Opticks (1721, 3rd Edition) — part 9 of 18

repeated the Experiment alfo with fuch an Ob- jeft-glals of another Telefcope as I had at hand. This was a double Convex ground on both fides to one and the fame Sphere, and its Fo- cus was diilantfrom it 834 Inches. And thence, if the Sines of Incidence and Refradion of the bright yellow Light be alTumed in proportion as II to 17, the Diameter of the Sphere to which the Glafs was figured will by computa- tion be found i8i Inches. This Glafs I laid upon a flat one, fo that the black Spot appear- ed in the middle of the Rings of Colours with- out any other PrelTure than that of the weight of the Glafs. And ndw meafuring the Diame- ter of the fifth dark Circle as accurately as I could, I found it the fifth part of an Inch pre- cifely. This meafure was taken with the points of a pair of CompafTes on the upper Surface on the upper Glafs, and my Eye was about eight or nine Inches diftant from the Glafs, almoft perpendicularly over it, and the Glafs was i of an Inch thick, and thence it is eafy to colled: that the true Diameter of the Ring be- tween the GlafTes was greater than its meafur'd Diameter above the Glafles in the Proportion of 80 to 79, or thereabouts, and by confequence equal to ■— part of an Inch, and its true Semi- diameter equal to ~ parts. Now as the Dia- meter of the Sphere (i8x Inches) is to the Se- mi-diameter of this fifth dark Ring (-^ parts of an Inch) fo is this Semi-diameter to the thick- nefs of the Air at this fifth dark Ring 5 which is

there-

r ill ]

therefore — — r: or -- — - parts of an IncH ; and

S^79jI 17/47^4 ^

the fifth part thereof, viz. the ^— - part of art

Inch, is the thicknefs of the Air at the firft of thefe dark Rings.

The fame Experiment I repeated with ano- ther double convex Objed-glais ground on both fides to one and the lame Sphere. Its Focus was diilant from it 1684 Inches, and therefore the Diameter of that Sphere was 184 Inches. The Glafs being laid upon the fame plain Glafs, the Diameter of the tifth of the dark Rings, when the black Spot in their center appear'd plainly without prefling the Glaffes, was by the meafure of the Compaires upon the upper Glafs

^^ parts of an Inch, and by confequence be- tween the Glaffes ft was g^ For the upper

Glafs was 4 of an Inch thick, and my Eye was diltant from it 8 Inches. And a third propor- tional to half this from the Diameter of the

Sphere is ^^^ parts of an Inch. This is there- fore the thicknefs of the Air at this Ring, and a fifth part thereof, viz. the ^-^th part of an

Inch is the thicknefs thereof at the firft of the Rings, as above.

I tried the fame thing by laying thefe Ob- jeft-glafTes upon flat pieces of a broken Look- ing-glafs, and found the fame Meafures of the Rings : Which makes me rely upon them till

N they

[ 178 ]

they can be determin'd more accurately by Glailes ground to larger Spheres, though in fuch GlaiFes greater care mult be taken of a true Plane.

Thefe Dimenfions were taken when my Eye was placed almoit perpendicularly over the GlalTes, being about an Inch, or an Inch and a quarter, diilant from the incident Rays, and eight Inches diltant from the Giafs ; fo that the Rays were inclined to the Glafs in an Angle of about four Degrees. Whence by the following Obfervation you will underiland, that had the Rays been perpendicular to the Glailes, the thicknefs of the Air at thefe Fungs would have been lefs in the proportion of the Radius to the Secant of four Degrees , that is of looco to 10024. Let the thicknefles found be therefore diminifli'd in this Proportion, and they will be- come 57^ and r-^ or (to ufe the nearefi:

round number ) the ~— th part of an Inch.

^ 09000 ^

This is the thicknefs of the Air at the darkeft part of the firll dark Ring made by perpendi- cular Rays, and half this thickneis multiplied by the Progreilion, i, 3, 5", 7? 9? n? ^c. gives the thicknelTes of the Air at the moll luminous

parts of all the brighteil Rings, viz,.

17^000 ^c. their arithmetical

l7booo? 1 7 800C;, ,17^000'

Means —r~-> —0^9 -^y ^^- being its

"**^ 178000' 178000' i/»ooo' c>

thicknefles at the darkeil parts of all the dark ones.

OH.

[ 179 j

t)bj. 7. The Rings were leaft when my Ey^ Was placed perpendicularly over the GlalFes in the Axis of the Rings : And when I view'd them obliquely they became bigger, continual- ly fwelling as I removed my Eye farther from the Axis. And partly by meafuring the Diame- ter of the fame Circle at feveral Obliquities of my Eye, partly by other means, as alfo by ma- king ufe of the two Prifms for very great Obli- quities, I found its Diameter, and confequenc- ly the thicknefs of the Air at its Perimeter in all thofe' Obliquities to be very nearly m the Pro- portions exprefs'd in this Table.

Angle of In-

Angle of Re-

'^Diameter

Thicknefs

cidence on

fra^ion in-

of the

of the

the Air.

to the Air.

Ring.

Air.

Degr. Min.

00 00

00 00

10

10

06 a6

10 00

IOt't '

• IOtV

IX 4f

2o 00

lOf

10^

18 49

30 00

IOt

114

24 30

40 00

I It

13

29 37

50 00

124

■1)4

33 5S

60 GO

14

20

IS 47

6$ GO

15-^

2^3^

11 19

70 00

i6|

^^.

38 33

75" 00

19^

37

39 2.7

80 GO

22j

5^-4

40 00

85- GO

^9

84VT

40 II

90 00

35"

12X4

Na

%.

[ i8o]

Fn the two firft Columns are exprefs'd the Obliquities of the incident and emergent Rays to the Plate of the Air, that is, their Angles of Incidence and Refradtion. In the third Column the Diameter of any colour'd Ring at thofe Ob- liquities is exprelTed in parts, of which ten con- Ititute that Diameter when the Rays are per- pendicular. And in the fourth Column the thicknefs of the Air at the circumference of that Ring is exprefTed in parts of which alfo ten conftitute its thicknefs when the Rays are perpendicular.

And from thefe Meafures I feem to gather this Rule : That the thicknefs of the Air is proportional to the fecant of an Angle, whofe Sine is a certain mean Proportional between the Sines of Incidence and Refradion. And that mean Proportional, fo far as by thefe Meafures I. can determine it, is the firfl of an hundred and fix arithmetical mean Proportionals be- tween thofCiSines counted from the bigger Sine,' that is, from the Sine of Refradion when the Refradion is made out of the Glafs into the Plate of Air, or from the Sine of Incidence when the Refradion is made out of the Plate of Air into the Glafs.

Obf.%. The dark Spot in the middle of the Rings increafed alfo by the Obliquation of the Eye, although almoit infenfibly. But if infte^ of the Objed'glaiTes the Prifms were made ufe of, its Increafe was more manifeft when view- ed fo obliquely that no Colours appear'd aboiit it. It was lead when the Rays were incident molt obliquely on the interjacent Air, and as

the

[ i8i ]

the obliquity dccreafed it increafed more and more until the colour'd Rings appear'd, and then decreafed again, but not lb much as it in- creafed before. And hence it is evident, that the Tranfparency was not only at the abfolute Contaft of the GlalTes, but alfo where they had fome little Interval. I have fometimes obferved the Diameter of that Spot to be between half and two fifth parts of the Diameter of the ex- terior Circumference of the red in the firfl Cir- cuit or Revolution of Colours when view'd al- moll perpendicularly ; whereas when view'd ob- liquely it hath wholly vanifh'd and become opake and white like the other parts of the Glafs ; whence it may be collefted that the GlalTes did then fcarcely, or not at all, touch one another, and that their hiterval at the perimeter of that Spot when view'd perpendicularly was about a fifth or fixth part of their Interval at the cir- cumference of the faid red.

Obf. 9. By looking through the two conti- guous Objeft glailes , I found that the interja- cent Air exhibited Rings of Colours, as well by tranfmitting Light as by refledfing it. The central Spot was now white, and from it the order of the Colours were yellowilli red ; black, violet, blue, white, yellow, red ; violet, blue, green, yellow, red, ^c. But thefe Colours were very faint and dilute, unlefs when the Light was trajedted very obliquely through the GlalTes : For by that means they became pretty vivid. Only the firlt yellowilli red, like the blue in the fourth Obfervation, was fo little and faint as fcarcely to be difcern'd. Comparing

N 3 thp

.:;;:#'■

[ 182 ]

the colour'd Rings made by Reflexion, with thefe made by tranfmiffion of the Light ; I found that white was oppofite to black, red to blue, yellow to violet, and green to a Compound of red and violet. That is , thofe parts of the Glafs were black when looked through, which when looked upon appear'd white, and on the contrary. And fo thofe which in one cafe ex- hibited blue, did in the other cafe exhibit red. And the like of the other Colours. The man- ner you have reprefented in the third Figure, where AB, CD, are the Surfaces of the Glaf- {cs contiguous at E, and the black Lines be- tween them are their Diitances in arithmetical Progreilion, and the Colours written above are feen by refleLled Light, and thofe below by Light tranfmitted. .

O^/ 10. Wetting the Objeft - glalTes a little at ^ their edges, the Water crept in llowly be- tween them, and the Circles thereby became lefs and the Colours more faint: Infomuch that as the Water crept along one half of them at which it firil: arrived would appear broken off from th^ other half, and contracted into a lefs Room. By meafuring them I found the Pro- portions otNheir Diameters to the Diameters of the like Circles made by Air to be about feven to eight, and confequently the Intervals of the Glades at like Circles, caufed by thofe two Me- diums Water and Air, are as about three to four. Perhaps it may be a general Rule, That if any other Medium more or lefs denfe than Water be comprefs'd ^between the Glaffes, their Inter- vals at the Rings caufed thereby v^ill be to their

Intervals

[ 183 ]

Intervals caufed by interjacent Air, as the Sines are which meafure the Refradion made out of that Medium into Air.

Obf.w. When the Water was between the GlalTes, if I preifed the upper Glafs varioufly 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 prefently vanifli. Its appearance w^as fuch as interjacent Air would have caufed, and it exhibited the fame Colours. But it was not Air, for where any Bubbles of Air were in the Water they would not vanilh. The Refle- xion muft have rather been caufed by a fubtiier Medium, which could recede through the GlalTes at the creeping in of the Water.

Obf. 11.. Thefe Obfervations were made in the open Air. But farther to examine the Ef- feds of colour'd Light falling on the Glailes, I darken'd the Room, and view'd them by Re- flexion of the Colours of a Prifm call on a Sheet of white Paper, my Eye being fo placed that I could fee the colour'd Paper by Reflexion in the GlalTes, as in a Looking-glafs. And by this means the Rings became dillinfter and vifible to a far greater number than in the open Air. I have fometimes feen more than twenty of them, whereas in the open Air I could not dif- cern above eight or nine.

Obf. 13. Appointing an Alliltant to move the Prifm to and fro about its Axis, that all the Colours might fuccellively fall on that part of the Paper which I faw by Reflexion from thar

N 4 pari

[ '84 ]

Part of the GlafTes, where the Circles appear 'd, fo that all the Colours might be fuccefTively re- fleded from the Circles to my E3^e whilll I held it immoveable, I found the Circles which the red Light made to be manifellly bigger than thofe which were made by the blue and violet. And it was very pleafant to fee them gradually fvvell or contraft accordingly as the Colour of the Light was changed. The Interval of the Glaf- fes at any of the Rings, when they were made by the utmoil red Light, was to their Interval at the fame Ring when made by the utmofl violet, gre::iter than as 3 to 2, and lefs than as 13 to 8. By the moil of my Obfervations it was as 14 to 9. And this Proportion fcem'd very nearly the fame in all Obliquities of my Eye ; unleis when two Prifms were made ufe of in- ftead of the Objeft-glalTes. For then at a cer- tain great obliquity of my Eye, the Rings made by the feverai Colours feem'd equal, and at a greater obliquity thofe made by the violet would be greater than the fame Rings made by the red : the RefradUon of the Prifm in this cafe caufing the moll refrangible Rays to fall more obliquely on that plate of the Air than the leaft refrangi- ble ones. Thus the Experiment fucceeded in the colour'd Light, which was fufficiently flrong and copious to make the Ritigs fenfible. And thence it may be gathered, that if the moft re- frangible and leail refrangible Rays had been co- pious enough to make the Rings fenfible with- out the mixture of other Rays, the Proportion \vhich here was 14 to 9 would have been a little greater, fuppofe 14^^ or 147 to 9,

• , . Qbf.

C 185 ]

Ohf. 14. Whilft the Prifm was turn'd about its Axis with an uniform Motion, to make all the feveral Colours fall fuccefTively upon the Objed- glafTes, and thereby to make the Rings contrad and dilate : The Contraction or Dila- tation of each Ring thus made by the variation of its Colour was fwifteft in the red, and flow- eil in the violet, and in the intermediate Colours it had intermediate degrees of Celerity. Com- paring the quantity of Contrac^tion and Dilata- tion made by all the degrees of each Colour, I found that it was greatell in the red ; lefs in the yellow, flill lefs in the blue, and lead in the violet. And to make as juif an Eilim.ation as I could of the Proportions of their Contradions or Dilatations, I obferv'd that the whole Con- tradion or Dilatation of the Diameter of any Ring made by all the degrees of red, was to that of the Diameter of the fame 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 greatefl Diame- ter of the fame Ring made by the outmolt red, and the lead Diameter thereof made by the outmofl violet ; contrary to what happens in the Colours of the oblong Spedrum made by the Refradion of a Prifm, where the red is mod contraded, the violet mod expanded, and in the midd of all the Colours is the Corn- fine of green and blue. And hence I feem to colled that the thicknedes of the Air between the GlafTes there, where the Ring is fucceflive-

5y

[ i80

ly made by the limits of the five principal Co- lours (red, yellow, green, blue, violet) in or- der (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 neady as the fix lengths of a Chord which found the Notes in a fixth Major, fol^ la^ mt, fa, fol, la. But it agrees fomething better with the Obfer- vation to fa3^ that the thicknefTes of the Air between the GlafTes there, where the Rings are fucceffively made by the limits of the feven Co- lours, red, orange, yellow, green, blue, indi- go, 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, fil, la, fa, fol, la, mi, fa fol; that is, as the Cube Roots of the Squares of the Numbers, i,

■S 5 5 ' 5 _9 I

:?? -K-y "+? 7) T5 xTJ 2.

Obf. 15-. Thefe Rings were not of various Colours like thofe made in the open Air,, but appeared all over of that prifmatick Colour on- ly with which they wxre illuminated. And by projeding the prii'matick Colours immediately upon the GlafTes, I found that the Light which fell on the dark ^Spaces which were between the colour'd Rings, v/as tranfmitted through the GlalTes without any variation of Colour. For on a white Paper placed behind, it would paint Rings of the fame Colour with thofe which were receded, and of the bignefs of their im- mediate Spaces. And from thence tlie origin

of

[ i87 ]

of thefe Rings is manifefl; namely, that the Air between the Glafles, according to its vari- ous thicknefs, is difpofed in fome places to re^ fled:, and in others to tranfmit the Light of a- ny one Colour (as you may fee repreiented in the fourth Figure) and in the fame place to re- flect that of one Colour where it tranfmits that of another.

Obf 1 6. The Squares of the Diameters of thefe Rings made by any prifmatick Colour were in arithmetical Progrellion, as in the fifth Ob- fervation. And the Diameter of the fixch Cir-» cle, when made by the citrine yellow, and

viewed almoit perpendicularly, was about —

parts of an Inch, or a little lefs, agreeable to the fixth Obfervation.

The precedent Obfervations were made with a rarer thin Medium, terminated by a denfer, fuch as was Air or Water comprefs'd between two GlalFes. In thofe that follow are fet down the Appearances of a denfer Medium thin'd within a rarer, fuch as are Plates of Mufcovy Glafs, Bubbles of Water, and fome other thin Subflances terminated on all iides with Air.

Obf.ij. If a Bubble be blown with Water firlt made tenacious by dillblving a little Soap in it, 'tis a common Obfervation, that after a while it will appear tinged with a great variety of Colours. To defend thefe Bubbles from be- ing agitated by the external Air (whereby their Colours are irregularly moved one among ano- ther, fo that no accurate Obfervation can be inadc of them,) as foon as I had blown any of

them

[ m ]

them I cover'd it with a clear Glafs, and by that means its Colours emerged in a very regular order, like fo many concentrick Rings encom- paffing the top of the Bubble. And as the Bub- ble grew thinner by the continual fubfiding of the Water, thefe Rings dilated flowly and over- fpread the whole Bubble, defcending in order to the bottom of it, where they vanifli'd fuc- ceffively. In the mean while, after all the Co- lours were emerged at the top, there grew in the center of the Rings a fmall round black Spot, like that in the tir(t Obfervation, which continually dilated it felf till it became fome- times more than 4 or 4 of an Inch in breadth before the Bubble broke. At firft I thought there had been no Light refle(^i:ed from the Wa- ter in that place, but obferving it more curi- oufly, I faw within it feveral fmaller round Spots, which appeared much blacker and dark- er than the reli:, whereby I knew that there was fome Reflexion at the other places which were not fo dark as thofe Spots. And by far- ther Tryal I found that I could fee the Images of fome things (as of a Candle or the Sun) ve- ry faintly retledted, not only from the great black Spot, but alfo from the little darker Spots which were within in it.

Befides the aforefaid colour'd Rings there would often appear fmall Spots of Colours, af- eending and defcending up and down the fides of the Bubble, by reafon of fome Inequalities in the fubfiding of the Water. And fometimes fmall black Spots generated at the fides would

afcend

[ '8^ ]

afcend up to the larger black Spot at the top of the Bubble, and unite with it.

Obf.i%. Becaufe the Colours of thefe Bubbles were more extended and lively than thofe oi the Air thinn'd between two GlalTes, and fo more eafy to be dillinguilli'd, I iliall here give you a farther defcription of their order, as they were obferv'd in viewing them by Reflexion of the Skies when of a white Colour, whilft a black fubftance was placed behind the Bubble. And they were thefe, red, blue ; red, blue ; red, blue; red, green; red, yellow, green, blue, purpje ; red, yellow, green, blue, violet ; red, yellow, white, blue, black.

The three firft Succeflions of red and blue were very dilute and dirty, efpecially the firft, where the red feem'd in a manner to be white. Among thefe there was fcarce any other Colour fenfible befides red and blue, only the blues (and principally the fecond blue) incUned a little to green.

The fourth red was alfo dilute and dirty, but not fo much as the former three ; after that fucceeded little or no yellow, but a copious green, which at firft inclined a little to yellow, and then became a pretty brisk and good wil- low green, and afterwards changed to a blueifh Colour ; but there fucceeded neither blue nor violet.

The fifth red at firft inchned very much to purple, and afterwards became more bright and brisk, but yet not very pure. This was fucceeded with a very bright and intenfe yel- low, which was but little in quantity, and foon

changed

3

[ i^o ]

chang'd to green : But that green was copious and fomething more pure, deep and lively, thait the former green. After that follow 'd an ex- cellent blue of a bright Sky-colour, and then a purple, which was lefs in quantity than the blue, and much inclin'd to red.

The fixth red was at firfl of a very fair and lively Scarlet, and foon after of a brighter Co- lour, being very pure and brisk, and the beft of all the reds. Then after a lively orange fol- low'd an intenfe bright and copious yellow ^ which was alfo the beft of all the yellows, and this changed firft to a greenifh yelloWj and then to a greenifli blue; but the green between the yellow and the blue, was very little and dilute, leeming rather a greenifli white than a green. The blue which fucceeded became very good, and of a very fair bright Sky-colour, but yet fomething inferior to the former blue ; and the violet was intenfe and deep with little or no rednefs in it. And lefs in quantity than the blue.

In the lafl: red appeared a tindure of fcarlet next to violet, which foon changed to a bright- er Colour, inclining to an orange ; and the yel- low which followed was at firft pretty good and lively, but afterwards it grew more dilute, until by degrees it ended in perfeft whitenefs* And this whitenefs, if the Water was very te^ nacious and well tempered, would flow ly fpread and dilate it felf over the greater part of the Bubble; continually growing paler at the top, where at length it would crack in many places, and thofe cracks, as they dilated, would appear

of

C 191 ]

of a pretty good, but yet obfcure and dark Sky- colour ; the white between the blue Spots diminifhing , untill it refembled the Threds of an irregular Net-work, and foon after vanifh'd and left all the upper part of the Bubble of the faid dark blue Colour. And this Colour, after the aforefaid manner, dilated it felf downwards, until fometimes it hath overfpread the whole Bubble. In the mean while at the top, which was of a darker blue than the bottom, and ap- pear'd alfo full of many round blue Spots, fome- thing darker than the reft, there would emerge one or more very black Spots, and within thofe, other Spots of an intenfer blacknefs, which I mention'd in the former Obfervation ; and thefe continually dilated themfelves until the Bubble broke.

If the Water was not very tenacious the black Spots would break forth in the white, without any fenfible intervention of the blue. And fometimes they would break forth within the precedent yellow, or red, or perhaps within the blue of the fecond order, before the inter- mediate Colours had time to difplay themfelves.

By this defcription you may perceive how great an affinity thefe Colours have with thofe of Air defcribed in the fourth Obfervation, al- though fet down in a contrary order, by reafon that they begin to appear when the Bubble is thickeft and are molt conveniently reckon'd from the lowelt and thickeft part of the Bubble upwards.

Obf. 19. Viewing in feveral oblique Pofitions pf myEye the Rings of Colours emerging on

the

[ 152 ]

the top of the Bubble> I found that they were fenfibly dilated by increafing the obliquity, but yet not fo much by far as thofe made by thinn'd Air in the feventh Obfervation. For there they were dilated (o much as, when view 'd moft ob- liquely, to arrive at a part of the Plate more than twelve times thicker than that where they appeared when viewed perpendicularly ; where- as in this cafe the thicknefs of the Water, at which they arriv'd when view'd molt obliquely, was to that thicknefs which exhibited them by perpendicular Rays, fomething lefs than as 8 to 5. By the bed of my Obfervations it was be- tween i^ and i5'4 to 10; an increafe about 14 times lefs than in the other cafe.

Sometimes the Bubble would become of an uniform thicknefs all over, except at the top of it near the black Spot, as I knew, becaufe it would exhibit the fame, appearance of Colours in all Politions of the Eye. And then the Co- lours which were feen at its apparent circumfe- rence by the obliquelt Rays would be different from thofe that were feen in other places, by Rays lefs obUque to it. And divers Spedators might fee the fame part of it of differing Co- lours, by viewing it at very differing Obliqui- ties. Now obferving how much the Colours at the fame places of the Bubble, or at divers pla- ces of equal thicknefs, were varied by the fe- veral Obliquities of the Rays ; by the affiltance of the 4th, 14th, 1 6th and 18 th Obfervations, as they are hereafter explain'd , I colled: the thicknefs of the Water requifite to exhibit any one and the fame Colour, at feveral Obliquities,

to

r ^93 ]

to be very nearly in the Proportion exprefled in this Table.

Incidence on

Refra^ion in-

Thickiiefs of

the JVater.

to the Water.

the Water.

Dcgr. Min.

Degr. Min.

GO CO

GO GO

10

15- 00

ii 11

lO:^

^0 00

22 I

104

45" 00

32 2

114

60 00

40 30

13

IS 00

46 -Ly

144

90 00

48 35-

TfT

In the two firft Columns are exprefs'd the Obliquities of the Rays to the Superficies of the Water, that is, their Angles of Incidence and Refradion. Where I fuppofe that the Sines which meafure them are in round Numbers, as 3 to 4, though probably the diflblution of Soaps in the Water, may a little alter its refra(^Hve Virtue. In the third Column the thicknefs of the Bubble, at which any one Colour is exhibit- ed in thofe feveral Obliquities, is exprefs'd in parts, of which ten conftitute its thicknefs when the Rays are perpendicular. And the Rule found by the feventh Obfervation agrees welt with thefe Meafures, if duly apply'd ; namely, that the thicknefs of a Plate of Water rcquifite to exhibit one and the fame Colour at feveral Obhquities of the Eye, is proportional to the fecant of an Angle whofe Sine is the firfl: of an hundred and fix arithmetical mean Proportio-

O nals

C 194 ]

nals between the Sines of Incidence 'and Refra- dion couiucd from the leflerSine, that is, from the Sine of Refradion when the Refradion is made out of Air into Water, otherwife from the Sine of Incidence.

I have fometimes obferv'd, that the Colours which arife on poHlh'd Steel by heating it, or on Bell- metal, and fome other metalUne Sub- ftances, when melted and pour'd on the ground, where they may cool in the open Air, have, hke the Colours of Water- bubbles, been a little changed by viewing them at divers Obliquities, and particularly that a deep blue, or violet, when vievv'd very obliquely, hath been changed to a deep red. But the Changes of thefe Co- , lours are not fo great and fenfible as of thofe made by Water. For the Scoria or vitrified part of the Metal, which molt Metals when heated or melted do continually protrude, and fend out to their Surface, and which by covering the Metals in form of a thin glaffy Skin, caufes thefe Colours, is much denfer than Water ; and 1 find that the Change made by the Obliquation of the Eyes is leaft in Colours of the denfelt thin Subilances.

Obf.ro. As in the ninth Obfervation, fo here, the Bubble, by tranfmitted 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 refleded from it, feem'd red at its apparent circumference, if the Clouds at the fame time, or immediately after, wereview'd through it, the Colour at its circumference would be blue. And, on the ' contrary.

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contrary, when by reflefted Light it appeared blue, it would appear red by tranfmitted Light.

Obf.xi. By wetting very thin Plates of Muf- covy Glafs, whofe thinnefs made the Hke Co- lours appear, the Colours became more faint and languid, efpecially by wetting the Plates on that fide oppofite to the Eye : But I could not perceive any variation of their Species. So then the thicknefs of a Plate requiiite to produce any Colour, depends only on the denfity of the Plate, and not on that of the ambient Medium. And hence, by the loth and i6th Obfervations, may be known the thicknefs which Bubbles of Water, or Plates of Mufcovy Glafs, or other Subflances, have at any Colour produced by them.

Obf. zz. A thin tranfparent Body, which is denfer than its ambient Medium, exhibits more brisk and vivid Colours than that which is fo much rarer ; as I have particularly obferved in the Air and Glafs. For blowing Glafs very thin at a Lamp Furnace, thofe Plates encompaiTed with Air did exhibit Colours much more vivid than thofe of Air made thin between two GlaiTes.

Obf 13. Comparing the quantity of Light reflefted from the feveral Rings, I found that it was moll copious from the firft or inmoft, and in the exterior Rings became gradually lefs and lefs. Alfo the whitenefs of the firfl Ring was Itronger than that refleded from thofe parts of the thin Medium or Plate v.^hich were without the Rings; as I could manifeltly per- ceive by viewing at a diftance the Rings made

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by the tu^o Objed-glafTes ; pr. by comparing two Bubbles of Water blown at diftant times , in the firft of which the whitenefs appear'd, which fucceeded all the Colours, and in the other, the whitenefs which preceded them all.

Obf.i^. When the two Objed-glafTes were lay'd upon one another, fo as to make the Rin^s of the Colours appear, though with my nake^' Eye I could not difcern above eight or nine of thofe Rings, yet by viewing them through a Prifm I have feen a far greater multitude, info- much that I could number more than forty, be- fides many others, that were fo very fmall and clofe together, that I could not keep my Eye itea- dy on them feverally fo as to number them, but by their Extent I have fometimes eftimated them to be more than an hundred. And I believe the Experiment may be improved to the difcovery qf far greater Numbers. For they feem to be really unUmited, though vifible only fo far as they can be feparated by the Refraftion of this Prifm as I lliall hereafter explain.

But it was but one fide of thefe Rings, name- ly, that towards which the Refradion was made, which by that Refradion was render'd dillinft, and the other fide became more confufed than when view'd by the naked Eye, infomuch that there I could not difcern above one or two, and fometimes none of thofe Rings, of which I couid difcern eight or nine with my naked Eye. And their Segments or Arcs, which on* the other fide appear'd fp numerous, for the moil part exceeded not the third part of a Cir- cle. If the Refradion, was very great, or the *-" '^' 4 Prifm

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Prifm very diftant from the Objeff^-glafTes, the middle part of thofe Arcs became aUb confu- fed, fo as to difappear and conftitute an even whitenefs, whilit on either fide their ends, as alfo the whole Arcs fartheft from the center, became diftinfter than before, appearing in the form as you fee them defign'd in the fifth Fi- gure.

The Arcs, where they feem'd diftin^iefl, were only white and black fucceffively, without any other Colours intermix'd. But in other places there appeared Colours, whofe order was in- verted by the Refradion in fuch manner, that if I firfl held the Prifm very near the Objed- glaffes, and then gradually removed it farther off towards my Eye, the Colours of the id, 3d, 4th, and following Rings fhrunk towards the white that emerged between them, until they wholly vanifli'd into it at the middle of the Arcs, and afterwards emerged again in a con- trary order. But at the ends of the Arcs they retain'd their order unchanged.

I have fometimes fo lay'd one Objed glafs upon the other, that to the naked Eye they have all over feem'd uniformly white, without the leaft appearance of any of the colour 'd Rings ; and yet by viewing them through a ^ Prifm, great multitudes of thofe Rings have difcover'd themfelves. And in Hke manner Plates of Mufcpvy Glafs, and Bubbles of Glafs blown at a Lamp Furnace, which were not fo thin as to exhibit any Colours to the naked Eye, have through the Prifm exhibited a great varie- ty of them ranged irregularly up and down in

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the form of Waves. And fo Bubbles of W^- ter, before they began to exhibit their Colours to the naked Eye of a By-ftander, have appear- ed through a Prifm, girded about with many parallel and horizontal Rings ; to produce which EfFeft, it was necelTary to hold the Prifm pa- rallel, or very nearly parallel to the Horizon^^ and to difpofe it fo that the Rays might be re-i fra(^ed upwards.

THE

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THE

SECOND BOOK

O P TIC K S.

P A R T II.

, Remarks ufon the foregoing Obfervation^.

A V IN G given my Obfervations of thefe Colours, before Lmake ufe of them to unfold the Caufes of the Colours of natural Bodies, it is convenient that by the fimpleft of them, fuch as are the 2d, 3d, 4th, 9th, 1 2th, 1 8th, 20th, and 24th, I firit ex-

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