Opticks Part 8

[K] _As is done in our_ Author's Lect. Optic. _Part_ I. _Sect._ III.

_and_ IV. _and Part_ II. _Sect._ II.

[L] _See our_ Author's Lect. Optic. _Part_ II. _Sect._ II. _pag._ 269, &c.

[M] _This is demonstrated in our_ Author's Lect. Optic. _Part_ I.

_Sect._ IV. _Prop._ 35 _and_ 36.

THE

SECOND BOOK

OF

OPTICKS

_PART I._

_Observations concerning the Reflexions, Refractions, and Colours of thin transparent Bodies._

It has been observed by others, that transparent Substances, as Gla.s.s, 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 const.i.tution 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 princ.i.p.al 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 Gla.s.s. 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 Gla.s.ses. And through this hole Objects that were beyond might be seen distinctly, which could not at all be seen through other parts of the Gla.s.ses where the Air was interjacent. Although the Gla.s.ses were a little convex, yet this transparent spot was of a considerable breadth, which breadth seemed princ.i.p.ally to proceed from the yielding inwards of the parts of the Gla.s.ses, 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 incompa.s.sing it, and afterwards continually grew more and more contracted.

[Ill.u.s.tration: 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 mult.i.tude.

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-gla.s.ses, 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 Gla.s.s from the lower to make them successively vanish again in the same place. The Colour, which by pressing the Gla.s.ses 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 circ.u.mference to the center and by compressing the Gla.s.ses still more, grow continually broader until a new Colour emerged in its center, and thereby it became a Ring encompa.s.sing that new Colour. And by compressing the Gla.s.ses still more, the diameter of this Ring would increase, and the breadth of its...o...b..t 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 encompa.s.sing the innermost Colour, the last of which was the black Spot. And, on the contrary, by lifting up the upper Gla.s.s 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 Quant.i.ty to be as followeth.

Next to the pellucid central Spot made by the contact of the Gla.s.ses succeeded blue, white, yellow, and red. The blue was so little in quant.i.ty, 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 encompa.s.sing 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 quant.i.ty, 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 Gla.s.ses 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.

[Ill.u.s.tration: FIG. 2.]

_Obs._ 5. To determine the interval of the Gla.s.ses, 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 Gla.s.ses 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 Gla.s.ses, 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...o...b..t was 58/100 parts of an Inch, and the Diameter of the Sphere on which the double convex Object-gla.s.s was ground was about 102 Feet, and hence I gathered the thickness of the Air or Aereal Interval of the Gla.s.ses 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 Gla.s.s was truly plane, and not something concave or convex on that side which I accounted plane; and whether I had not pressed the Gla.s.ses together, as I often did, to make them touch; (For by pressing such Gla.s.ses together their parts easily yield inwards, and the Rings thereby become sensibly broader than they would be, did the Gla.s.ses 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-gla.s.s 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 a.s.sumed in proportion as 11 to 17, the Diameter of the Sphere to which the Gla.s.s was figured will by computation be found 182 Inches. This Gla.s.s 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 Gla.s.s. 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 Compa.s.ses on the upper Surface on the upper Gla.s.s, and my Eye was about eight or nine Inches distance from the Gla.s.s, almost perpendicularly over it, and the Gla.s.s was 1/6 of an Inch thick, and thence it is easy to collect that the true Diameter of the Ring between the Gla.s.ses was greater than its measur'd Diameter above the Gla.s.ses 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-gla.s.s 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 Gla.s.s being laid upon the same plain Gla.s.s, the Diameter of the fifth of the dark Rings, when the black Spot in their Center appear'd plainly without pressing the Gla.s.ses, was by the measure of the Compa.s.ses upon the upper Gla.s.s 121/600 Parts of an Inch, and by consequence between the Gla.s.ses it was 1222/6000: For the upper Gla.s.s 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-gla.s.ses upon flat Pieces of a broken Looking-gla.s.s, and found the same Measures of the Rings: Which makes me rely upon them till they can be determin'd more accurately by Gla.s.ses ground to larger Spheres, though in such Gla.s.ses greater care must be taken of a true Plane.

These Dimensions were taken, when my Eye was placed almost perpendicularly over the Gla.s.ses, being about an Inch, or an Inch and a quarter, distant from the incident Rays, and eight Inches distant from the Gla.s.s; so that the Rays were inclined to the Gla.s.s in an Angle of about four Degrees. Whence by the following Observation you will understand, that had the Rays been perpendicular to the Gla.s.ses, 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 Gla.s.ses 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 const.i.tute that Diameter when the Rays are perpendicular. And in the fourth Column the Thickness of the Air at the Circ.u.mference of that Ring is expressed in Parts, of which also ten const.i.tute 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 Gla.s.s into the Plate of Air, or from the Sine of Incidence when the Refraction is made out of the Plate of Air into the Gla.s.s.

_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-gla.s.ses 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 Gla.s.ses, 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 Circ.u.mference 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 Gla.s.s; whence it may be collected that the Gla.s.ses 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 circ.u.mference of the said red.

_Obs._ 9. By looking through the two contiguous Object-gla.s.ses, 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 Gla.s.ses: 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 Gla.s.s 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 Gla.s.ses 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-gla.s.ses 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 Gla.s.ses 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 Gla.s.ses, 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 Gla.s.ses, if I pressed the upper Gla.s.s 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 Gla.s.ses 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 Gla.s.ses, 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 Gla.s.ses, as in a Looking-gla.s.s. 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.

[Ill.u.s.tration: FIG. 3.]

_Obs._ 13. Appointing an a.s.sistant 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 Gla.s.ses, 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 Gla.s.ses 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-gla.s.ses. 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-gla.s.ses, 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 quant.i.ty 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 Gla.s.ses there, where the Ring is successively made by the limits of the five princ.i.p.al 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 Gla.s.ses 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 Gla.s.ses, I found that the Light which fell on the dark s.p.a.ces which were between the Colour'd Rings was transmitted through the Gla.s.ses 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 s.p.a.ces. And from thence the origin of these Rings is manifest; namely, that the Air between the Gla.s.ses, 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.

[Ill.u.s.tration: 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 Gla.s.ses. In those that follow are set down the Appearances of a denser Medium thin'd within a rarer, such as are Plates of Muscovy Gla.s.s, 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 Gla.s.s, and by that means its Colours emerged in a very regular order, like so many concentrick Rings encompa.s.sing 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.