Collateral Rays, that crouding into the Eye,
together with the direct ones, would confound and disturb it, partly by
mixing and interfering with the direct Rays, and partly (or rather
chiefly) by too much enlightning the fund of the Eye, wherein Vision is
truly (tho' then imperfectly) made.
On this is founded the Art of making _Spectacles_ without Glasses; (as
well as _Tubes_) which is done by putting into the Glass-holes (instead
of Glasses) two short _Tubes_ of between three and four Inches long (for
their length is to be vary'd according to the Age or Eye of the
Beholder, and so also is the Diameter of the extream ends) which _Tubes_
being made of _Spanish Leather_ (or Past-board, or some such like
Matter) and black'd on the inside, are so to be placed, as that the
visual Rays, receiv'd thro' them, may meet in one point (or rather issue
out from one Point) of the Object standing at such a due distance, as
the Person may clearly and distinctly see it, or according to his length
of Sight (as ABC, in the 4th _Fig. Tab._ 3.)
And these _Spectacles_ may be suppos'd better for preserving the Sight,
than the ordinary ones with Glasses, because they represent the Object
more naturally, and withal more clearly and distinctly to the Eye, than
the other, whose refracted Rays being collected together with the right
ones in the Glasses, do somewhat confound good Vision, as before:
Especially if the visive Power be strong enough to be sufficiently
determin'd by the right Rays alone.
For I speak now of preserving a good Eye by these Spectacles, which
holds in proportion true also of a bad one. Because those Rays (both
right and refracted) being collected and brought so near the Eye
(whether good or bad) as the Spectacles are usually plac'd, do too much
affect it, both by their own brightness, and also by the brightness of
the Colours of the Object (when they are bright) which is brought very
near also; whereby the Eye is dazl'd and confounded, unless there be a
strong attention and _conatus_ of the Spirits, whereto the bright Rays
do certainly engage them, which of necessity weakens Vision, especially
if these Glass-spectacles be much us'd.
Wherefore the now describ'd new Tube-spectacles, contributing so much to
the help and preservation of Sight, may well be counted an improvement
of _Direct Vision_, because they convey the Rays to the Eye without any
kind of Refraction whatsoever. Seeing the same Object also through
various holes, plac'd at certain distances, does somewhat alter Vision;
but of this perhaps more hereafter.
Now as _Direct Vision_ has thus been improved, so likewise _Direct
Hearing_ partly has already receiv'd, and partly may by the Doctrine
whereof we are treating, (if well cultivated) farther receive as great
and notable Improvements, both _ex parte_ Objecti, and _ex parte_ Organi
_vel_ Medii.
1. As to the _Object_ of Hearing, which is Sound, improvement has been
and may be made, both as to the _Begetting_, and as to the _Conveying_
and _Propagating_ (which is a kind of _Conserving_) of Sounds.
1. As to the _Begetting_ of Sounds. The Art of imitating any Sound,
whether by _Speaking_ (that is pronouncing) any kind of Language, (which
really is an Art, and the _Art of Speaking_, perhaps one of the
greatest) or by _Whistling_, or by _Singing_ (which are allow'd Arts) or
by _Hollowing_ or _Luring_ (which the Huntsman or Faulkner would have to
be an Art also) or by imitating with the Mouth (or otherwise) the Voice
of any Animal, as of _Quails_, _Cats_, and the like; or by
_representing_ any Sound begotten by the Collision of Solid Bodies, or
after any other manner; these are all _Improvements_ of _Direct
Hearing_, and may be improv'd.
Moreover the Skill to make all sorts of _Musical Instruments_, both
Ancient and Modern, whether _Wind_ Instruments or _String'd_, or of any
other sort, whereof there are very many (as _Drums_, _Bells_, the
_Systrum_ of the _Egyptians_, and the like) that beget (and not only
propagate) Sounds; the Skill of making these, I say, is an Art, that has
as much improv'd _Direct Hearing_, as an Harmonious Sound exceeds a
single and rude one, that is, an immusical _Tone_; which Art is yet
capable of farther improvement. And I do hope, that by the Rules, which
may happily be laid down concerning the _Nature_, _Propagation_ and
_Proportion_, or _Adapting_ of Sounds, a way may be found out, both to
improve _Musical Instruments_ already in use, and to invent new ones,
that shall be more sweet and luscious, than any yet known. Besides that,
by the same means _Instruments_ may be made, that shall imitate any
Sound in Nature, that is not Articulate, be it of Bird, Beast, or what
thing else soever.
2. The _Conveying_ and _Propagating_ (which is a kind of _Conserving_)
of Sounds, is much help'd by _duly placing the Sonorous Body_, and also
by the _Medium_.
For if the Medium be _Thin_ and _Quiescent_, and the Sounding Body
_plac'd conveniently_, the Sound will be easily and regularly
propagated, and mightily conserv'd. I say,
1. If the _Medium be Thin and Quiescent_, because it otherwise causes a
_Refracted Sound_, of which afterwards. Hence in a _still Evening_, or
the _dead of the Night_ (when the Wind ceases) a Sound is better sent
out, and to a greater distance than otherwise, tho' much of this may be
ascrib'd to its _Refraction_ also.
2. I say, that the _Sonorous Body must be plac'd conveniently_, near a
_Smooth Wall_, near _Water_, or _a Plain_, whose Surface is even.
1. Near a _Smooth Wall_, either Plain or Arch'd (Cycloidically or
Elliptically, rather than otherwise, tho' a Circular or any Arch will
do, but not so well.)
Hence in a Church, the nearer the Preacher stands to the Wall (and
certainly 'tis much the best way to place Pulpits near the Wall) the
better is he heard, especially by those who stand near the Wall also,
though at a greater distance from the Pulpit; those at the remotest end
of the Church, by laying their Ears somewhat close to the Wall, may hear
him easier than those in the middle.
Hence also do arise _Whispering Places_. For the Voice being apply'd to
one end of an Arch, easily rowls to the other. And indeed were the
_Motion_ and _Propagation_ of Sounds but rightly understood, 'twould be
no hard matter to contrive _Whispering Places_ of infinite variety and
use. And perhaps there could be no better or more pleasant hearing a
_Consort of Musick_ than at such a place as this, where the Sounds
rowling along together, before they come to the Ear, must needs
consolidate and imbody into one; which becomes a true composition of
Sounds, and is the very Life and Soul of Consort.
2. If the Sonorous Body be plac'd near _Water_, the Sound will easily be
convey'd, yet mollified; as Experience teacheth us from a Ring of
_Bells_ near a River, and a great _Gun_ shot off at Sea, which yet
differ much in the strength, and softness and continuance, or
propagation of their Sounds, from the same at Land, where the Sound is
more harsh and more perishing, or much sooner decays.
3. In a _Plain_ a Voice may be heard at a far greater distance than in
uneven Ground.
The _Reason_ of all which last nam'd _Phænomena_ is the same; because
the Sonorous Air meeting with little or no resistance upon a _Plane_
(much less upon an Arch'd) smooth Superficies, easily rowls along it,
without being let or hinder'd in its Motion, and consequently without
having its parts disfigured, and put into another kind of Revolution,
than what they had at the first begetting of the Sound. Which is the
true cause of its _Preservation_ or _Progression_, and fails much when
the Air passes over an uneven Surface, according to the degrees of its
inequality, and somewhat also, when it passes over the plain Superficies
of a Body that is hard and resisting.
Wherefore the smooth Top of the Water (by reason of its yielding to the
Arch'd Air, and gently arising again with a kind of Resurge, like to
_Elasticity_, tho' it be not so, by which Resurge it quickens and
hastens the motion of the Air rowling over it, and by its yielding
preserves it in its Arch'd Cycloidical or Elliptical Figure) the smooth
Top of the Water, I say, for these Reasons, and by these Means, conveys
a Sound more entire, and to a greater distance than the plain Surface of
a piece of Ground, a Wall, or any other Solid Body whatever, can do.
As for the _Speaking Trumpet_, by which a Voice may be convey'd to a
considerable distance, I refer its consideration to that of _Refracted
Sounds_, or _Refracted Audition_.
Thus much of the Improvements of Hearing, that respect its _Object_,
which is _Sound_.
2. The _Organ_ and _Medium_ are to be consider'd. And, 1. The _Organ_,
which is the _Ear_, is helpt much by placing it near a Wall (especially
at one end of an Arch, the Sound being begotten at the other) or near
the Surface of Water, or of the Earth, along which the Sounds are most
easily and naturally convey'd, as was before declar'd. And 'tis
incredible how far a Sound made upon the Earth (by the trampling of a
Troop of Horses, for Example) may be heard in a still Night, if a Man
lays his Ear close to the Ground in a large Plain.
_Otacousticks_ here come in for helping the _Ear_; which may be so
contriv'd (by a right understanding the _Progression of Sounds_, which
is the principal thing to be known for the due regulating all such kinds
of Instruments) as that the Sound might enter the Ear without any
Refraction, but as now they are generally made I refer them to
_Refracted Audition_.
2. As to the _Medium_, I know not how that, by any contrivance of Art,
can advantage _Direct Hearing_, otherwise than I have declar'd already
in the propagation or conveyance of Sounds, though to the Refracting or
Reflecting of them it may very much conduce; of which presently.
And so I have done with the first part of my present undertaking, which
is the _Comparison of Direct Vision and Audition_, as to their
Improvements from Art. The rest follow. Wherefore,
II. Concerning _Refracted Vision_ and its Comparison, I observe, That
_Refracted Vision_ is always made _Ex parte Medii_, as _Reflected_ is
_ex parte Objecti_. And therefore, though _Direct Vision_ may be help'd
_ex parte Objecti, Medii vel Organi_, yet _Refracted_ can be
improv'd only _ex parte Medii_, and _Reflected_ _ex parte Corporis
oppositi_ alone. Unless it be in a mixt or compound Vision, that is
_Refracto-Reflext_, when the reflext Rays pass to the Eye through a
refracting Medium, such as the _Medium Internum_, contain'd in the Body
of the Eye, always is. So that in truth, all Vision is _Refracted_ by an
internal Refraction made in _ipso Oculo_.
And all that I have spoken of _Vision_ holds true of _Hearing_ also,
both _Refracted_ and _Reflext_, and therefore need not be repeated.
_Refracted Vision_ arises from the different _Density_, _Figure_, and
_Magnitude_ of the Medium, which is somewhat alter'd also by the diverse
incidence of the visible Rays. And so it is in _Refracted Hearing_, all
these Causes concur to its Production, and some others to be hereafter
consider'd.
Now as any Object (a Man for example) seen through a thicken'd Air, by
_Refraction_ appears greater than really he is: So likewise a _Sound_,
heard through the same thicken'd part of the _Atmosphere_, will be
considerably vary'd from what it would seem to be, if heard through a
thinner Medium.
And this I call a _Refracted Sound_: But what this Refraction of Sound
is, and how caus'd, may hereafter be discuss'd, when the Nature, and
Motion, or Progression of Sounds are well stated.
For the Improvement of _Refracted Vision_ artificial Instruments have
been made, by grinding or blowing Glasses, into a certain Figure, and
placing them at due distances, whereby the Object may be (as 'twere)
enabled to send forth its Rays more vigorously, and the Visive Faculty
impower'd the better to receive them. And thus also Instruments may be
contriv'd for the assisting both the _Sonorous Body_, to send forth its
Sound more strongly, and the _Acoustick Faculty_, to receive and discern
it more easily and clearly. _For_,
1. As a fine _Glass Bubble_, fill'd with clear Water, and placed before
a burning Candle or Lamp, does help it to dart forth its Rays to a
prodigious Length and Brightness: So an _Instrument_ may be invented,
that apply'd to the Mouth (or any Sonorous Body) shall send forth the
Voice distinctly to as prodigious a Distance and Loudness.
For if the _Stentoro-phonecon_ (which is but a rude and unartificial
Instrument) does such great feats, what might be done with one compos'd
according to the Rules of Art? whose make should comply with the Laws of
_Sonorous Motion_ (which that does not) and therefore not so much
_Refract_, as to alter and confound the _Tone_ of the _Voice_ and Words
(as that somewhat does.)
Now of what use such an Instrument might be for speaking clearly and
articulately at a distance (and that without altering the Tone of the
Voice) whether it be at Sea or at Land (but especially at Sea in
tempestuous Weather and in the Night) is obvious to any Man to conceive.
2. As Instruments have been invented to help the Eye, So likewise are
there some, and more such there may be, for the Ear.
_For_,
1. As _Spectacles_ and other _Glasses_ are made to help the _Purblind_
and weak _Eyes_, to see at any competent distance: So there are
_Otacousticks_ (and better may be made) to help weak _Ears_ to hear at a
reasonable distance also. Which would be as great a help to the
infirmity of Old Age, as the other invention of Spectacles is, and
perhaps greater; forasmuch as the Hearing what's spoken is of more daily
use and concern to such Men, then to be able to _read Books_ or to _view
Pictures_.
2. As _Perspective-Glasses_ and Telescopes help the Eye to see Objects
at a very great distance, which otherwise would not be discernable; in
like manner may a sort of _Otacousticks_ be so contriv'd, as that they
shall receive in _Sounds_ made at a very great distance also, but with
so much advantage, that the Ear shall be able to hear them, which
otherwise would have been _inaudible_.
And these _Otacousticks_ in some respects would be of greater use than
_Perspectives_. For whereas at Land _Perspectives_ are many times
render'd almost useless, by the interposition of Woods and Mountains,
which hinder the Sight from reaching very far: Our _Otacousticks_ would,
notwithstanding these Obstacles, take in a Sound made some Leagues off.
Which might be of notable use in the time of War, for discovering the
Enemy at a good distance, when he marches or lyes incamp'd behind a
Mountain or Wood, or any such place of shelter.
Yea, even at Sea also, where _Perspectives_ are of most use, by reason
of the plainess of the Surface of the Water; yet sometimes there
_Otacousticks_ may be of more benefit, when in dark hazy Weather the Air
is too thick, or in Stormy Tempestuous Weather the Waves arise too high
for the _Perspective_ to be made use of.
But, whether at Sea or Land, _Perspectives_ become altogether
insignificant in the Night-time (unless it be for viewing the Stars)
which is the chief time for using _Otacousticks_; as it is generally,
for Soldiers to take their March, when they would surprise their Enemies.
And therefore this sort of _Otacousticks_ have then their chief use,
when _Perspectives_ are of no use at all; besides that they may be
imploy'd in the Day-time, as well as _Perspectives_, whence they may
(not unfitly) be term'd _the most useful Instrument_ of the two.
3. As _Microscopes_ or _Magnifying-Glasses_ help the Eye to see near
Objects, that by reason of their smallness were _Invisible_ before;
which Objects they Magnify to a strange greatness: So _Microphones_ or
_Micracousticks_, that is Magnifying Ear Instruments, may be contriv'd
after that manner, that they shall render the most minute Sound in
nature distinctly audible, by Magnifying it to an unconceivable loudness.
By the help hereof we may hear the different Cries and Tones, as well as
by _Microscopes_ see the divers Shapes and Figures of the smallest
Animals.
4. As by _Polyscopes_ or _Multiplying-Glasses_, one thing is represented
to the Eye as many, whether in the same or different Shapes (for so
Multiplying-Glasses may be contriv'd:) So by a _Polyphone_ or
_Polyacoustick_ well order'd, one _Sound_ may be heard as many, either
of the same or a different Note. Insomuch, that who uses this
Instrument, he shall, at the Sound of a single Viol, seem to hear a
whole Consort and all true Harmony. By which means this Instrument has
much the advantage of the _Polyscope_.
And thus much may suffice for comparing the Improvements made upon
_Refracted Seeing and Hearing_; I call it _Refracted Hearing_, because
made through a _Medium_, viz. thick Air, or an Instrument, through which
the Sound passing is broken or refracted.
III. _Reflected Vision_ has been improv'd by the Invention of
_Looking-glasses_ and _Polished Metals_, whether _Plane_, _Concave_, or
_Convex_; and these two last, either _Spherical_, _Oval_, _Cylindrical_,
_Conical_, _Hyperbolical_, or of several other shapes; all which cause a
different Reflection, and vary the _Phænomena_.
Thus also _Reflext Audition_, made by _Ecchoes_, may be improv'd, by
contriving several sorts of _Artificial Ecchoes_; as 'tis no hard matter
to do in almost any place.
For (speaking in the general) _Any Sound, falling directly or obliquely
upon any dense Body, of a smooth (whether Plane or Arch'd) Superficies,
is beat back again and reflected, or does eccho more or less_.
I say (1.) _falling directly or obliquely_; because, if the Sound be
sent out and propagated parallel to the Surface of the _Dense Body_, or
be made so _far off_ and so _weak_, that it cannot reach it, there will
be no Reflection of Sound, no _Eccho_.
I say (2.) _upon a Body of a smooth Superficies_; because if the Surface
of the _Corpus Obstans_ be uneven, the Air by reverberation will be put
out of its regular Motion, and the Sound thereby broken and
extinguish'd: So that tho' in this case also the Air be beaten back
again, yet Sound is not reflected, nor is there any Eccho.
I say (3.) _it does eccho more or less_, to shew, that when all things
are, as is before describ'd, there is still an Ecchoing, though it be
not always heard; either because the _direct Sound_ is too weak to be
beaten quite back again to him that made it; or that it does return home
to him, but so weak, that without the help of a good _Otacoustick_, it
cannot be discern'd; or that he stands in a wrong place, to receive the
reflected Sound, which passes over his head, under his Feet, or to one
side of him; which therefore may be heard by a Man standing in that
place, where the reflected Sound will come, provided no interpos'd Body
does intercept it; but not by him, that first made it.
I shall further make out the comparison 'twixt _Reflex'd Vision_ and
_Audition_, by these following _Propositions_.
1. As a _Plain Speculum_ reflects the _Object_ in its due _Dimensions_
and _Colours_; allowing for their difference of appearance, according to
their distance: So a _Plane Corpus Obstans_ reflects the _Sound_ back in
its due _Tone_ and _Loudness_; if allowance be likewise made for the
proportionable decrease of the Sound, according to its distance.
2. As a _Convex Speculum_ reflects the _Object less_, but somewhat
_brighter_ or clearer: So a _Convex Corpus Obstans_ repels the Sound
(insensibly) _smaller_; but somewhat _quicker_ (though _weaker_) than
otherwise it would be.
3. As a _Concave Speculum_ reflects the Object _bigger_, more obscure
and _Inverted_: So a _Concave Corpus Obstans_ ecchoes back the Sound
(insensibly) _bigger_, _slower_ (though _stronger_) and also _inverted_;
but never according to the order of Words. Nor do I think it possible
for the Art of Man to contrive a _Single Eccho_, that shall invert the
Sound, and repeat backwards; because then the Words last spoken, that
is, which do last occur to the _Corpus Obstans_, must first be repell'd;
which cannot be: For where, in the mean time, should the first Words
hang, and be conceal'd, or lie dormant? Or how, after such a pause, be
reviv'd and animated again into Motion? Yet in complicated or _Compound
Ecchoes_, where many receive from one another, I know not whether
something that way may not be done.
From the determinate _Concavity_ or _Archedness_ of these reflecting
Bodies, it comes to pass, that some of them, from a certain distance or
posture, will eccho back but one determinate Note, and from no other
place will they reverberate any; because of the undue Position of the
sounding Body. Such an one (as I remember) is the Vault in _Merton_
College in _Oxford_.
4. As a _Speculum_ takes in and reflects more of its Object, when plac'd
at a great distance from it, than when nearer; because it reflects
according to the apparent Magnitude of the Body at such a distance,
which is less: So also the _Ecchoing Body_, being remov'd farther off,
reflects more of the Sound, than when nearer. And this is the reason,
why some Ecchoes repeat but one Syllable, some one Word, and some many.
5. As _Specula's_ may be so plac'd, that reflecting one upon or into the
other, either directly or obliquely, one Object shall appear many; as in
Sir _Samuel Moreland_'s Glass-room: After the same manner _Ecchoing
Bodies_ may be so contriv'd and plac'd, as that reflecting the Sound
from one to the other, either directly and mutually, or obliquely and by
Succession, out of one Sound shall many Ecchoes be begotten; which in
the first case will be all together, and somewhat involv'd or swallow'd
up of each other, and thereby confus'd (as a Face in Looking-glasses
obverted) in the other they will be distinct, separate, and succeeding
one another; as most _multiple Ecchoes_ do.
Moreover a _Multiple-Eccho_ may be made, by so placing the _Ecchoing
Bodies_, at unequal distances, that they reflect all one way, and not
one on the other; by which means a manifold successive Sound will be
heard (not without astonishment) one Clap of the hands like many, one
Ha, like a laughter, one single Word like many of the same Tone and
Accent, and so one Viol like many of the same kind imitating each other.
Furthermore, as _Specula's_ may be so order'd, that by Reflection they
shall make one single thing appear many different things; as one single
Man to seem many Men, differing as to Shape and Complexion (or a company
of Men) which I think Sir _Samuel Moreland_'s Contrivance does not: So
may _Ecchoing Bodies_ also be order'd, that from any one Sound given,
they shall produce many Ecchoes, different both as to their _Tone_ and
_Intension_. (The ground whereof has elsewhere been laid down in a
Treatise concerning the _Sympathy of Lute-strings_.)
By this means a _Musical Room_ may be so contriv'd, that not only one
Instrument, play'd on in it shall seem many of the same _sort_ and
_size_; but even a Consort of (somewhat) different ones; only by placing
certain _Ecchoing Bodies_ so, as that any Note (play'd) shall be
return'd by them in 3_ds_, 5_ths_, and 8_ths_, which is possible to be
done otherwise than was mention'd before in _Refracted Audition_.
I have now done with my _Comparison_ of the two Noblest _Senses_, and
_Sciences_, as to their _Improvements_; wherein I have been thus large,
that I might give you a little prospect into the _Excellency_ and
_Usefulness_ of _Acousticks_; and that thereby I might excite all that
hear me, to bend their Thoughts towards the making of Experiments for
the compleating this (yet very imperfect, tho' noble) Science; a
_Specimen_ whereof I will give you in three _Problems_, and then present
you with the _Semiplane_ of an _Acoustick or Phonical Sphere_, as an
Attempt to explicate the great _Principle_ in this Science, which is
_The Progression of Sounds_.
_The Problems are these:_
1. _Sonum intendere quousque velis_; or, _Datum sonum ad datum gradum
intendere_.
2. _Sonum extendere quousque velis_; or, _Datum sonum ad datum
distantiam extendere seu propagare_.