_PART I._
_Observations concerning the Inflexions of the Rays of Light, and the
Colours made thereby._
Grimaldo has inform'd us, that if a beam of the Sun's Light be let into
a dark Room through a very small hole, the Shadows of things in this
Light will be larger than they ought to be if the Rays went on by the
Bodies in straight Lines, and that these Shadows have three parallel
Fringes, Bands or Ranks of colour'd Light adjacent to them. But if the
Hole be enlarged the Fringes grow broad and run into one another, so
that they cannot be distinguish'd. These broad Shadows and Fringes have
been reckon'd by some to proceed from the ordinary refraction of the
Air, but without due examination of the Matter. For the circumstances of
the Phaenomenon, so far as I have observed them, are as follows.
_Obs._ 1. I made in a piece of Lead a small Hole with a Pin, whose
breadth was the 42d part of an Inch. For 21 of those Pins laid together
took up the breadth of half an Inch. Through this Hole I let into my
darken'd Chamber a beam of the Sun's Light, and found that the Shadows
of Hairs, Thred, Pins, Straws, and such like slender Substances placed
in this beam of Light, were considerably broader than they ought to be,
if the Rays of Light passed on by these Bodies in right Lines. And
particularly a Hair of a Man's Head, whose breadth was but the 280th
part of an Inch, being held in this Light, at the distance of about
twelve Feet from the Hole, did cast a Shadow which at the distance of
four Inches from the Hair was the sixtieth part of an Inch broad, that
is, above four times broader than the Hair, and at the distance of two
Feet from the Hair was about the eight and twentieth part of an Inch
broad, that is, ten times broader than the Hair, and at the distance of
ten Feet was the eighth part of an Inch broad, that is 35 times broader.
Nor is it material whether the Hair be encompassed with Air, or with any
other pellucid Substance. For I wetted a polish'd Plate of Glass, and
laid the Hair in the Water upon the Glass, and then laying another
polish'd Plate of Glass upon it, so that the Water might fill up the
space between the Glasses, I held them in the aforesaid beam of Light,
so that the Light might pass through them perpendicularly, and the
Shadow of the Hair was at the same distances as big as before. The
Shadows of Scratches made in polish'd Plates of Glass were also much
broader than they ought to be, and the Veins in polish'd Plates of Glass
did also cast the like broad Shadows. And therefore the great breadth of
these Shadows proceeds from some other cause than the Refraction of the
Air.
Let the Circle X [in _Fig._ 1.] represent the middle of the Hair; ADG,
BEH, CFI, three Rays passing by one side of the Hair at several
distances; KNQ, LOR, MPS, three other Rays passing by the other side of
the Hair at the like distances; D, E, F, and N, O, P, the places where
the Rays are bent in their passage by the Hair; G, H, I, and Q, R, S,
the places where the Rays fall on a Paper GQ; IS the breadth of the
Shadow of the Hair cast on the Paper, and TI, VS, two Rays passing to
the Points I and S without bending when the Hair is taken away. And it's
manifest that all the Light between these two Rays TI and VS is bent in
passing by the Hair, and turned aside from the Shadow IS, because if any
part of this Light were not bent it would fall on the Paper within the
Shadow, and there illuminate the Paper, contrary to experience. And
because when the Paper is at a great distance from the Hair, the Shadow
is broad, and therefore the Rays TI and VS are at a great distance from
one another, it follows that the Hair acts upon the Rays of Light at a
good distance in their passing by it. But the Action is strongest on the
Rays which pass by at least distances, and grows weaker and weaker
accordingly as the Rays pass by at distances greater and greater, as is
represented in the Scheme: For thence it comes to pass, that the Shadow
of the Hair is much broader in proportion to the distance of the Paper
from the Hair, when the Paper is nearer the Hair, than when it is at a
great distance from it.
_Obs._ 2. The Shadows of all Bodies (Metals, Stones, Glass, Wood, Horn,
Ice, &c.) in this Light were border'd with three Parallel Fringes or
Bands of colour'd Light, whereof that which was contiguous to the Shadow
was broadest and most luminous, and that which was remotest from it was
narrowest, and so faint, as not easily to be visible. It was difficult
to distinguish the Colours, unless when the Light fell very obliquely
upon a smooth Paper, or some other smooth white Body, so as to make them
appear much broader than they would otherwise do. And then the Colours
were plainly visible in this Order: The first or innermost Fringe was
violet and deep blue next the Shadow, and then light blue, green, and
yellow in the middle, and red without. The second Fringe was almost
contiguous to the first, and the third to the second, and both were blue
within, and yellow and red without, but their Colours were very faint,
especially those of the third. The Colours therefore proceeded in this
order from the Shadow; violet, indigo, pale blue, green, yellow, red;
blue, yellow, red; pale blue, pale yellow and red. The Shadows made by
Scratches and Bubbles in polish'd Plates of Glass were border'd with the
like Fringes of colour'd Light. And if Plates of Looking-glass sloop'd
off near the edges with a Diamond-cut, be held in the same beam of
Light, the Light which passes through the parallel Planes of the Glass
will be border'd with the like Fringes of Colours where those Planes
meet with the Diamond-cut, and by this means there will sometimes appear
four or five Fringes of Colours. Let AB, CD [in _Fig._ 2.] represent the
parallel Planes of a Looking-glass, and BD the Plane of the Diamond-cut,
making at B a very obtuse Angle with the Plane AB. And let all the Light
between the Rays ENI and FBM pass directly through the parallel Planes
of the Glass, and fall upon the Paper between I and M, and all the Light
between the Rays GO and HD be refracted by the oblique Plane of the
Diamond-cut BD, and fall upon the Paper between K and L; and the Light
which passes directly through the parallel Planes of the Glass, and
falls upon the Paper between I and M, will be border'd with three or
more Fringes at M.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
So by looking on the Sun through a Feather or black Ribband held close
to the Eye, several Rain-bows will appear; the Shadows which the Fibres
or Threds cast on the _Tunica Retina_, being border'd with the like
Fringes of Colours.
_Obs._ 3. When the Hair was twelve Feet distant from this Hole, and its
Shadow fell obliquely upon a flat white Scale of Inches and Parts of an
Inch placed half a Foot beyond it, and also when the Shadow fell
perpendicularly upon the same Scale placed nine Feet beyond it; I
measured the breadth of the Shadow and Fringes as accurately as I could,
and found them in Parts of an Inch as follows.
-------------------------------------------+-----------+--------
| half a | Nine
At the Distance of | Foot | Feet
-------------------------------------------+-----------+--------
The breadth of the Shadow | 1/54 | 1/9
-------------------------------------------+-----------+--------
The breadth between the Middles of the | 1/38 |
brightest Light of the innermost Fringes | or |
on either side the Shadow | 1/39 | 7/50
-------------------------------------------+-----------+--------
The breadth between the Middles of the | |
brightest Light of the middlemost Fringes| |
on either side the Shadow | 1/23-1/2 | 4/17
-------------------------------------------+-----------+--------
The breadth between the Middles of the | 1/18 |
brightest Light of the outmost Fringes | or |
on either side the Shadow | 1/18-1/2 | 3/10
-------------------------------------------+-----------+--------
The distance between the Middles of the | |
brightest Light of the first and second | |
Fringes | 1/120 | 1/21
-------------------------------------------+-----------+--------
The distance between the Middles of the | |
brightest Light of the second and third | |
Fringes | 1/170 | 1/31
-------------------------------------------+-----------+--------
The breadth of the luminous Part (green, | |
white, yellow, and red) of the first | |
Fringe | 1/170 | 1/32
-------------------------------------------+-----------+--------
The breadth of the darker Space between | |
the first and second Fringes | 1/240 | 1/45
-------------------------------------------+-----------+--------
The breadth of the luminous Part of the | |
second Fringe | 1/290 | 1/55
-------------------------------------------+-----------+--------
The breadth of the darker Space between | |
the second and third Fringes | 1/340 | 1/63
-------------------------------------------+-----------+--------
These Measures I took by letting the Shadow of the Hair, at half a Foot
distance, fall so obliquely on the Scale, as to appear twelve times
broader than when it fell perpendicularly on it at the same distance,
and setting down in this Table the twelfth part of the Measures I then
took.
_Obs._ 4. When the Shadow and Fringes were cast obliquely upon a smooth
white Body, and that Body was removed farther and farther from the Hair,
the first Fringe began to appear and look brighter than the rest of the
Light at the distance of less than a quarter of an Inch from the Hair,
and the dark Line or Shadow between that and the second Fringe began to
appear at a less distance from the Hair than that of the third part of
an Inch. The second Fringe began to appear at a distance from the Hair
of less than half an Inch, and the Shadow between that and the third
Fringe at a distance less than an inch, and the third Fringe at a
distance less than three Inches. At greater distances they became much
more sensible, but kept very nearly the same proportion of their
breadths and intervals which they had at their first appearing. For the
distance between the middle of the first, and middle of the second
Fringe, was to the distance between the middle of the second and middle
of the third Fringe, as three to two, or ten to seven. And the last of
these two distances was equal to the breadth of the bright Light or
luminous part of the first Fringe. And this breadth was to the breadth
of the bright Light of the second Fringe as seven to four, and to the
dark Interval of the first and second Fringe as three to two, and to
the like dark Interval between the second and third as two to one. For
the breadths of the Fringes seem'd to be in the progression of the
Numbers 1, sqrt(1/3), sqrt(1/5), and their Intervals to be in the
same progression with them; that is, the Fringes and their Intervals
together to be in the continual progression of the Numbers 1,
sqrt(1/2), sqrt(1/3), sqrt(1/4), sqrt(1/5), or thereabouts. And
these Proportions held the same very nearly at all distances from the
Hair; the dark Intervals of the Fringes being as broad in proportion to
the breadth of the Fringes at their first appearance as afterwards at
great distances from the Hair, though not so dark and distinct.
_Obs._ 5. The Sun shining into my darken'd Chamber through a hole a
quarter of an Inch broad, I placed at the distance of two or three Feet
from the Hole a Sheet of Pasteboard, which was black'd all over on both
sides, and in the middle of it had a hole about three quarters of an
Inch square for the Light to pass through. And behind the hole I
fasten'd to the Pasteboard with Pitch the blade of a sharp Knife, to
intercept some part of the Light which passed through the hole. The
Planes of the Pasteboard and blade of the Knife were parallel to one
another, and perpendicular to the Rays. And when they were so placed
that none of the Sun's Light fell on the Pasteboard, but all of it
passed through the hole to the Knife, and there part of it fell upon the
blade of the Knife, and part of it passed by its edge; I let this part
of the Light which passed by, fall on a white Paper two or three Feet
beyond the Knife, and there saw two streams of faint Light shoot out
both ways from the beam of Light into the shadow, like the Tails of
Comets. But because the Sun's direct Light by its brightness upon the
Paper obscured these faint streams, so that I could scarce see them, I
made a little hole in the midst of the Paper for that Light to pass
through and fall on a black Cloth behind it; and then I saw the two
streams plainly. They were like one another, and pretty nearly equal in
length, and breadth, and quantity of Light. Their Light at that end next
the Sun's direct Light was pretty strong for the space of about a
quarter of an Inch, or half an Inch, and in all its progress from that
direct Light decreased gradually till it became insensible. The whole
length of either of these streams measured upon the paper at the
distance of three Feet from the Knife was about six or eight Inches; so
that it subtended an Angle at the edge of the Knife of about 10 or 12,
or at most 14 Degrees. Yet sometimes I thought I saw it shoot three or
four Degrees farther, but with a Light so very faint that I could scarce
perceive it, and suspected it might (in some measure at least) arise
from some other cause than the two streams did. For placing my Eye in
that Light beyond the end of that stream which was behind the Knife, and
looking towards the Knife, I could see a line of Light upon its edge,
and that not only when my Eye was in the line of the Streams, but also
when it was without that line either towards the point of the Knife, or
towards the handle. This line of Light appear'd contiguous to the edge
of the Knife, and was narrower than the Light of the innermost Fringe,
and narrowest when my Eye was farthest from the direct Light, and
therefore seem'd to pass between the Light of that Fringe and the edge
of the Knife, and that which passed nearest the edge to be most bent,
though not all of it.
_Obs._ 6. I placed another Knife by this, so that their edges might be
parallel, and look towards one another, and that the beam of Light might
fall upon both the Knives, and some part of it pass between their edges.
And when the distance of their edges was about the 400th part of an
Inch, the stream parted in the middle, and left a Shadow between the two
parts. This Shadow was so black and dark that all the Light which passed
between the Knives seem'd to be bent, and turn'd aside to the one hand
or to the other. And as the Knives still approach'd one another the
Shadow grew broader, and the streams shorter at their inward ends which
were next the Shadow, until upon the contact of the Knives the whole
Light vanish'd, leaving its place to the Shadow.
And hence I gather that the Light which is least bent, and goes to the
inward ends of the streams, passes by the edges of the Knives at the
greatest distance, and this distance when the Shadow begins to appear
between the streams, is about the 800th part of an Inch. And the Light
which passes by the edges of the Knives at distances still less and
less, is more and more bent, and goes to those parts of the streams
which are farther and farther from the direct Light; because when the
Knives approach one another till they touch, those parts of the streams
vanish last which are farthest from the direct Light.
_Obs._ 7. In the fifth Observation the Fringes did not appear, but by
reason of the breadth of the hole in the Window became so broad as to
run into one another, and by joining, to make one continued Light in the
beginning of the streams. But in the sixth, as the Knives approached one
another, a little before the Shadow appeared between the two streams,
the Fringes began to appear on the inner ends of the Streams on either
side of the direct Light; three on one side made by the edge of one
Knife, and three on the other side made by the edge of the other Knife.
They were distinctest when the Knives were placed at the greatest
distance from the hole in the Window, and still became more distinct by
making the hole less, insomuch that I could sometimes see a faint
lineament of a fourth Fringe beyond the three above mention'd. And as
the Knives continually approach'd one another, the Fringes grew
distincter and larger, until they vanish'd. The outmost Fringe vanish'd
first, and the middlemost next, and the innermost last. And after they
were all vanish'd, and the line of Light which was in the middle between
them was grown very broad, enlarging it self on both sides into the
streams of Light described in the fifth Observation, the above-mention'd
Shadow began to appear in the middle of this line, and divide it along
the middle into two lines of Light, and increased until the whole Light
vanish'd. This enlargement of the Fringes was so great that the Rays
which go to the innermost Fringe seem'd to be bent above twenty times
more when this Fringe was ready to vanish, than when one of the Knives
was taken away.
And from this and the former Observation compared, I gather, that the
Light of the first Fringe passed by the edge of the Knife at a distance
greater than the 800th part of an Inch, and the Light of the second
Fringe passed by the edge of the Knife at a greater distance than the
Light of the first Fringe did, and that of the third at a greater
distance than that of the second, and that of the streams of Light
described in the fifth and sixth Observations passed by the edges of the
Knives at less distances than that of any of the Fringes.
_Obs._ 8. I caused the edges of two Knives to be ground truly strait,
and pricking their points into a Board so that their edges might look
towards one another, and meeting near their points contain a rectilinear
Angle, I fasten'd their Handles together with Pitch to make this Angle
invariable. The distance of the edges of the Knives from one another at
the distance of four Inches from the angular Point, where the edges of
the Knives met, was the eighth part of an Inch; and therefore the Angle
contain'd by the edges was about one Degree 54: The Knives thus fix'd
together I placed in a beam of the Sun's Light, let into my darken'd
Chamber through a Hole the 42d Part of an Inch wide, at the distance of
10 or 15 Feet from the Hole, and let the Light which passed between
their edges fall very obliquely upon a smooth white Ruler at the
distance of half an Inch, or an Inch from the Knives, and there saw the
Fringes by the two edges of the Knives run along the edges of the
Shadows of the Knives in Lines parallel to those edges without growing
sensibly broader, till they met in Angles equal to the Angle contained
by the edges of the Knives, and where they met and joined they ended
without crossing one another. But if the Ruler was held at a much
greater distance from the Knives, the Fringes where they were farther
from the Place of their Meeting, were a little narrower, and became
something broader and broader as they approach'd nearer and nearer to
one another, and after they met they cross'd one another, and then
became much broader than before.
Whence I gather that the distances at which the Fringes pass by the
Knives are not increased nor alter'd by the approach of the Knives, but
the Angles in which the Rays are there bent are much increased by that
approach; and that the Knife which is nearest any Ray determines which
way the Ray shall be bent, and the other Knife increases the bent.
_Obs._ 9. When the Rays fell very obliquely upon the Ruler at the
distance of the third Part of an Inch from the Knives, the dark Line
between the first and second Fringe of the Shadow of one Knife, and the
dark Line between the first and second Fringe of the Shadow of the other
knife met with one another, at the distance of the fifth Part of an Inch
from the end of the Light which passed between the Knives at the
concourse of their edges. And therefore the distance of the edges of the
Knives at the meeting of these dark Lines was the 160th Part of an Inch.
For as four Inches to the eighth Part of an Inch, so is any Length of
the edges of the Knives measured from the point of their concourse to
the distance of the edges of the Knives at the end of that Length, and
so is the fifth Part of an Inch to the 160th Part. So then the dark
Lines above-mention'd meet in the middle of the Light which passes
between the Knives where they are distant the 160th Part of an Inch, and
the one half of that Light passes by the edge of one Knife at a distance
not greater than the 320th Part of an Inch, and falling upon the Paper
makes the Fringes of the Shadow of that Knife, and the other half passes
by the edge of the other Knife, at a distance not greater than the 320th
Part of an Inch, and falling upon the Paper makes the Fringes of the
Shadow of the other Knife. But if the Paper be held at a distance from
the Knives greater than the third Part of an Inch, the dark Lines
above-mention'd meet at a greater distance than the fifth Part of an
Inch from the end of the Light which passed between the Knives at the
concourse of their edges; and therefore the Light which falls upon the
Paper where those dark Lines meet passes between the Knives where the
edges are distant above the 160th part of an Inch.
For at another time, when the two Knives were distant eight Feet and
five Inches from the little hole in the Window, made with a small Pin as
above, the Light which fell upon the Paper where the aforesaid dark
lines met, passed between the Knives, where the distance between their
edges was as in the following Table, when the distance of the Paper from
the Knives was also as follows.
-----------------------------+------------------------------
| Distances between the edges
Distances of the Paper | of the Knives in millesimal
from the Knives in Inches. | parts of an Inch.
-----------------------------+------------------------------
1-1/2. | 0'012
3-1/3. | 0'020
8-3/5. | 0'034
32. | 0'057
96. | 0'081
131. | 0'087
_____________________________|______________________________
And hence I gather, that the Light which makes the Fringes upon the
Paper is not the same Light at all distances of the Paper from the
Knives, but when the Paper is held near the Knives, the Fringes are made
by Light which passes by the edges of the Knives at a less distance, and
is more bent than when the Paper is held at a greater distance from the
Knives.
[Illustration: FIG. 3.]
_Obs._ 10. When the Fringes of the Shadows of the Knives fell
perpendicularly upon a Paper at a great distance from the Knives, they
were in the form of Hyperbola's, and their Dimensions were as follows.
Let CA, CB [in _Fig._ 3.] represent Lines drawn upon the Paper parallel
to the edges of the Knives, and between which all the Light would fall,
if it passed between the edges of the Knives without inflexion; DE a
Right Line drawn through C making the Angles ACD, BCE, equal to one
another, and terminating all the Light which falls upon the Paper from
the point where the edges of the Knives meet; _eis_, _fkt_, and _glv_,
three hyperbolical Lines representing the Terminus of the Shadow of one
of the Knives, the dark Line between the first and second Fringes of
that Shadow, and the dark Line between the second and third Fringes of
the same Shadow; _xip_, _ykq_, and _zlr_, three other hyperbolical Lines
representing the Terminus of the Shadow of the other Knife, the dark
Line between the first and second Fringes of that Shadow, and the dark
line between the second and third Fringes of the same Shadow. And
conceive that these three Hyperbola's are like and equal to the former
three, and cross them in the points _i_, _k_, and _l_, and that the
Shadows of the Knives are terminated and distinguish'd from the first
luminous Fringes by the lines _eis_ and _xip_, until the meeting and
crossing of the Fringes, and then those lines cross the Fringes in the
form of dark lines, terminating the first luminous Fringes within side,
and distinguishing them from another Light which begins to appear at
_i_, and illuminates all the triangular space _ip_DE_s_ comprehended by
these dark lines, and the right line DE. Of these Hyperbola's one
Asymptote is the line DE, and their other Asymptotes are parallel to the
lines CA and CB. Let _rv_ represent a line drawn any where upon the
Paper parallel to the Asymptote DE, and let this line cross the right
lines AC in _m_, and BC in _n_, and the six dark hyperbolical lines in
_p_, _q_, _r_; _s_, _t_, _v_; and by measuring the distances _ps_, _qt_,
_rv_, and thence collecting the lengths of the Ordinates _np_, _nq_,
_nr_ or _ms_, _mt_, _mv_, and doing this at several distances of the
line _rv_ from the Asymptote DD, you may find as many points of these
Hyperbola's as you please, and thereby know that these curve lines are
Hyperbola's differing little from the conical Hyperbola. And by
measuring the lines C_i_, C_k_, C_l_, you may find other points of these
Curves.
For instance; when the Knives were distant from the hole in the Window
ten Feet, and the Paper from the Knives nine Feet, and the Angle
contained by the edges of the Knives to which the Angle ACB is equal,
was subtended by a Chord which was to the Radius as 1 to 32, and the
distance of the line _rv_ from the Asymptote DE was half an Inch: I
measured the lines _ps_, _qt_, _rv_, and found them 0'35, 0'65, 0'98
Inches respectively; and by adding to their halfs the line 1/2 _mn_,
(which here was the 128th part of an Inch, or 0'0078 Inches,) the Sums
_np_, _nq_, _nr_, were 0'1828, 0'3328, 0'4978 Inches. I measured also
the distances of the brightest parts of the Fringes which run between
_pq_ and _st_, _qr_ and _tv_, and next beyond _r_ and _v_, and found
them 0'5, 0'8, and 1'17 Inches.
_Obs._ 11. The Sun shining into my darken'd Room through a small round
hole made in a Plate of Lead with a slender Pin, as above; I placed at
the hole a Prism to refract the Light, and form on the opposite Wall the
Spectrum of Colours, described in the third Experiment of the first
Book. And then I found that the Shadows of all Bodies held in the
colour'd Light between the Prism and the Wall, were border'd with
Fringes of the Colour of that Light in which they were held. In the full
red Light they were totally red without any sensible blue or violet, and
in the deep blue Light they were totally blue without any sensible red
or yellow; and so in the green Light they were totally green, excepting
a little yellow and blue, which were mixed in the green Light of the
Prism. And comparing the Fringes made in the several colour'd Lights, I
found that those made in the red Light were largest, those made in the
violet were least, and those made in the green were of a middle bigness.
For the Fringes with which the Shadow of a Man's Hair were bordered,
being measured cross the Shadow at the distance of six Inches from the
Hair, the distance between the middle and most luminous part of the
first or innermost Fringe on one side of the Shadow, and that of the
like Fringe on the other side of the Shadow, was in the full red Light
1/37-1/4 of an Inch, and in the full violet 7/46. And the like distance
between the middle and most luminous parts of the second Fringes on
either side the Shadow was in the full red Light 1/22, and in the violet
1/27 of an Inch. And these distances of the Fringes held the same
proportion at all distances from the Hair without any sensible
variation.
So then the Rays which made these Fringes in the red Light passed by the
Hair at a greater distance than those did which made the like Fringes in
the violet; and therefore the Hair in causing these Fringes acted alike
upon the red Light or least refrangible Rays at a greater distance, and
upon the violet or most refrangible Rays at a less distance, and by
those actions disposed the red Light into Larger Fringes, and the violet
into smaller, and the Lights of intermediate Colours into Fringes of
intermediate bignesses without changing the Colour of any sort of Light.
When therefore the Hair in the first and second of these Observations
was held in the white beam of the Sun's Light, and cast a Shadow which
was border'd with three Fringes of coloured Light, those Colours arose
not from any new modifications impress'd upon the Rays of Light by the
Hair, but only from the various inflexions whereby the several Sorts of
Rays were separated from one another, which before separation, by the
mixture of all their Colours, composed the white beam of the Sun's
Light, but whenever separated compose Lights of the several Colours
which they are originally disposed to exhibit. In this 11th Observation,
where the Colours are separated before the Light passes by the Hair, the
least refrangible Rays, which when separated from the rest make red,
were inflected at a greater distance from the Hair, so as to make three
red Fringes at a greater distance from the middle of the Shadow of the
Hair; and the most refrangible Rays which when separated make violet,
were inflected at a less distance from the Hair, so as to make three
violet Fringes at a less distance from the middle of the Shadow of the
Hair. And other Rays of intermediate degrees of Refrangibility were
inflected at intermediate distances from the Hair, so as to make Fringes
of intermediate Colours at intermediate distances from the middle of the
Shadow of the Hair. And in the second Observation, where all the Colours
are mix'd in the white Light which passes by the Hair, these Colours are
separated by the various inflexions of the Rays, and the Fringes which
they make appear all together, and the innermost Fringes being
contiguous make one broad Fringe composed of all the Colours in due
order, the violet lying on the inside of the Fringe next the Shadow, the
red on the outside farthest from the Shadow, and the blue, green, and
yellow, in the middle. And, in like manner, the middlemost Fringes of
all the Colours lying in order, and being contiguous, make another broad
Fringe composed of all the Colours; and the outmost Fringes of all the
Colours lying in order, and being contiguous, make a third broad Fringe
composed of all the Colours. These are the three Fringes of colour'd
Light with which the Shadows of all Bodies are border'd in the second
Observation.
When I made the foregoing Observations, I design'd to repeat most of
them with more care and exactness, and to make some new ones for
determining the manner how the Rays of Light are bent in their passage
by Bodies, for making the Fringes of Colours with the dark lines between
them. But I was then interrupted, and cannot now think of taking these
things into farther Consideration. And since I have not finish'd this
part of my Design, I shall conclude with proposing only some Queries, in
order to a farther search to be made by others.
_Query_ 1. Do not Bodies act upon Light at a distance, and by their
action bend its Rays; and is not this action (_caeteris paribus_)
strongest at the least distance?
_Qu._ 2. Do not the Rays which differ in Refrangibility differ also in
Flexibity; and are they not by their different Inflexions separated from
one another, so as after separation to make the Colours in the three
Fringes above described? And after what manner are they inflected to
make those Fringes?
_Qu._ 3. Are not the Rays of Light in passing by the edges and sides of
Bodies, bent several times backwards and forwards, with a motion like
that of an Eel? And do not the three Fringes of colour'd Light
above-mention'd arise from three such bendings?
_Qu._ 4. Do not the Rays of Light which fall upon Bodies, and are
reflected or refracted, begin to bend before they arrive at the Bodies;
and are they not reflected, refracted, and inflected, by one and the
same Principle, acting variously in various Circumstances?
_Qu._ 5. Do not Bodies and Light act mutually upon one another; that is
to say, Bodies upon Light in emitting, reflecting, refracting and
inflecting it, and Light upon Bodies for heating them, and putting their
parts into a vibrating motion wherein heat consists?
_Qu._ 6. Do not black Bodies conceive heat more easily from Light than
those of other Colours do, by reason that the Light falling on them is
not reflected outwards, but enters the Bodies, and is often reflected
and refracted within them, until it be stifled and lost?
_Qu._ 7. Is not the strength and vigor of the action between Light and
sulphureous Bodies observed above, one reason why sulphureous Bodies
take fire more readily, and burn more vehemently than other Bodies do?
_Qu._ 8. Do not all fix'd Bodies, when heated beyond a certain degree,
emit Light and shine; and is not this Emission perform'd by the
vibrating motions of their parts? And do not all Bodies which abound
with terrestrial parts, and especially with sulphureous ones, emit Light
as often as those parts are sufficiently agitated; whether that
agitation be made by Heat, or by Friction, or Percussion, or
Putrefaction, or by any vital Motion, or any other Cause? As for
instance; Sea-Water in a raging Storm; Quick-silver agitated in _vacuo_;
the Back of a Cat, or Neck of a Horse, obliquely struck or rubbed in a
dark place; Wood, Flesh and Fish while they putrefy; Vapours arising
from putrefy'd Waters, usually call'd _Ignes Fatui_; Stacks of moist Hay
or Corn growing hot by fermentation; Glow-worms and the Eyes of some
Animals by vital Motions; the vulgar _Phosphorus_ agitated by the
attrition of any Body, or by the acid Particles of the Air; Amber and
some Diamonds by striking, pressing or rubbing them; Scrapings of Steel
struck off with a Flint; Iron hammer'd very nimbly till it become so hot
as to kindle Sulphur thrown upon it; the Axletrees of Chariots taking
fire by the rapid rotation of the Wheels; and some Liquors mix'd with
one another whose Particles come together with an Impetus, as Oil of
Vitriol distilled from its weight of Nitre, and then mix'd with twice
its weight of Oil of Anniseeds. So also a Globe of Glass about 8 or 10
Inches in diameter, being put into a Frame where it may be swiftly
turn'd round its Axis, will in turning shine where it rubs against the
palm of ones Hand apply'd to it: And if at the same time a piece of
white Paper or white Cloth, or the end of ones Finger be held at the
distance of about a quarter of an Inch or half an Inch from that part of
the Glass where it is most in motion, the electrick Vapour which is
excited by the friction of the Glass against the Hand, will by dashing
against the white Paper, Cloth or Finger, be put into such an agitation
as to emit Light, and make the white Paper, Cloth or Finger, appear
lucid like a Glowworm; and in rushing out of the Glass will sometimes
push against the finger so as to be felt. And the same things have been
found by rubbing a long and large Cylinder or Glass or Amber with a
Paper held in ones hand, and continuing the friction till the Glass grew
warm.
_Qu._ 9. Is not Fire a Body heated so hot as to emit Light copiously?
For what else is a red hot Iron than Fire? And what else is a burning
Coal than red hot Wood?
_Qu._ 10. Is not Flame a Vapour, Fume or Exhalation heated red hot, that
is, so hot as to shine? For Bodies do not flame without emitting a
copious Fume, and this Fume burns in the Flame. The _Ignis Fatuus_ is a
Vapour shining without heat, and is there not the same difference
between this Vapour and Flame, as between rotten Wood shining without
heat and burning Coals of Fire? In distilling hot Spirits, if the Head
of the Still be taken off, the Vapour which ascends out of the Still
will take fire at the Flame of a Candle, and turn into Flame, and the
Flame will run along the Vapour from the Candle to the Still. Some
Bodies heated by Motion, or Fermentation, if the heat grow intense, fume
copiously, and if the heat be great enough the Fumes will shine and
become Flame. Metals in fusion do not flame for want of a copious Fume,
except Spelter, which fumes copiously, and thereby flames. All flaming
Bodies, as Oil, Tallow, Wax, Wood, fossil Coals, Pitch, Sulphur, by
flaming waste and vanish into burning Smoke, which Smoke, if the Flame
be put out, is very thick and visible, and sometimes smells strongly,
but in the Flame loses its smell by burning, and according to the nature
of the Smoke the Flame is of several Colours, as that of Sulphur blue,
that of Copper open'd with sublimate green, that of Tallow yellow, that
of Camphire white. Smoke passing through Flame cannot but grow red hot,
and red hot Smoke can have no other appearance than that of Flame. When
Gun-powder takes fire, it goes away into Flaming Smoke. For the Charcoal
and Sulphur easily take fire, and set fire to the Nitre, and the Spirit
of the Nitre being thereby rarified into Vapour, rushes out with
Explosion much after the manner that the Vapour of Water rushes out of
an AEolipile; the Sulphur also being volatile is converted into Vapour,
and augments the Explosion. And the acid Vapour of the Sulphur (namely
that which distils under a Bell into Oil of Sulphur,) entring violently
into the fix'd Body of the Nitre, sets loose the Spirit of the Nitre,
and excites a great Fermentation, whereby the Heat is farther augmented,
and the fix'd Body of the Nitre is also rarified into Fume, and the
Explosion is thereby made more vehement and quick. For if Salt of Tartar
be mix'd with Gun-powder, and that Mixture be warm'd till it takes fire,
the Explosion will be more violent and quick than that of Gun-powder
alone; which cannot proceed from any other cause than the action of the
Vapour of the Gun-powder upon the Salt of Tartar, whereby that Salt is
rarified. The Explosion of Gun-powder arises therefore from the violent
action whereby all the Mixture being quickly and vehemently heated, is
rarified and converted into Fume and Vapour: which Vapour, by the
violence of that action, becoming so hot as to shine, appears in the
form of Flame.
_Qu._ 11. Do not great Bodies conserve their heat the longest, their
parts heating one another, and may not great dense and fix'd Bodies,
when heated beyond a certain degree, emit Light so copiously, as by the
Emission and Re-action of its Light, and the Reflexions and Refractions
of its Rays within its Pores to grow still hotter, till it comes to a
certain period of heat, such as is that of the Sun? And are not the Sun
and fix'd Stars great Earths vehemently hot, whose heat is conserved by
the greatness of the Bodies, and the mutual Action and Reaction between
them, and the Light which they emit, and whose parts are kept from
fuming away, not only by their fixity, but also by the vast weight and
density of the Atmospheres incumbent upon them; and very strongly
compressing them, and condensing the Vapours and Exhalations which arise
from them? For if Water be made warm in any pellucid Vessel emptied of
Air, that Water in the _Vacuum_ will bubble and boil as vehemently as it
would in the open Air in a Vessel set upon the Fire till it conceives a
much greater heat. For the weight of the incumbent Atmosphere keeps down
the Vapours, and hinders the Water from boiling, until it grow much
hotter than is requisite to make it boil _in vacuo_. Also a mixture of
Tin and Lead being put upon a red hot Iron _in vacuo_ emits a Fume and
Flame, but the same Mixture in the open Air, by reason of the incumbent
Atmosphere, does not so much as emit any Fume which can be perceived by
Sight. In like manner the great weight of the Atmosphere which lies upon
the Globe of the Sun may hinder Bodies there from rising up and going
away from the Sun in the form of Vapours and Fumes, unless by means of a
far greater heat than that which on the Surface of our Earth would very
easily turn them into Vapours and Fumes. And the same great weight may
condense those Vapours and Exhalations as soon as they shall at any time
begin to ascend from the Sun, and make them presently fall back again
into him, and by that action increase his Heat much after the manner
that in our Earth the Air increases the Heat of a culinary Fire. And the
same weight may hinder the Globe of the Sun from being diminish'd,
unless by the Emission of Light, and a very small quantity of Vapours
and Exhalations.
_Qu._ 12. Do not the Rays of Light in falling upon the bottom of the Eye
excite Vibrations in the _Tunica Retina_? Which Vibrations, being
propagated along the solid Fibres of the optick Nerves into the Brain,
cause the Sense of seeing. For because dense Bodies conserve their Heat
a long time, and the densest Bodies conserve their Heat the longest, the
Vibrations of their parts are of a lasting nature, and therefore may be
propagated along solid Fibres of uniform dense Matter to a great
distance, for conveying into the Brain the impressions made upon all the
Organs of Sense. For that Motion which can continue long in one and the
same part of a Body, can be propagated a long way from one part to
another, supposing the Body homogeneal, so that the Motion may not be
reflected, refracted, interrupted or disorder'd by any unevenness of the
Body.
_Qu._ 13. Do not several sorts of Rays make Vibrations of several
bignesses, which according to their bignesses excite Sensations of
several Colours, much after the manner that the Vibrations of the Air,
according to their several bignesses excite Sensations of several
Sounds? And particularly do not the most refrangible Rays excite the
shortest Vibrations for making a Sensation of deep violet, the least
refrangible the largest for making a Sensation of deep red, and the
several intermediate sorts of Rays, Vibrations of several intermediate
bignesses to make Sensations of the several intermediate Colours?
_Qu._ 14. May not the harmony and discord of Colours arise from the
proportions of the Vibrations propagated through the Fibres of the
optick Nerves into the Brain, as the harmony and discord of Sounds arise
from the proportions of the Vibrations of the Air? For some Colours, if
they be view'd together, are agreeable to one another, as those of Gold
and Indigo, and others disagree.
_Qu._ 15. Are not the Species of Objects seen with both Eyes united
where the optick Nerves meet before they come into the Brain, the Fibres
on the right side of both Nerves uniting there, and after union going
thence into the Brain in the Nerve which is on the right side of the
Head, and the Fibres on the left side of both Nerves uniting in the same
place, and after union going into the Brain in the Nerve which is on the
left side of the Head, and these two Nerves meeting in the Brain in such
a manner that their Fibres make but one entire Species or Picture, half
of which on the right side of the Sensorium comes from the right side of
both Eyes through the right side of both optick Nerves to the place
where the Nerves meet, and from thence on the right side of the Head
into the Brain, and the other half on the left side of the Sensorium
comes in like manner from the left side of both Eyes. For the optick
Nerves of such Animals as look the same way with both Eyes (as of Men,
Dogs, Sheep, Oxen, &c.) meet before they come into the Brain, but the
optick Nerves of such Animals as do not look the same way with both Eyes
(as of Fishes, and of the Chameleon,) do not meet, if I am rightly
inform'd.
_Qu._ 16. When a Man in the dark presses either corner of his Eye with
his Finger, and turns his Eye away from his Finger, he will see a Circle
of Colours like those in the Feather of a Peacock's Tail. If the Eye and
the Finger remain quiet these Colours vanish in a second Minute of Time,
but if the Finger be moved with a quavering Motion they appear again. Do
not these Colours arise from such Motions excited in the bottom of the
Eye by the Pressure and Motion of the Finger, as, at other times are
excited there by Light for causing Vision? And do not the Motions once
excited continue about a Second of Time before they cease? And when a
Man by a stroke upon his Eye sees a flash of Light, are not the like
Motions excited in the _Retina_ by the stroke? And when a Coal of Fire
moved nimbly in the circumference of a Circle, makes the whole
circumference appear like a Circle of Fire; is it not because the
Motions excited in the bottom of the Eye by the Rays of Light are of a
lasting nature, and continue till the Coal of Fire in going round
returns to its former place? And considering the lastingness of the
Motions excited in the bottom of the Eye by Light, are they not of a
vibrating nature?
_Qu._ 17. If a stone be thrown into stagnating Water, the Waves excited
thereby continue some time to arise in the place where the Stone fell
into the Water, and are propagated from thence in concentrick Circles
upon the Surface of the Water to great distances. And the Vibrations or
Tremors excited in the Air by percussion, continue a little time to move
from the place of percussion in concentrick Spheres to great distances.
And in like manner, when a Ray of Light falls upon the Surface of any
pellucid Body, and is there refracted or reflected, may not Waves of
Vibrations, or Tremors, be thereby excited in the refracting or
reflecting Medium at the point of Incidence, and continue to arise
there, and to be propagated from thence as long as they continue to
arise and be propagated, when they are excited in the bottom of the Eye
by the Pressure or Motion of the Finger, or by the Light which comes
from the Coal of Fire in the Experiments above-mention'd? and are not
these Vibrations propagated from the point of Incidence to great
distances? And do they not overtake the Rays of Light, and by overtaking
them successively, do they not put them into the Fits of easy Reflexion
and easy Transmission described above? For if the Rays endeavour to
recede from the densest part of the Vibration, they may be alternately
accelerated and retarded by the Vibrations overtaking them.
_Qu._ 18. If in two large tall cylindrical Vessels of Glass inverted,
two little Thermometers be suspended so as not to touch the Vessels, and
the Air be drawn out of one of these Vessels, and these Vessels thus
prepared be carried out of a cold place into a warm one; the Thermometer
_in vacuo_ will grow warm as much, and almost as soon as the Thermometer
which is not _in vacuo_. And when the Vessels are carried back into the
cold place, the Thermometer _in vacuo_ will grow cold almost as soon as
the other Thermometer. Is not the Heat of the warm Room convey'd through
the _Vacuum_ by the Vibrations of a much subtiler Medium than Air, which
after the Air was drawn out remained in the _Vacuum_? And is not this
Medium the same with that Medium by which Light is refracted and
reflected, and by whose Vibrations Light communicates Heat to Bodies,
and is put into Fits of easy Reflexion and easy Transmission? And do not
the Vibrations of this Medium in hot Bodies contribute to the
intenseness and duration of their Heat? And do not hot Bodies
communicate their Heat to contiguous cold ones, by the Vibrations of
this Medium propagated from them into the cold ones? And is not this
Medium exceedingly more rare and subtile than the Air, and exceedingly
more elastick and active? And doth it not readily pervade all Bodies?
And is it not (by its elastick force) expanded through all the Heavens?
_Qu._ 19. Doth not the Refraction of Light proceed from the different
density of this AEthereal Medium in different places, the Light receding
always from the denser parts of the Medium? And is not the density
thereof greater in free and open Spaces void of Air and other grosser
Bodies, than within the Pores of Water, Glass, Crystal, Gems, and other
compact Bodies? For when Light passes through Glass or Crystal, and
falling very obliquely upon the farther Surface thereof is totally
reflected, the total Reflexion ought to proceed rather from the density
and vigour of the Medium without and beyond the Glass, than from the
rarity and weakness thereof.
_Qu._ 20. Doth not this AEthereal Medium in passing out of Water, Glass,
Crystal, and other compact and dense Bodies into empty Spaces, grow
denser and denser by degrees, and by that means refract the Rays of
Light not in a point, but by bending them gradually in curve Lines? And
doth not the gradual condensation of this Medium extend to some distance
from the Bodies, and thereby cause the Inflexions of the Rays of Light,
which pass by the edges of dense Bodies, at some distance from the
Bodies?
_Qu._ 21. Is not this Medium much rarer within the dense Bodies of the
Sun, Stars, Planets and Comets, than in the empty celestial Spaces
between them? And in passing from them to great distances, doth it not
grow denser and denser perpetually, and thereby cause the gravity of
those great Bodies towards one another, and of their parts towards the
Bodies; every Body endeavouring to go from the denser parts of the
Medium towards the rarer? For if this Medium be rarer within the Sun's
Body than at its Surface, and rarer there than at the hundredth part of
an Inch from its Body, and rarer there than at the fiftieth part of an
Inch from its Body, and rarer there than at the Orb of _Saturn_; I see
no reason why the Increase of density should stop any where, and not
rather be continued through all distances from the Sun to _Saturn_, and
beyond. And though this Increase of density may at great distances be
exceeding slow, yet if the elastick force of this Medium be exceeding
great, it may suffice to impel Bodies from the denser parts of the
Medium towards the rarer, with all that power which we call Gravity. And
that the elastick force of this Medium is exceeding great, may be
gather'd from the swiftness of its Vibrations. Sounds move about 1140
_English_ Feet in a second Minute of Time, and in seven or eight Minutes
of Time they move about one hundred _English_ Miles. Light moves from
the Sun to us in about seven or eight Minutes of Time, which distance is
about 70,000,000 _English_ Miles, supposing the horizontal Parallax of
the Sun to be about 12''. And the Vibrations or Pulses of this Medium,
that they may cause the alternate Fits of easy Transmission and easy
Reflexion, must be swifter than Light, and by consequence above 700,000
times swifter than Sounds. And therefore the elastick force of this
Medium, in proportion to its density, must be above 700000 x 700000
(that is, above 490,000,000,000) times greater than the elastick force
of the Air is in proportion to its density. For the Velocities of the
Pulses of elastick Mediums are in a subduplicate _Ratio_ of the
Elasticities and the Rarities of the Mediums taken together.
As Attraction is stronger in small Magnets than in great ones in
proportion to their Bulk, and Gravity is greater in the Surfaces of
small Planets than in those of great ones in proportion to their bulk,
and small Bodies are agitated much more by electric attraction than
great ones; so the smallness of the Rays of Light may contribute very
much to the power of the Agent by which they are refracted. And so if
any one should suppose that _AEther_ (like our Air) may contain Particles
which endeavour to recede from one another (for I do not know what this
_AEther_ is) and that its Particles are exceedingly smaller than those of
Air, or even than those of Light: The exceeding smallness of its
Particles may contribute to the greatness of the force by which those
Particles may recede from one another, and thereby make that Medium
exceedingly more rare and elastick than Air, and by consequence
exceedingly less able to resist the motions of Projectiles, and
exceedingly more able to press upon gross Bodies, by endeavouring to
expand it self.
_Qu._ 22. May not Planets and Comets, and all gross Bodies, perform
their Motions more freely, and with less resistance in this AEthereal
Medium than in any Fluid, which fills all Space adequately without
leaving any Pores, and by consequence is much denser than Quick-silver
or Gold? And may not its resistance be so small, as to be
inconsiderable? For instance; If this _AEther_ (for so I will call it)
should be supposed 700000 times more elastick than our Air, and above
700000 times more rare; its resistance would be above 600,000,000 times
less than that of Water. And so small a resistance would scarce make any
sensible alteration in the Motions of the Planets in ten thousand
Years. If any one would ask how a Medium can be so rare, let him tell me
how the Air, in the upper parts of the Atmosphere, can be above an
hundred thousand thousand times rarer than Gold. Let him also tell me,
how an electrick Body can by Friction emit an Exhalation so rare and
subtile, and yet so potent, as by its Emission to cause no sensible
Diminution of the weight of the electrick Body, and to be expanded
through a Sphere, whose Diameter is above two Feet, and yet to be able
to agitate and carry up Leaf Copper, or Leaf Gold, at the distance of
above a Foot from the electrick Body? And how the Effluvia of a Magnet
can be so rare and subtile, as to pass through a Plate of Glass without
any Resistance or Diminution of their Force, and yet so potent as to
turn a magnetick Needle beyond the Glass?
_Qu._ 23. Is not Vision perform'd chiefly by the Vibrations of this
Medium, excited in the bottom of the Eye by the Rays of Light, and
propagated through the solid, pellucid and uniform Capillamenta of the
optick Nerves into the place of Sensation? And is not Hearing perform'd
by the Vibrations either of this or some other Medium, excited in the
auditory Nerves by the Tremors of the Air, and propagated through the
solid, pellucid and uniform Capillamenta of those Nerves into the place
of Sensation? And so of the other Senses.
_Qu._ 24. Is not Animal Motion perform'd by the Vibrations of this
Medium, excited in the Brain by the power of the Will, and propagated
from thence through the solid, pellucid and uniform Capillamenta of the
Nerves into the Muscles, for contracting and dilating them? I suppose
that the Capillamenta of the Nerves are each of them solid and uniform,
that the vibrating Motion of the AEthereal Medium may be propagated along
them from one end to the other uniformly, and without interruption: For
Obstructions in the Nerves create Palsies. And that they may be
sufficiently uniform, I suppose them to be pellucid when view'd singly,
tho' the Reflexions in their cylindrical Surfaces may make the whole
Nerve (composed of many Capillamenta) appear opake and white. For
opacity arises from reflecting Surfaces, such as may disturb and
interrupt the Motions of this Medium.
[Sidenote: _See the following Scheme, p. 356._]
_Qu._ 25. Are there not other original Properties of the Rays of Light,
besides those already described? An instance of another original
Property we have in the Refraction of Island Crystal, described first by
_Erasmus Bartholine_, and afterwards more exactly by _Hugenius_, in his
Book _De la Lumiere_. This Crystal is a pellucid fissile Stone, clear as
Water or Crystal of the Rock, and without Colour; enduring a red Heat
without losing its transparency, and in a very strong Heat calcining
without Fusion. Steep'd a Day or two in Water, it loses its natural
Polish. Being rubb'd on Cloth, it attracts pieces of Straws and other
light things, like Ambar or Glass; and with _Aqua fortis_ it makes an
Ebullition. It seems to be a sort of Talk, and is found in form of an
oblique Parallelopiped, with six parallelogram Sides and eight solid
Angles. The obtuse Angles of the Parallelograms are each of them 101
Degrees and 52 Minutes; the acute ones 78 Degrees and 8 Minutes. Two of
the solid Angles opposite to one another, as C and E, are compassed each
of them with three of these obtuse Angles, and each of the other six
with one obtuse and two acute ones. It cleaves easily in planes parallel
to any of its Sides, and not in any other Planes. It cleaves with a
glossy polite Surface not perfectly plane, but with some little
unevenness. It is easily scratch'd, and by reason of its softness it
takes a Polish very difficultly. It polishes better upon polish'd
Looking-glass than upon Metal, and perhaps better upon Pitch, Leather or
Parchment. Afterwards it must be rubb'd with a little Oil or white of an
Egg, to fill up its Scratches; whereby it will become very transparent
and polite. But for several Experiments, it is not necessary to polish
it. If a piece of this crystalline Stone be laid upon a Book, every
Letter of the Book seen through it will appear double, by means of a
double Refraction. And if any beam of Light falls either
perpendicularly, or in any oblique Angle upon any Surface of this
Crystal, it becomes divided into two beams by means of the same double
Refraction. Which beams are of the same Colour with the incident beam of
Light, and seem equal to one another in the quantity of their Light, or
very nearly equal. One of these Refractions is perform'd by the usual
Rule of Opticks, the Sine of Incidence out of Air into this Crystal
being to the Sine of Refraction, as five to three. The other
Refraction, which may be called the unusual Refraction, is perform'd by
the following Rule.
[Illustration: FIG. 4.]
Let ADBC represent the refracting Surface of the Crystal, C the biggest
solid Angle at that Surface, GEHF the opposite Surface, and CK a
perpendicular on that Surface. This perpendicular makes with the edge of
the Crystal CF, an Angle of 19 Degr. 3'. Join KF, and in it take KL, so
that the Angle KCL be 6 Degr. 40'. and the Angle LCF 12 Degr. 23'. And
if ST represent any beam of Light incident at T in any Angle upon the
refracting Surface ADBC, let TV be the refracted beam determin'd by the
given Portion of the Sines 5 to 3, according to the usual Rule of
Opticks. Draw VX parallel and equal to KL. Draw it the same way from V
in which L lieth from K; and joining TX, this line TX shall be the other
refracted beam carried from T to X, by the unusual Refraction.
If therefore the incident beam ST be perpendicular to the refracting
Surface, the two beams TV and TX, into which it shall become divided,
shall be parallel to the lines CK and CL; one of those beams going
through the Crystal perpendicularly, as it ought to do by the usual Laws
of Opticks, and the other TX by an unusual Refraction diverging from the
perpendicular, and making with it an Angle VTX of about 6-2/3 Degrees,
as is found by Experience. And hence, the Plane VTX, and such like
Planes which are parallel to the Plane CFK, may be called the Planes of
perpendicular Refraction. And the Coast towards which the lines KL and
VX are drawn, may be call'd the Coast of unusual Refraction.
In like manner Crystal of the Rock has a double Refraction: But the
difference of the two Refractions is not so great and manifest as in
Island Crystal.
When the beam ST incident on Island Crystal is divided into two beams TV
and TX, and these two beams arrive at the farther Surface of the Glass;
the beam TV, which was refracted at the first Surface after the usual
manner, shall be again refracted entirely after the usual manner at the
second Surface; and the beam TX, which was refracted after the unusual
manner in the first Surface, shall be again refracted entirely after the
unusual manner in the second Surface; so that both these beams shall
emerge out of the second Surface in lines parallel to the first incident
beam ST.
And if two pieces of Island Crystal be placed one after another, in such
manner that all the Surfaces of the latter be parallel to all the
corresponding Surfaces of the former: The Rays which are refracted after
the usual manner in the first Surface of the first Crystal, shall be
refracted after the usual manner in all the following Surfaces; and the
Rays which are refracted after the unusual manner in the first Surface,
shall be refracted after the unusual manner in all the following
Surfaces. And the same thing happens, though the Surfaces of the
Crystals be any ways inclined to one another, provided that their Planes
of perpendicular Refraction be parallel to one another.
And therefore there is an original difference in the Rays of Light, by
means of which some Rays are in this Experiment constantly refracted
after the usual manner, and others constantly after the unusual manner:
For if the difference be not original, but arises from new Modifications
impress'd on the Rays at their first Refraction, it would be alter'd by
new Modifications in the three following Refractions; whereas it suffers
no alteration, but is constant, and has the same effect upon the Rays in
all the Refractions. The unusual Refraction is therefore perform'd by an
original property of the Rays. And it remains to be enquired, whether
the Rays have not more original Properties than are yet discover'd.
_Qu._ 26. Have not the Rays of Light several sides, endued with several
original Properties? For if the Planes of perpendicular Refraction of
the second Crystal be at right Angles with the Planes of perpendicular
Refraction of the first Crystal, the Rays which are refracted after the
usual manner in passing through the first Crystal, will be all of them
refracted after the unusual manner in passing through the second
Crystal; and the Rays which are refracted after the unusual manner in
passing through the first Crystal, will be all of them refracted after
the usual manner in passing through the second Crystal. And therefore
there are not two sorts of Rays differing in their nature from one
another, one of which is constantly and in all Positions refracted after
the usual manner, and the other constantly and in all Positions after
the unusual manner. The difference between the two sorts of Rays in the
Experiment mention'd in the 25th Question, was only in the Positions of
the Sides of the Rays to the Planes of perpendicular Refraction. For one
and the same Ray is here refracted sometimes after the usual, and
sometimes after the unusual manner, according to the Position which its
Sides have to the Crystals. If the Sides of the Ray are posited the same
way to both Crystals, it is refracted after the same manner in them
both: But if that side of the Ray which looks towards the Coast of the
unusual Refraction of the first Crystal, be 90 Degrees from that side of
the same Ray which looks toward the Coast of the unusual Refraction of
the second Crystal, (which may be effected by varying the Position of
the second Crystal to the first, and by consequence to the Rays of
Light,) the Ray shall be refracted after several manners in the several
Crystals. There is nothing more required to determine whether the Rays
of Light which fall upon the second Crystal shall be refracted after
the usual or after the unusual manner, but to turn about this Crystal,
so that the Coast of this Crystal's unusual Refraction may be on this or
on that side of the Ray. And therefore every Ray may be consider'd as
having four Sides or Quarters, two of which opposite to one another
incline the Ray to be refracted after the unusual manner, as often as
either of them are turn'd towards the Coast of unusual Refraction; and
the other two, whenever either of them are turn'd towards the Coast of
unusual Refraction, do not incline it to be otherwise refracted than
after the usual manner. The two first may therefore be call'd the Sides
of unusual Refraction. And since these Dispositions were in the Rays
before their Incidence on the second, third, and fourth Surfaces of the
two Crystals, and suffered no alteration (so far as appears,) by the
Refraction of the Rays in their passage through those Surfaces, and the
Rays were refracted by the same Laws in all the four Surfaces; it
appears that those Dispositions were in the Rays originally, and
suffer'd no alteration by the first Refraction, and that by means of
those Dispositions the Rays were refracted at their Incidence on the
first Surface of the first Crystal, some of them after the usual, and
some of them after the unusual manner, accordingly as their Sides of
unusual Refraction were then turn'd towards the Coast of the unusual
Refraction of that Crystal, or sideways from it.
Every Ray of Light has therefore two opposite Sides, originally endued
with a Property on which the unusual Refraction depends, and the other
two opposite Sides not endued with that Property. And it remains to be
enquired, whether there are not more Properties of Light by which the
Sides of the Rays differ, and are distinguished from one another.
In explaining the difference of the Sides of the Rays above mention'd, I
have supposed that the Rays fall perpendicularly on the first Crystal.
But if they fall obliquely on it, the Success is the same. Those Rays
which are refracted after the usual manner in the first Crystal, will be
refracted after the unusual manner in the second Crystal, supposing the
Planes of perpendicular Refraction to be at right Angles with one
another, as above; and on the contrary.
If the Planes of the perpendicular Refraction of the two Crystals be
neither parallel nor perpendicular to one another, but contain an acute
Angle: The two beams of Light which emerge out of the first Crystal,
will be each of them divided into two more at their Incidence on the
second Crystal. For in this case the Rays in each of the two beams will
some of them have their Sides of unusual Refraction, and some of them
their other Sides turn'd towards the Coast of the unusual Refraction of
the second Crystal.