_PART I._
_Observations concerning the Reflexions, Refractions, and Colours of
thin transparent Bodies._
It has been observed by others, that transparent Substances, as Glass,
Water, Air, &c. when made very thin by being blown into Bubbles, or
otherwise formed into Plates, do exhibit various Colours according to
their various thinness, altho' at a greater thickness they appear very
clear and colourless. In the former Book I forbore to treat of these
Colours, because they seemed of a more difficult Consideration, and were
not necessary for establishing the Properties of Light there discoursed
of. But because they may conduce to farther Discoveries for compleating
the Theory of Light, especially as to the constitution of the parts of
natural Bodies, on which their Colours or Transparency depend; I have
here set down an account of them. To render this Discourse short and
distinct, I have first described the principal of my Observations, and
then consider'd and made use of them. The Observations are these.
_Obs._ 1. Compressing two Prisms hard together that their sides (which
by chance were a very little convex) might somewhere touch one another:
I found the place in which they touched to become absolutely
transparent, as if they had there been one continued piece of Glass. For
when the Light fell so obliquely on the Air, which in other places was
between them, as to be all reflected; it seemed in that place of contact
to be wholly transmitted, insomuch that when look'd upon, it appeared
like a black or dark spot, by reason that little or no sensible Light
was reflected from thence, as from other places; and when looked through
it seemed (as it were) a hole in that Air which was formed into a thin
Plate, by being compress'd between the Glasses. And through this hole
Objects that were beyond might be seen distinctly, which could not at
all be seen through other parts of the Glasses where the Air was
interjacent. Although the Glasses were a little convex, yet this
transparent spot was of a considerable breadth, which breadth seemed
principally to proceed from the yielding inwards of the parts of the
Glasses, by reason of their mutual pressure. For by pressing them very
hard together it would become much broader than otherwise.
_Obs._ 2. When the Plate of Air, by turning the Prisms about their
common Axis, became so little inclined to the incident Rays, that some
of them began to be transmitted, there arose in it many slender Arcs of
Colours which at first were shaped almost like the Conchoid, as you see
them delineated in the first Figure. And by continuing the Motion of the
Prisms, these Arcs increased and bended more and more about the said
transparent spot, till they were compleated into Circles or Rings
incompassing it, and afterwards continually grew more and more
contracted.
[Illustration: FIG. 1.]
These Arcs at their first appearance were of a violet and blue Colour,
and between them were white Arcs of Circles, which presently by
continuing the Motion of the Prisms became a little tinged in their
inward Limbs with red and yellow, and to their outward Limbs the blue
was adjacent. So that the order of these Colours from the central dark
spot, was at that time white, blue, violet; black, red, orange, yellow,
white, blue, violet, &c. But the yellow and red were much fainter than
the blue and violet.
The Motion of the Prisms about their Axis being continued, these Colours
contracted more and more, shrinking towards the whiteness on either
side of it, until they totally vanished into it. And then the Circles in
those parts appear'd black and white, without any other Colours
intermix'd. But by farther moving the Prisms about, the Colours again
emerged out of the whiteness, the violet and blue at its inward Limb,
and at its outward Limb the red and yellow. So that now their order from
the central Spot was white, yellow, red; black; violet, blue, white,
yellow, red, &c. contrary to what it was before.
_Obs._ 3. When the Rings or some parts of them appeared only black and
white, they were very distinct and well defined, and the blackness
seemed as intense as that of the central Spot. Also in the Borders of
the Rings, where the Colours began to emerge out of the whiteness, they
were pretty distinct, which made them visible to a very great multitude.
I have sometimes number'd above thirty Successions (reckoning every
black and white Ring for one Succession) and seen more of them, which by
reason of their smalness I could not number. But in other Positions of
the Prisms, at which the Rings appeared of many Colours, I could not
distinguish above eight or nine of them, and the Exterior of those were
very confused and dilute.
In these two Observations to see the Rings distinct, and without any
other Colour than Black and white, I found it necessary to hold my Eye
at a good distance from them. For by approaching nearer, although in the
same inclination of my Eye to the Plane of the Rings, there emerged a
bluish Colour out of the white, which by dilating it self more and more
into the black, render'd the Circles less distinct, and left the white a
little tinged with red and yellow. I found also by looking through a
slit or oblong hole, which was narrower than the pupil of my Eye, and
held close to it parallel to the Prisms, I could see the Circles much
distincter and visible to a far greater number than otherwise.
_Obs._ 4. To observe more nicely the order of the Colours which arose
out of the white Circles as the Rays became less and less inclined to
the Plate of Air; I took two Object-glasses, the one a Plano-convex for
a fourteen Foot Telescope, and the other a large double Convex for one
of about fifty Foot; and upon this, laying the other with its plane side
downwards, I pressed them slowly together, to make the Colours
successively emerge in the middle of the Circles, and then slowly lifted
the upper Glass from the lower to make them successively vanish again in
the same place. The Colour, which by pressing the Glasses together,
emerged last in the middle of the other Colours, would upon its first
appearance look like a Circle of a Colour almost uniform from the
circumference to the center and by compressing the Glasses still more,
grow continually broader until a new Colour emerged in its center, and
thereby it became a Ring encompassing that new Colour. And by
compressing the Glasses still more, the diameter of this Ring would
increase, and the breadth of its Orbit or Perimeter decrease until
another new Colour emerged in the center of the last: And so on until a
third, a fourth, a fifth, and other following new Colours successively
emerged there, and became Rings encompassing the innermost Colour, the
last of which was the black Spot. And, on the contrary, by lifting up
the upper Glass from the lower, the diameter of the Rings would
decrease, and the breadth of their Orbit increase, until their Colours
reached successively to the center; and then they being of a
considerable breadth, I could more easily discern and distinguish their
Species than before. And by this means I observ'd their Succession and
Quantity to be as followeth.
Next to the pellucid central Spot made by the contact of the Glasses
succeeded blue, white, yellow, and red. The blue was so little in
quantity, that I could not discern it in the Circles made by the Prisms,
nor could I well distinguish any violet in it, but the yellow and red
were pretty copious, and seemed about as much in extent as the white,
and four or five times more than the blue. The next Circuit in order of
Colours immediately encompassing these were violet, blue, green, yellow,
and red: and these were all of them copious and vivid, excepting the
green, which was very little in quantity, and seemed much more faint and
dilute than the other Colours. Of the other four, the violet was the
least in extent, and the blue less than the yellow or red. The third
Circuit or Order was purple, blue, green, yellow, and red; in which the
purple seemed more reddish than the violet in the former Circuit, and
the green was much more conspicuous, being as brisk and copious as any
of the other Colours, except the yellow, but the red began to be a
little faded, inclining very much to purple. After this succeeded the
fourth Circuit of green and red. The green was very copious and lively,
inclining on the one side to blue, and on the other side to yellow. But
in this fourth Circuit there was neither violet, blue, nor yellow, and
the red was very imperfect and dirty. Also the succeeding Colours became
more and more imperfect and dilute, till after three or four revolutions
they ended in perfect whiteness. Their form, when the Glasses were most
compress'd so as to make the black Spot appear in the center, is
delineated in the second Figure; where _a_, _b_, _c_, _d_, _e_: _f_,
_g_, _h_, _i_, _k_: _l_, _m_, _n_, _o_, _p_: _q_, _r_: _s_, _t_: _v_,
_x_: _y_, _z_, denote the Colours reckon'd in order from the center,
black, blue, white, yellow, red: violet, blue, green, yellow, red:
purple, blue, green, yellow, red: green, red: greenish blue, red:
greenish blue, pale red: greenish blue, reddish white.
[Illustration: FIG. 2.]
_Obs._ 5. To determine the interval of the Glasses, or thickness of the
interjacent Air, by which each Colour was produced, I measured the
Diameters of the first six Rings at the most lucid part of their Orbits,
and squaring them, I found their Squares to be in the arithmetical
Progression of the odd Numbers, 1, 3, 5, 7, 9, 11. And since one of
these Glasses was plane, and the other spherical, their Intervals at
those Rings must be in the same Progression. I measured also the
Diameters of the dark or faint Rings between the more lucid Colours, and
found their Squares to be in the arithmetical Progression of the even
Numbers, 2, 4, 6, 8, 10, 12. And it being very nice and difficult to
take these measures exactly; I repeated them divers times at divers
parts of the Glasses, that by their Agreement I might be confirmed in
them. And the same method I used in determining some others of the
following Observations.
_Obs._ 6. The Diameter of the sixth Ring at the most lucid part of its
Orbit was 58/100 parts of an Inch, and the Diameter of the Sphere on
which the double convex Object-glass was ground was about 102 Feet, and
hence I gathered the thickness of the Air or Aereal Interval of the
Glasses at that Ring. But some time after, suspecting that in making
this Observation I had not determined the Diameter of the Sphere with
sufficient accurateness, and being uncertain whether the Plano-convex
Glass was truly plane, and not something concave or convex on that side
which I accounted plane; and whether I had not pressed the Glasses
together, as I often did, to make them touch; (For by pressing such
Glasses together their parts easily yield inwards, and the Rings thereby
become sensibly broader than they would be, did the Glasses keep their
Figures.) I repeated the Experiment, and found the Diameter of the sixth
lucid Ring about 55/100 parts of an Inch. I repeated the Experiment also
with such an Object-glass of another Telescope as I had at hand. This
was a double Convex ground on both sides to one and the same Sphere, and
its Focus was distant from it 83-2/5 Inches. And thence, if the Sines of
Incidence and Refraction of the bright yellow Light be assumed in
proportion as 11 to 17, the Diameter of the Sphere to which the Glass
was figured will by computation be found 182 Inches. This Glass I laid
upon a flat one, so that the black Spot appeared in the middle of the
Rings of Colours without any other Pressure than that of the weight of
the Glass. And now measuring the Diameter of the fifth dark Circle as
accurately as I could, I found it the fifth part of an Inch precisely.
This Measure was taken with the points of a pair of Compasses on the
upper Surface on the upper Glass, and my Eye was about eight or nine
Inches distance from the Glass, almost perpendicularly over it, and the
Glass was 1/6 of an Inch thick, and thence it is easy to collect that
the true Diameter of the Ring between the Glasses was greater than its
measur'd Diameter above the Glasses in the Proportion of 80 to 79, or
thereabouts, and by consequence equal to 16/79 parts of an Inch, and its
true Semi-diameter equal to 8/79 parts. Now as the Diameter of the
Sphere (182 Inches) is to the Semi-diameter of this fifth dark Ring
(8/79 parts of an Inch) so is this Semi-diameter to the thickness of the
Air at this fifth dark Ring; which is therefore 32/567931 or
100/1774784. Parts of an Inch; and the fifth Part thereof, _viz._ the
1/88739 Part of an Inch, is the Thickness of the Air at the first of
these dark Rings.
The same Experiment I repeated with another double convex Object-glass
ground on both sides to one and the same Sphere. Its Focus was distant
from it 168-1/2 Inches, and therefore the Diameter of that Sphere was
184 Inches. This Glass being laid upon the same plain Glass, the
Diameter of the fifth of the dark Rings, when the black Spot in their
Center appear'd plainly without pressing the Glasses, was by the measure
of the Compasses upon the upper Glass 121/600 Parts of an Inch, and by
consequence between the Glasses it was 1222/6000: For the upper Glass
was 1/8 of an Inch thick, and my Eye was distant from it 8 Inches. And a
third proportional to half this from the Diameter of the Sphere is
5/88850 Parts of an Inch. This is therefore the Thickness of the Air at
this Ring, and a fifth Part thereof, _viz._ the 1/88850th Part of an
Inch is the Thickness thereof at the first of the Rings, as above.
I tried the same Thing, by laying these Object-glasses upon flat Pieces
of a broken Looking-glass, and found the same Measures of the Rings:
Which makes me rely upon them till they can be determin'd more
accurately by Glasses ground to larger Spheres, though in such Glasses
greater care must be taken of a true Plane.
These Dimensions were taken, when my Eye was placed almost
perpendicularly over the Glasses, being about an Inch, or an Inch and a
quarter, distant from the incident Rays, and eight Inches distant from
the Glass; so that the Rays were inclined to the Glass in an Angle of
about four Degrees. Whence by the following Observation you will
understand, that had the Rays been perpendicular to the Glasses, the
Thickness of the Air at these Rings would have been less in the
Proportion of the Radius to the Secant of four Degrees, that is, of
10000 to 10024. Let the Thicknesses found be therefore diminish'd in
this Proportion, and they will become 1/88952 and 1/89063, or (to use
the nearest round Number) the 1/89000th Part of an Inch. This is the
Thickness of the Air at the darkest Part of the first dark Ring made by
perpendicular Rays; and half this Thickness multiplied by the
Progression, 1, 3, 5, 7, 9, 11, &c. gives the Thicknesses of the Air at
the most luminous Parts of all the brightest Rings, _viz._ 1/178000,
3/178000, 5/178000, 7/178000, &c. their arithmetical Means 2/178000,
4/178000, 6/178000, &c. being its Thicknesses at the darkest Parts of
all the dark ones.
_Obs._ 7. The Rings were least, when my Eye was placed perpendicularly
over the Glasses in the Axis of the Rings: And when I view'd them
obliquely they became bigger, continually swelling as I removed my Eye
farther from the Axis. And partly by measuring the Diameter of the same
Circle at several Obliquities of my Eye, partly by other Means, as also
by making use of the two Prisms for very great Obliquities, I found its
Diameter, and consequently the Thickness of the Air at its Perimeter in
all those Obliquities to be very nearly in the Proportions express'd in
this Table.
-------------------+--------------------+----------+----------
Angle of Incidence |Angle of Refraction |Diameter |Thickness
on | into | of the | of the
the Air. | the Air. | Ring. | Air.
-------------------+--------------------+----------+----------
Deg. Min. | | |
| | |
00 00 | 00 00 | 10 | 10
| | |
06 26 | 10 00 | 10-1/13 | 10-2/13
| | |
12 45 | 20 00 | 10-1/3 | 10-2/3
| | |
18 49 | 30 00 | 10-3/4 | 11-1/2
| | |
24 30 | 40 00 | 11-2/5 | 13
| | |
29 37 | 50 00 | 12-1/2 | 15-1/2
| | |
33 58 | 60 00 | 14 | 20
| | |
35 47 | 65 00 | 15-1/4 | 23-1/4
| | |
37 19 | 70 00 | 16-4/5 | 28-1/4
| | |
38 33 | 75 00 | 19-1/4 | 37
| | |
39 27 | 80 00 | 22-6/7 | 52-1/4
| | |
40 00 | 85 00 | 29 | 84-1/12
| | |
40 11 | 90 00 | 35 | 122-1/2
-------------------+--------------------+----------+----------
In the two first Columns are express'd the Obliquities of the incident
and emergent Rays to the Plate of the Air, that is, their Angles of
Incidence and Refraction. In the third Column the Diameter of any
colour'd Ring at those Obliquities is expressed in Parts, of which ten
constitute that Diameter when the Rays are perpendicular. And in the
fourth Column the Thickness of the Air at the Circumference of that Ring
is expressed in Parts, of which also ten constitute its Thickness when
the Rays are perpendicular.
And from these Measures I seem to gather this Rule: That the Thickness
of the Air is proportional to the Secant of an Angle, whose Sine is a
certain mean Proportional between the Sines of Incidence and Refraction.
And that mean Proportional, so far as by these Measures I can determine
it, is the first of an hundred and six arithmetical mean Proportionals
between those Sines counted from the bigger Sine, that is, from the Sine
of Refraction when the Refraction is made out of the Glass into the
Plate of Air, or from the Sine of Incidence when the Refraction is made
out of the Plate of Air into the Glass.
_Obs._ 8. The dark Spot in the middle of the Rings increased also by the
Obliquation of the Eye, although almost insensibly. But, if instead of
the Object-glasses the Prisms were made use of, its Increase was more
manifest when viewed so obliquely that no Colours appear'd about it. It
was least when the Rays were incident most obliquely on the interjacent
Air, and as the obliquity decreased it increased more and more until the
colour'd Rings appear'd, and then decreased again, but not so much as it
increased before. And hence it is evident, that the Transparency was
not only at the absolute Contact of the Glasses, but also where they had
some little Interval. I have sometimes observed the Diameter of that
Spot to be between half and two fifth parts of the Diameter of the
exterior Circumference of the red in the first Circuit or Revolution of
Colours when view'd almost perpendicularly; whereas when view'd
obliquely it hath wholly vanish'd and become opake and white like the
other parts of the Glass; whence it may be collected that the Glasses
did then scarcely, or not at all, touch one another, and that their
Interval at the perimeter of that Spot when view'd perpendicularly was
about a fifth or sixth part of their Interval at the circumference of
the said red.
_Obs._ 9. By looking through the two contiguous Object-glasses, I found
that the interjacent Air exhibited Rings of Colours, as well by
transmitting Light as by reflecting it. The central Spot was now white,
and from it the order of the Colours were yellowish red; black, violet,
blue, white, yellow, red; violet, blue, green, yellow, red, &c. But
these Colours were very faint and dilute, unless when the Light was
trajected very obliquely through the Glasses: For by that means they
became pretty vivid. Only the first yellowish red, like the blue in the
fourth Observation, was so little and faint as scarcely to be discern'd.
Comparing the colour'd Rings made by Reflexion, with these made by
transmission of the Light; I found that white was opposite to black, red
to blue, yellow to violet, and green to a Compound of red and violet.
That is, those parts of the Glass were black when looked through, which
when looked upon appeared white, and on the contrary. And so those which
in one case exhibited blue, did in the other case exhibit red. And the
like of the other Colours. The manner you have represented in the third
Figure, where AB, CD, are the Surfaces of the Glasses contiguous at E,
and the black Lines between them are their Distances in arithmetical
Progression, and the Colours written above are seen by reflected Light,
and those below by Light transmitted (p. 209).
_Obs._ 10. Wetting the Object-glasses a little at their edges, the Water
crept in slowly between them, and the Circles thereby became less and
the Colours more faint: Insomuch that as the Water crept along, one half
of them at which it first arrived would appear broken off from the other
half, and contracted into a less Room. By measuring them I found the
Proportions of their Diameters to the Diameters of the like Circles made
by Air to be about seven to eight, and consequently the Intervals of the
Glasses at like Circles, caused by those two Mediums Water and Air, are
as about three to four. Perhaps it may be a general Rule, That if any
other Medium more or less dense than Water be compress'd between the
Glasses, their Intervals at the Rings caused thereby will be to their
Intervals caused by interjacent Air, as the Sines are which measure the
Refraction made out of that Medium into Air.
_Obs._ 11. When the Water was between the Glasses, if I pressed the
upper Glass variously at its edges to make the Rings move nimbly from
one place to another, a little white Spot would immediately follow the
center of them, which upon creeping in of the ambient Water into that
place would presently vanish. Its appearance was such as interjacent Air
would have caused, and it exhibited the same Colours. But it was not
air, for where any Bubbles of Air were in the Water they would not
vanish. The Reflexion must have rather been caused by a subtiler Medium,
which could recede through the Glasses at the creeping in of the Water.
_Obs._ 12. These Observations were made in the open Air. But farther to
examine the Effects of colour'd Light falling on the Glasses, I darken'd
the Room, and view'd them by Reflexion of the Colours of a Prism cast on
a Sheet of white Paper, my Eye being so placed that I could see the
colour'd Paper by Reflexion in the Glasses, as in a Looking-glass. And
by this means the Rings became distincter and visible to a far greater
number than in the open Air. I have sometimes seen more than twenty of
them, whereas in the open Air I could not discern above eight or nine.
[Illustration: FIG. 3.]
_Obs._ 13. Appointing an Assistant to move the Prism to and fro about
its Axis, that all the Colours might successively fall on that part of
the Paper which I saw by Reflexion from that part of the Glasses, where
the Circles appear'd, so that all the Colours might be successively
reflected from the Circles to my Eye, whilst I held it immovable, I
found the Circles which the red Light made to be manifestly bigger than
those which were made by the blue and violet. And it was very pleasant
to see them gradually swell or contract accordingly as the Colour of the
Light was changed. The Interval of the Glasses at any of the Rings when
they were made by the utmost red Light, was to their Interval at the
same Ring when made by the utmost violet, greater than as 3 to 2, and
less than as 13 to 8. By the most of my Observations it was as 14 to 9.
And this Proportion seem'd very nearly the same in all Obliquities of my
Eye; unless when two Prisms were made use of instead of the
Object-glasses. For then at a certain great obliquity of my Eye, the
Rings made by the several Colours seem'd equal, and at a greater
obliquity those made by the violet would be greater than the same Rings
made by the red: the Refraction of the Prism in this case causing the
most refrangible Rays to fall more obliquely on that plate of the Air
than the least refrangible ones. Thus the Experiment succeeded in the
colour'd Light, which was sufficiently strong and copious to make the
Rings sensible. And thence it may be gather'd, that if the most
refrangible and least refrangible Rays had been copious enough to make
the Rings sensible without the mixture of other Rays, the Proportion
which here was 14 to 9 would have been a little greater, suppose 14-1/4
or 14-1/3 to 9.
_Obs._ 14. Whilst the Prism was turn'd about its Axis with an uniform
Motion, to make all the several Colours fall successively upon the
Object-glasses, and thereby to make the Rings contract and dilate: The
Contraction or Dilatation of each Ring thus made by the variation of its
Colour was swiftest in the red, and slowest in the violet, and in the
intermediate Colours it had intermediate degrees of Celerity. Comparing
the quantity of Contraction and Dilatation made by all the degrees of
each Colour, I found that it was greatest in the red; less in the
yellow, still less in the blue, and least in the violet. And to make as
just an Estimation as I could of the Proportions of their Contractions
or Dilatations, I observ'd that the whole Contraction or Dilatation of
the Diameter of any Ring made by all the degrees of red, was to that of
the Diameter of the same Ring made by all the degrees of violet, as
about four to three, or five to four, and that when the Light was of the
middle Colour between yellow and green, the Diameter of the Ring was
very nearly an arithmetical Mean between the greatest Diameter of the
same Ring made by the outmost red, and the least Diameter thereof made
by the outmost violet: Contrary to what happens in the Colours of the
oblong Spectrum made by the Refraction of a Prism, where the red is most
contracted, the violet most expanded, and in the midst of all the
Colours is the Confine of green and blue. And hence I seem to collect
that the thicknesses of the Air between the Glasses there, where the
Ring is successively made by the limits of the five principal Colours
(red, yellow, green, blue, violet) in order (that is, by the extreme
red, by the limit of red and yellow in the middle of the orange, by the
limit of yellow and green, by the limit of green and blue, by the limit
of blue and violet in the middle of the indigo, and by the extreme
violet) are to one another very nearly as the sixth lengths of a Chord
which found the Notes in a sixth Major, _sol_, _la_, _mi_, _fa_, _sol_,
_la_. But it agrees something better with the Observation to say, that
the thicknesses of the Air between the Glasses there, where the Rings
are successively made by the limits of the seven Colours, red, orange,
yellow, green, blue, indigo, violet in order, are to one another as the
Cube Roots of the Squares of the eight lengths of a Chord, which found
the Notes in an eighth, _sol_, _la_, _fa_, _sol_, _la_, _mi_, _fa_,
_sol_; that is, as the Cube Roots of the Squares of the Numbers, 1, 8/9,
5/6, 3/4, 2/3, 3/5, 9/16, 1/2.
_Obs._ 15. These Rings were not of various Colours like those made in
the open Air, but appeared all over of that prismatick Colour only with
which they were illuminated. And by projecting the prismatick Colours
immediately upon the Glasses, I found that the Light which fell on the
dark Spaces which were between the Colour'd Rings was transmitted
through the Glasses without any variation of Colour. For on a white
Paper placed behind, it would paint Rings of the same Colour with those
which were reflected, and of the bigness of their immediate Spaces. And
from thence the origin of these Rings is manifest; namely, that the Air
between the Glasses, according to its various thickness, is disposed in
some places to reflect, and in others to transmit the Light of any one
Colour (as you may see represented in the fourth Figure) and in the same
place to reflect that of one Colour where it transmits that of another.
[Illustration: FIG. 4.]
_Obs._ 16. The Squares of the Diameters of these Rings made by any
prismatick Colour were in arithmetical Progression, as in the fifth
Observation. And the Diameter of the sixth Circle, when made by the
citrine yellow, and viewed almost perpendicularly was about 58/100 parts
of an Inch, or a little less, agreeable to the sixth Observation.
The precedent Observations were made with a rarer thin Medium,
terminated by a denser, such as was Air or Water compress'd between two
Glasses. In those that follow are set down the Appearances of a denser
Medium thin'd within a rarer, such as are Plates of Muscovy Glass,
Bubbles of Water, and some other thin Substances terminated on all sides
with air.
_Obs._ 17. If a Bubble be blown with Water first made tenacious by
dissolving a little Soap in it, 'tis a common Observation, that after a
while it will appear tinged with a great variety of Colours. To defend
these Bubbles from being agitated by the external Air (whereby their
Colours are irregularly moved one among another, so that no accurate
Observation can be made of them,) as soon as I had blown any of them I
cover'd it with a clear Glass, and by that means its Colours emerged in
a very regular order, like so many concentrick Rings encompassing the
top of the Bubble. And as the Bubble grew thinner by the continual
subsiding of the Water, these Rings dilated slowly and overspread the
whole Bubble, descending in order to the bottom of it, where they
vanish'd successively. In the mean while, after all the Colours were
emerged at the top, there grew in the center of the Rings a small round
black Spot, like that in the first Observation, which continually
dilated it self till it became sometimes more than 1/2 or 3/4 of an Inch
in breadth before the Bubble broke. At first I thought there had been no
Light reflected from the Water in that place, but observing it more
curiously, I saw within it several smaller round Spots, which appeared
much blacker and darker than the rest, whereby I knew that there was
some Reflexion at the other places which were not so dark as those
Spots. And by farther Tryal I found that I could see the Images of some
things (as of a Candle or the Sun) very faintly reflected, not only from
the great black Spot, but also from the little darker Spots which were
within it.
Besides the aforesaid colour'd Rings there would often appear small
Spots of Colours, ascending and descending up and down the sides of the
Bubble, by reason of some Inequalities in the subsiding of the Water.
And sometimes small black Spots generated at the sides would ascend up
to the larger black Spot at the top of the Bubble, and unite with it.
_Obs._ 18. Because the Colours of these Bubbles were more extended and
lively than those of the Air thinn'd between two Glasses, and so more
easy to be distinguish'd, I shall here give you a farther description of
their order, as they were observ'd in viewing them by Reflexion of the
Skies when of a white Colour, whilst a black substance was placed
behind the Bubble. And they were these, red, blue; red, blue; red, blue;
red, green; red, yellow, green, blue, purple; red, yellow, green, blue,
violet; red, yellow, white, blue, black.
The three first Successions of red and blue were very dilute and dirty,
especially the first, where the red seem'd in a manner to be white.
Among these there was scarce any other Colour sensible besides red and
blue, only the blues (and principally the second blue) inclined a little
to green.
The fourth red was also dilute and dirty, but not so much as the former
three; after that succeeded little or no yellow, but a copious green,
which at first inclined a little to yellow, and then became a pretty
brisk and good willow green, and afterwards changed to a bluish Colour;
but there succeeded neither blue nor violet.
The fifth red at first inclined very much to purple, and afterwards
became more bright and brisk, but yet not very pure. This was succeeded
with a very bright and intense yellow, which was but little in quantity,
and soon chang'd to green: But that green was copious and something more
pure, deep and lively, than the former green. After that follow'd an
excellent blue of a bright Sky-colour, and then a purple, which was less
in quantity than the blue, and much inclined to red.
The sixth red was at first of a very fair and lively scarlet, and soon
after of a brighter Colour, being very pure and brisk, and the best of
all the reds. Then after a lively orange follow'd an intense bright and
copious yellow, which was also the best of all the yellows, and this
changed first to a greenish yellow, and then to a greenish blue; but the
green between the yellow and the blue, was very little and dilute,
seeming rather a greenish white than a green. The blue which succeeded
became very good, and of a very bright Sky-colour, but yet something
inferior to the former blue; and the violet was intense and deep with
little or no redness in it. And less in quantity than the blue.
In the last red appeared a tincture of scarlet next to violet, which
soon changed to a brighter Colour, inclining to an orange; and the
yellow which follow'd was at first pretty good and lively, but
afterwards it grew more dilute until by degrees it ended in perfect
whiteness. And this whiteness, if the Water was very tenacious and
well-temper'd, would slowly spread and dilate it self over the greater
part of the Bubble; continually growing paler at the top, where at
length it would crack in many places, and those cracks, as they dilated,
would appear of a pretty good, but yet obscure and dark Sky-colour; the
white between the blue Spots diminishing, until it resembled the Threds
of an irregular Net-work, and soon after vanish'd, and left all the
upper part of the Bubble of the said dark blue Colour. And this Colour,
after the aforesaid manner, dilated it self downwards, until sometimes
it hath overspread the whole Bubble. In the mean while at the top, which
was of a darker blue than the bottom, and appear'd also full of many
round blue Spots, something darker than the rest, there would emerge
one or more very black Spots, and within those, other Spots of an
intenser blackness, which I mention'd in the former Observation; and
these continually dilated themselves until the Bubble broke.
If the Water was not very tenacious, the black Spots would break forth
in the white, without any sensible intervention of the blue. And
sometimes they would break forth within the precedent yellow, or red, or
perhaps within the blue of the second order, before the intermediate
Colours had time to display themselves.
By this description you may perceive how great an affinity these Colours
have with those of Air described in the fourth Observation, although set
down in a contrary order, by reason that they begin to appear when the
Bubble is thickest, and are most conveniently reckon'd from the lowest
and thickest part of the Bubble upwards.
_Obs._ 19. Viewing in several oblique Positions of my Eye the Rings of
Colours emerging on the top of the Bubble, I found that they were
sensibly dilated by increasing the obliquity, but yet not so much by far
as those made by thinn'd Air in the seventh Observation. For there they
were dilated so much as, when view'd most obliquely, to arrive at a part
of the Plate more than twelve times thicker than that where they
appear'd when viewed perpendicularly; whereas in this case the thickness
of the Water, at which they arrived when viewed most obliquely, was to
that thickness which exhibited them by perpendicular Rays, something
less than as 8 to 5. By the best of my Observations it was between 15
and 15-1/2 to 10; an increase about 24 times less than in the other
case.
Sometimes the Bubble would become of an uniform thickness all over,
except at the top of it near the black Spot, as I knew, because it would
exhibit the same appearance of Colours in all Positions of the Eye. And
then the Colours which were seen at its apparent circumference by the
obliquest Rays, would be different from those that were seen in other
places, by Rays less oblique to it. And divers Spectators might see the
same part of it of differing Colours, by viewing it at very differing
Obliquities. Now observing how much the Colours at the same places of
the Bubble, or at divers places of equal thickness, were varied by the
several Obliquities of the Rays; by the assistance of the 4th, 14th,
16th and 18th Observations, as they are hereafter explain'd, I collect
the thickness of the Water requisite to exhibit any one and the same
Colour, at several Obliquities, to be very nearly in the Proportion
expressed in this Table.
-----------------+------------------+----------------
Incidence on | Refraction into | Thickness of
the Water. | the Water. | the Water.
-----------------+------------------+----------------
Deg. Min. | Deg. Min. |
| |
00 00 | 00 00 | 10
| |
15 00 | 11 11 | 10-1/4
| |
30 00 | 22 1 | 10-4/5
| |
45 00 | 32 2 | 11-4/5
| |
60 00 | 40 30 | 13
| |
75 00 | 46 25 | 14-1/2
| |
90 00 | 48 35 | 15-1/5
-----------------+------------------+----------------
In the two first Columns are express'd the Obliquities of the Rays to
the Superficies of the Water, that is, their Angles of Incidence and
Refraction. Where I suppose, that the Sines which measure them are in
round Numbers, as 3 to 4, though probably the Dissolution of Soap in the
Water, may a little alter its refractive Virtue. In the third Column,
the Thickness of the Bubble, at which any one Colour is exhibited in
those several Obliquities, is express'd in Parts, of which ten
constitute its Thickness when the Rays are perpendicular. And the Rule
found by the seventh Observation agrees well with these Measures, if
duly apply'd; namely, that the Thickness of a Plate of Water requisite
to exhibit one and the same Colour at several Obliquities of the Eye, is
proportional to the Secant of an Angle, whose Sine is the first of an
hundred and six arithmetical mean Proportionals between the Sines of
Incidence and Refraction counted from the lesser Sine, that is, from the
Sine of Refraction when the Refraction is made out of Air into Water,
otherwise from the Sine of Incidence.
I have sometimes observ'd, that the Colours which arise on polish'd
Steel by heating it, or on Bell-metal, and some other metalline
Substances, when melted and pour'd on the Ground, where they may cool in
the open Air, have, like the Colours of Water-bubbles, been a little
changed by viewing them at divers Obliquities, and particularly that a
deep blue, or violet, when view'd very obliquely, hath been changed to a
deep red. But the Changes of these Colours are not so great and
sensible as of those made by Water. For the Scoria, or vitrified Part of
the Metal, which most Metals when heated or melted do continually
protrude, and send out to their Surface, and which by covering the
Metals in form of a thin glassy Skin, causes these Colours, is much
denser than Water; and I find that the Change made by the Obliquation of
the Eye is least in Colours of the densest thin Substances.
_Obs._ 20. As in the ninth Observation, so here, the Bubble, by
transmitted Light, appear'd of a contrary Colour to that, which it
exhibited by Reflexion. Thus when the Bubble being look'd on by the
Light of the Clouds reflected from it, seemed red at its apparent
Circumference, if the Clouds at the same time, or immediately after,
were view'd through it, the Colour at its Circumference would be blue.
And, on the contrary, when by reflected Light it appeared blue, it would
appear red by transmitted Light.
_Obs._ 21. By wetting very thin Plates of _Muscovy_ Glass, whose
thinness made the like Colours appear, the Colours became more faint and
languid, especially by wetting the Plates on that side opposite to the
Eye: But I could not perceive any variation of their Species. So then
the thickness of a Plate requisite to produce any Colour, depends only
on the density of the Plate, and not on that of the ambient Medium. And
hence, by the 10th and 16th Observations, may be known the thickness
which Bubbles of Water, or Plates of _Muscovy_ Glass, or other
Substances, have at any Colour produced by them.
_Obs._ 22. A thin transparent Body, which is denser than its ambient
Medium, exhibits more brisk and vivid Colours than that which is so much
rarer; as I have particularly observed in the Air and Glass. For blowing
Glass very thin at a Lamp Furnace, those Plates encompassed with Air did
exhibit Colours much more vivid than those of Air made thin between two
Glasses.
_Obs._ 23. Comparing the quantity of Light reflected from the several
Rings, I found that it was most copious from the first or inmost, and in
the exterior Rings became gradually less and less. Also the whiteness of
the first Ring was stronger than that reflected from those parts of the
thin Medium or Plate which were without the Rings; as I could manifestly
perceive by viewing at a distance the Rings made by the two
Object-glasses; or by comparing two Bubbles of Water blown at distant
Times, in the first of which the Whiteness appear'd, which succeeded all
the Colours, and in the other, the Whiteness which preceded them all.
_Obs._ 24. When the two Object-glasses were lay'd upon one another, so
as to make the Rings of the Colours appear, though with my naked Eye I
could not discern above eight or nine of those Rings, yet by viewing
them through a Prism I have seen a far greater Multitude, insomuch that
I could number more than forty, besides many others, that were so very
small and close together, that I could not keep my Eye steady on them
severally so as to number them, but by their Extent I have sometimes
estimated them to be more than an hundred. And I believe the Experiment
may be improved to the Discovery of far greater Numbers. For they seem
to be really unlimited, though visible only so far as they can be
separated by the Refraction of the Prism, as I shall hereafter explain.
[Illustration: FIG. 5.]
But it was but one side of these Rings, namely, that towards which the
Refraction was made, which by that Refraction was render'd distinct, and
the other side became more confused than when view'd by the naked Eye,
insomuch that there I could not discern above one or two, and sometimes
none of those Rings, of which I could discern eight or nine with my
naked Eye. And their Segments or Arcs, which on the other side appear'd
so numerous, for the most part exceeded not the third Part of a Circle.
If the Refraction was very great, or the Prism very distant from the
Object-glasses, the middle Part of those Arcs became also confused, so
as to disappear and constitute an even Whiteness, whilst on either side
their Ends, as also the whole Arcs farthest from the Center, became
distincter than before, appearing in the Form as you see them design'd
in the fifth Figure.
The Arcs, where they seem'd distinctest, were only white and black
successively, without any other Colours intermix'd. But in other Places
there appeared Colours, whose Order was inverted by the refraction in
such manner, that if I first held the Prism very near the
Object-glasses, and then gradually removed it farther off towards my
Eye, the Colours of the 2d, 3d, 4th, and following Rings, shrunk towards
the white that emerged between them, until they wholly vanish'd into it
at the middle of the Arcs, and afterwards emerged again in a contrary
Order. But at the Ends of the Arcs they retain'd their Order unchanged.
I have sometimes so lay'd one Object-glass upon the other, that to the
naked Eye they have all over seem'd uniformly white, without the least
Appearance of any of the colour'd Rings; and yet by viewing them through
a Prism, great Multitudes of those Rings have discover'd themselves. And
in like manner Plates of _Muscovy_ Glass, and Bubbles of Glass blown at
a Lamp-Furnace, which were not so thin as to exhibit any Colours to the
naked Eye, have through the Prism exhibited a great Variety of them
ranged irregularly up and down in the Form of Waves. And so Bubbles of
Water, before they began to exhibit their Colours to the naked Eye of a
Bystander, have appeared through a Prism, girded about with many
parallel and horizontal Rings; to produce which Effect, it was necessary
to hold the Prism parallel, or very nearly parallel to the Horizon, and
to dispose it so that the Rays might be refracted upwards.