Opticks (1721, 3rd Edition) — part 13 of 18

Obf. 6. Placing my Eye where thefe Rings appeared plained, I faw the Speculum tinged all over with Waves of Colours (red, yellow, green, blue ;) like thofe which in the Obfervations of the firft Part of this Book appeared between • the Objed glaffes and upon Bubbles of Water,, but much larger. And after the manner of thofe, they were of various Magnitudes in various Po- rtions of the Eye, fwelling and flirinking as I moved my Eye this way and that way. They w^ere formed like Arcs of concentrick Circles as thofe were, and when my Eye was over againil: the center of the concavity of the Speculum (that is, 5 Feet and 10 Inches diftant from the Specu- lum) their common center was in a right Line with that center of concavity, and with the hole in the Window. But in other poftures of my Eye their center had other pofitions. They appear'd by the Light of the Clouds propagated to the Speculum through the hole in the \Vin- dow, and when the Sun ilione, through that hole upon the Speculum, his Light upon it

was

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Was of the Colour of the Ring whereon it fell, but by its fplendor obfcured the Rings made by the Light of the Clouds, unlefs when the Spe- culum ^vas removed to a great diftance from the Window, fo that his Light upon it might be broad and faint. By varying the pofition of my Eye, and moving it nearer to or farther from the dire6l beam of the Sun's Light, the Colour of the Sun's reflected Light conftantly varied upon the Speculum, as it did upon my Eye, the fame Colour always appearing to a By llander upon my Eye which to me appear'd upon the Speculum. And thence I knew that the Rings of Colours upon the Chart were made by thefe refled:ed Colours propagated thither from the Speculum in feveral Angles, and that their produftion depended not upon the ter- mination of Light and Shadow. .■ Obf 7. By the Analogy of all thefe Phseno- mena with thofe of the like Rings of Colours defcribed in the firft Part of this Book, it feem- ed to me that thefe Colours were produced by this thick Plate of Glafs, much after the manner that thofe were produced by very thin Plates. For, upon tryal, I found that if the Quick-fil- ver were rubb'd off from the backfide of the Speculum, the Glafs alone would caufe the fame Rings of Colours, but much more faint than before; and therefore the Phaenomenon depends not upon the Quick-filver, unlefs fo far as the Quick-filver by increafmgthe Reflexion of the backfide of the Giafs increafes the Light of the Rings of Colours. I found alfo that a Speculum of Metal without Glafs made fome

Years

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Years fince for optical ufes, and very wel wrought, produced none of thofe Rings ; aii thence I underftood that thefe Rings arife not from one fpecular Surface alone, but depend upon the two Surfaces of the Plate of Glafs whereof the Speculum was made, and upon the thicknefs of the Glafs between them. For as in the 7th and 19th Obfervations of the firfl Part of this Book a thin Plate of Air, Water, or Glafs of an even thicknefs appeared of one Colour when the Rays were perpendicular to it, of another when they were a Uttle oblique, of another when more oblique, of another wheii ftill more oblique, and fo on; fo here, in the fixth Obfervation ,. the Light which emerged out of the Glafs in feveral Obliquities, made the Glafs appear of feveral Colours, and being pro- pagated in thofe Obliquities to the Chart, there painted Rings of thofe Colours. And as the reafon why a thin Plate appeared of feveral Co- lours in feveral Obliquities of the Rays,' was, that the Rays of one and the fame fort are re- fleded. by the thin Plate at one obliquity and tranfmitted at another, and thofe of other forts tranfmitted where thefe are reflected, and re- lieved where thefe are tranfmitted: So the reafon why the thick Plate of Glafs whereof the Speculum was made did appear of various Colours in various Obliquities , and in thofe Obliquities propagated thofe Colours to the Chart, was, that the Rays of one and the fame fort did at one Obliquity emerge out of the Glafs, at another did not emerge but Were refleiSed back towards the Quick-filver

T b?

[ 274 ] .

by the hither Surface of the Glafs, and according- ly as the Obliquity became greater and greater emerged and were reflefted alternately for ma- ny Succeffions, and that in one and the fame ■ Obliquity the Rays of one fort were retleded, and thofe of another tranfmitted. This is ma- nifeit by the fifth Obfervation of this Part of this Book. For in that Obfervation, when the Spe- culum was illuminated by any one of the prif- matick Colours, that Light made many Rings of the fame Colour upon the Chart with dark Intervals, and therefore at its emergence out of the Speculum was alternately tranfmitted and not tranfmitted from the Speculum to the Chart for many Succeflions, according to the various Obliquities of its Emergence. And when the Colour caft on the Speculum by the Prifm was varied, the Rings became of the Colour call on it, and varied their bignefs with 'their Colour, and therefore the Light was now alternately tranfmitted and not tranfmitted from the Spe- culum to the Chart at other Obliquities than before. It feemed to me therefore that thefe Rings were of one and the fame original with thofe of thin Plates, but yet with this difference, that thofe of thin Plates are made by the alter- nate Reflexions and Tranfmiflions of the Rays at the fecond Surface of the Plate after one paf- fage through it, but here the Rays go twice through the Plate before they are alternately re- flected and tranfmitted. Firft, they go through it from the firtl Surface to the Quick-lilver, and then return through it from the Quick-filver to the iiril Surface, and there are either tranf- mitted

• [ 275 ]

mitted to the Chart or refleded back to the Quick-filver , accordingly as they are in their Fits of eafy Reflexion or Tranfmiflion when they arrive at that Surface. For the Intervals of the Fits of the Rays which fall perpendicu- larly on the Speculum , and are refleded back in the fame perpendicular Lines, by reafon of the equality of thefe Angles and Lines, are of the fame length and number within the Glafs after Reflexion as before by the 19th Propofi- tion of the third Part of this Book. And there- fore fmce all the Rays that enter through the firil Surface are in their Fits of eafy Tranfmif- fion at their entrance, and as many of thefe as are reflefted by the fecond are in their Fits of eafy Reflexion there, all thefe muit be again in their Fits of eafy Tranfmiffion at their return to the firfl:, and by confequence there go out of the Glafs to rhe Chart, and form upon it the white Spot of Light in the center of the Rings. For the reafon holds good in all forts of Rays, and therefore all forts muit go out promifcu- oufly to that Spot, and by their mixture caufe it to be white. But the Intervals of the Fits of thofe Rays which are refleded more obliquely than they enter, muil be greater after Reflexion than before by the 15th and ^oih. Propofitions. And thence it may happen that the Rays at their return to the flrlt Surface, may in certain Ob^ Equities be in Fits of eafy Reflexion, and return back to the Quick-fiiver, and in other inteririC- diate Obliquities be again in Fits of eafy Tranf- miiTion, and fo go out to the Chart, and paint on it the Rings of Colours about the white Spot;

T i And

[ 27^ ] .

And becaufe the Intervals of the Fits at equal Obhquities are greater and fewer in the iefs re- frangible Rays, and Iefs and more numerous in the more refrangible, therefore the Iefs re- frangible at equal Obliquities fliall make fewer Rings than the more refrangible, and the Rings made by thofe Ihall be larger than the like number of Rings made by thefe; that is, the red Rings ihall be larger than the yellow, the yellow than the green, the green than the blue, and the blue than the violet, as they were real-' ly found to be in the fifth Obfervation. And therefore the firH: Ring of all Colours encom- pafling the white Spot of Light Ihall be red without any violet wdthin, and yellow and green and blue in the middle, as it was found in the fecond Obfervation ; and thefe Colours in the fecond Ring, and thofe that follow fliall be more expanded till they fpread into one a- nother, and blend one another by interfering.

Thefe feem to be theReafons of thefe Rings in general ; and this put me upon obferving the thicknefs of the Glafs, and confidering whether the Dimenfions and Proportions of the Rings may be truly derived from it by computation.

Obf.%. I meafured therefore the thicknefs of this concavo convex Plate of Glafs, and found it every where ^ of an Inch precifely. Now, by the fixth Obfervation of the firft Part of this Book, a thin Plate of Air tranfmits the brighteft Light of the firft Ring, that is the bright yel- low, when its thicknefs is the o-^th part of an ' 09000 •••

Inch, and by the tenth Obfervation of the fame,

Part

[ 277 ]

Part, a thin Plate of Glafs tranfmits the fame Light of the fame Ring when its thicknefs is lels in proportion of the Sine of Refradion to the Sine of Incidence, that is, when its thick- nefs is the ,~^th or Tj^jth part of an Inch,

fuppofing the Sines are as ii to 17. And if this thicknefs be doubled it tranfmits the fame bright Light of the Second Ring, if tripled it tranf- mits that of the third, and fo on, the bright yellow Light in all thefe cafes being in its Fits of Trahfmiflion. And therefore if its thicknefs be multiplied 34386 times fo as to become ^ of an Inch it transmits the fame bright Light of the 54386th Ring. Suppofe this be the bright yellow Light tranfmitted perpendicularly from the reilefting convex fide of the Glafs through the concave fide to the white Spot in the cen- ter of the Rings of Colours on the Chart : And by a Rule in the 7th and 19th Obfervations in the firll Part of this Book, and by the ifth and ioth Propofitions of the third Part of this Book, if the Rays be made oblique to the Glafs, the thicknefs of the Glafs requifite to tranfmit the fame bright Light of the fame Ring in any Ob- liquity is to this thicknefs of ^ of an Inch, as the , Secant of a certain Angle to the Radius , the Sine of which Angle is the firfl of an hundred and fix arithmetical Means between the Sines of Incidence and Refraftion, counted from the Sine of Incidence when the Refraftion is made out of any plated Body into any Medium en- compaffing it, that is, in this cafe, out of Glafs into Air. Now if the thicknefs of the Glafs be

T 3 increafed

[ 278 ]

increafed by degrees, fo as to bear to its firft thicknefs, (viz. that of a quarter of an Inch) the Proportions which 34386 (the number of Fits of the perpendicular Rays in going through the Glafs towards the white Spot in the center of the Rings,) hath to 34385-, 34384, 34383 and 34381 the numbers of the Fits of the obhque Rays in going through the Glafs towards the firft, fecond, third and fourth Rings of Co- lours) and if the firll thicknefs be divided in- to loooooooo equal parts, the increaled thick- nefles will be 100001908, 100005816, 1001008725' and loooi 1 63 3, and the Angles of which thefe thicknelTes are fecants will be 26^ 13'^ 37' 5^^^, 45^,6'^ and 5'2'' i(/\ the Radius being loooooooo; and the Sines of thefe Angles are 762, 1079, 1 321 and 1515', and the proportional Sines of Refraction 1171, 1659, 2031 and 2345-, the Ra- (^ius being looooo. For fmce the Sines of In- cidence out of Glafs into Air are to the Sines of Refradion as 11 to 17, and to the above- inentioned Secants as 11 to the firft of 106 arith- metical Means between ii and 17, that is, as

\i to II j-^ thofe Secants will be to the Sines

of Refra6lion as 11 7^ to 17, and by this Ana-

Ipgy will give thefe Sines. So then if the Ob- " Kquities of the Rays to the concave Surface of the Glafs be fuch that the Sines of their Refra- djon in palling out of the Glafs through that Surface into the Air be 1172, 1659, 2031, 2345-, the bright Light of the 34386th Ring iliall e- merge at the thickneifes of the Glafs which are

[ 279 ]

tov^ of an Inch as 343^6 to 34385-, 34384, 34383, 343 8x, refpedively. And therefore if the thick- nefs in all thefe cafes be ~ of an Inch (as it is in the Glafs of which the Speculum was made) the bright Light of the 34385'th Ring fliall e- merge where the Sine of Refraction is 1172,, and that of the 34384th,. 34383th and 3438xth Ring where the Sine is 165-9, 2.031, and ^s^s refpedtively. And in thefe Angles of Refra- dion the Light of thefe Rings fhall be propaga- ted from the Speculum to the Chart;, and there paint Rings about the white central round Spot of Light which we faid was the Light of the 34386th Ring. And the Semidiameters of thefe Rings fliall fubtend the Angles of Refradion made at the concave Surface of the Speculmn, and by confequence their Diameters fliall be to the diitance of the Chart from the Speculum cs thofe Sines of Refradion doubled are to the Radius, that is, as 1171, 165-9, 2.031, and 2,345", doubled are to looooo. And therefore if thej diitance of the Chart from the. concave Surface of the Speculum be fix Feet (as it was in the third of thefe Obfervations) the Diameters of the Rings of this bright yellow Light upon the Chart fliall be i'688, z's^^, 2,'9X5', f^y^ Inches. For thefe Diameters are to fix Feet, as the a- bovemention'd Sines doubled are to the Ra- dius. Now thefe Diameters of the bright yel- low Rings, thus found by computation are the very fame with thofe found in the third of thefe Obfervations by meafuring them, V!z. with i44, 2.T, 144-, and 34 Inches, and therefore the Theory of deriving thefe R-ings-from the thick-

T 4 nefs

[ 280 ]

pefs of the Plate of Glafs of which the Specu- _ Jum was made, and from the Obliquity of the emerging Rays agrees with the Obfervation. In this computation 1 have equalled the Diameters of the bright Rings made by Light of all Co^ lours, to the Diameters of the Rings made by the bright yellow. For this yellow makes the brighteit part of the Rings of all Colours. If you defire the Diameters of the Rings- made by the Light of any other unmix'd Colour, you may find them readily by putting them to the Dia- meters of the bright yellow ones in a fubdupH- cate proportion of the Intervals of the Fits of the Rays of thofe Colours when equally inclined to the refrafting or reflefting Surface which caufed thofe Fits, that is by putting the Diame- ters of the Rings made by the Rays in the Ex- tremities and Limits of the feven Colours, red, orange, yellow, green, blue indigo, violet, pro- portional to the Cube-roots of the Numbers, i, J, -h -?» 7» T» tV, 4, which exprefs the lengths of a Monochord founding the Notes in an Eighth : For by this means the Diameters of the Rings of theie Colours will be found pretty nearly in the fame proportion to one another, which they ought to have by the fifth of thefe Obfer- vations.

And thus I fatisfy'd my felf that thefe Rings were of the fame kind and original with thofe of thin Plates, and by confequence that the Fits or alternate Difpofitions of the Rays to be refleded and tranfmitted are propagated to great diftances from every reflefting and re- frading Surface. But ypt to put the mat- ter

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ter out of doubt, I added the following Ob- fervation.

Obf. 9. If thefe Rings thus depend on the thicknefs of the Plate of Glafs, their Diameters at equal diftances from feveral Speculums made of fuch concavo-convex Plates of Glafs as are ground on the fame Sphere, ought to be reci- procally in a fubduplicate proportion of the thickneiTes of the Plates of Glafs. And if this Proportion be found true by experience it will amount to a demonftration that thefe Rings (like thofe formed in thin Plates) do depend on the thicknefs of the Glafs. I procured there- fore" another concavo-convex Plate of Glafs ground on both fides to the fame Sphere with the former Plate. Its thicknefs was -s-V parts of an Inch ; and the Diameters of the three firft bright Rings meafured between the brigheft parts of their Orbits at the diftance of fix Feet from the Glafs were 3. 4^. 54. Inches. Now the thicknefs of the other Glafs being t of an Inch was to the thicknefs of this Glafs as ^ to ^, that is as 31 to 10, or 310000000 to loooooooo, and the Roots of thefe Numbers are 17607 and loooo, and in the proportion of the firft of thefe Roots to the fecond are the Diameters of the bright Rings made in this Obfervation by the thinner Glafs, 3. 44. 54) to the Diameters of the fame Rings made in the third of thefe Ob- fervations by the thicker Glafs i44. i. 244-, that is, the Diameters of the Rings .are reciprocally in a fubduplicate proportion of the thicknefTes of the Plates of Glafs.

So then in Plates of Glafs which are alike

con-

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concave on one fide, and alike convex on the other fide, and aUke quick-filver'd on the con- vex fides, and differ in nothing but their thick- nefs, the Diameters of the Rings are recipro- cally in a fubduplicate proportion of the thick- neffes of the Plates. And this fliews fufficient- ly that the Rings depend on both the Surfaces of the Glafs. They depend on the convex Sur- face becaufe they are more luminous when that Surface is quick -filver'd over than when it is without Quick filver. They depend alfo upon the concave Surface, becaule without that Sur- face a Speculum makes them not. They de- pend on both Surfaces and on the diftances be- tween them, becaufe their bignefs is varied by varying only that didance. And this depen- dence is of the fame kind with that which the Colours of thin Plates have on the diitance of the Surfaces of thofe Plates, becaufe the big- •nefs of the Rings and their proportion to one another, and the variation of their bignefs ari- fing from the variation of the thicknefs of the Glafs, and the orders of their Colours, is fuch as ought to refult from the Propofitions in the end of the third Part of this Book, derived from the Phaenomena of the Colours of thin Plates fet down in the firft Part.

There are yet other Phsenomena of thefe Rings of Colours but fuch as follow from the fame Propofitions, and therefore confirm both the truth of thofe Propofitions, and the Analo- gy between thefe Rings and the Rings of Co- lours made by very thin Plates. I fliall fubjoin fome of them.

Obf.

[ 283 ]

Ohf. 10. When the beam of the Sun's Light was reflefted back from the Speculum not di- redly to the hole in the Window, but to a place a little didant from it, the common center of that Spot, and of all the Rings of Colours fell in the middle way between the beam of the in- cident Light, and the beam of the refleded Light, and by confequence in the center of the fpherical concavity of the Speculum, ^^^henever the Chart on which the Rings of Colours fell was placed at that center. And as the beam of refleded Light by inclining the Speculum re- ceded more and more from the beam of inci- dent Light and from the common center of the colour'd Rings between them, thofe Rings grew bigger and bigger, and fo alfo did the white round Spot, and new Rings of Colours emer- ged fucceffively out of their common center, and the white Spot became a white Ring en- compafTmg them ; and the incident and retleft- ed beams of Light always fell iipon the oppo-* fite parts of this white Ring, illuminating its Perimeter like two mock Suns in the oppofite parts of an Iris. So then the Diameter of this .Ring, meafured from the middle of its Light on one fide to the middle of its Light on the other fide, was always equal to the diitance be- ' tween the middle of the incident beam of Light, and the middle of the refieded beam meafured at the Chart on which the Rings ap- peared: And the Rays which form'd this Ring were reflefted by the Speculum in Angles equal to their Angles of Incidence, and by confe- quence to their Angles of Refraction at their

entrance

[ 284 ]

entrance into the Glafs, but yet their Angles of Reflexion were not in the fame Planes with their Angles of Incidence.

Obf.w. The Colours of the new Rings were in a contrary order to thofe of the former, and arofe after this manner. " The white round Spot of Light in the middle of the Rings continued white to the center till the diftance of the in- cident and refleded beams at the Chart was a- bout 4 parts of an Inch, and then it began to grow dark in the middle. And'when that di- Itance was about i-V of an Inch, the white Spot was become a Ring encompafling a dark round Spot which in the middle incUned to violet and indigo. And the luminous Rings encompafling it were grown equal to thofe dark ones which in the four firlt Obfervations encompafTed them, that is to fay, the white Spot was grown a white Ring equal to the firll of thofe dark Rings, and the firll of thofe luminous Rings 'was now grown equal to the fecond of thofe dark ones, and the fecond of thofe luminous ones to the third of thofe dark ones, and fo on. For the Diameters of the luminous Rings were now ItV, 2,tV, 2.TJ 3^j ^C' Inches.

When the diflance between the incident and ' refleded beams of Light became a little big- ger, there emerged out of the middle of the dark Spot after the indigo a blue, and then out of that blue a pale green, and foon after a yel- low and red. And when the Colour at the center was brightefl, being between yellow and red, the bright Rings were grown equal to thofe Rings which in the four firft Obfervations next

encom-

[ 285 ]

encompaffed them; that is to fay, the white Spot in the middle of thofe Rings was now be- come a white Ring equal to the firft of thofe bright Rings, and the firlt of thofe bright ones was now come equal to the fecond of thofe and fo on. For the Diameters of the white Ring, and of the other luminous Rings encom- paffing it, were now i44, 2,4, 2.44, 3^y ^c. or. thereabouts.

When the diftance of the two beams of Light at the Chart was a Httle more increafed, there emerged out of the middle in order after the red, a purple, a blue, a green, a yellow, and a red inclining much to purple, and when the Colour was brighteft being between yellow and red, the former indigo, blue, green, yel- low and red, were become an Iris or Ring of Colours equal to the firft of thofe luminous Rings which appeared in the four firft Obfer- vations, and the white 'Ring which w^as now become the fecond of the luminous Rings was grown equal to the .fecond of thofe , and the firft of 'thofe which was now become the third Ring was become equal to the third of thofe, and fo .on. For their Diameters were i44, 2.4, i44, 34 Inches, the diftance of the two beams of Light, and the Diameter of the white Ring being 24 Inches.

When thefe two beams became more diftant there emerged out of the middle of the pur- plifli red, firft a darker round Spot, and then out of the middle of that Spot a brighter. And now the former Colours (purple, blue, green, yellow, and purpliih red) were become a Ring

equal

C 28^ ]

equal to the firft of the bright Rings mention- ed in the four firft Obfervations, and the Rings about this Ring were grown equal to the Rings about that refpeftively; the diitance between the two beams of Light and the Diameter of the white Ring (which was now become the third Ring) being about 3 Inches.

The Colours of the Rings in the middle be- gan now to grow very dilute, and if the di- itance between the two Beams was increafed half an Inch, or an Inch more, they vanilh'd whilil the white Ring, with one or two of the Rings next it on either fide, continued ftill vi- fible. But if the diftance of the two beams of Light was ftill more iricreafed, thefe alfo va- nifhed : For the Light which coming from fe- veral parts of the hole in the Window fell up- on the Speculum in feveral Angles of Incidence, made Rings of feveral bigneffes, which diluted and blotted out one another, as I knew by in- tercepting fome part of 'that Light. For if I intercepted that part which was neareft to the Axis of the Speculum the Rings would 'be lefs, if the other part which was remoteit from it they would be bigger.

Obf. II. When the Colours of the Prifm were cait fucceflively on the Speculum, that Ring which in the two lalt Obfervations was white, was of the fame bignefs in all the Co- Jours, but the Rings without it were greater in the green than in the blue, and l^ill greater in the yellow, and greateft in the red. And, on the contrary, the Rings within that white Circle were lefs in the green than in the blue, and ftill

lefs

[ 287 ]

lefs in the yellow, and lead in the red. For the Angles of Reflexion of thofe Rays which made this Ring, being equal to their Angles of Incidence, the Fits of every refleded Ray within the Glafs after Reflexion are equal in length and number to the Fits of the fame Ray with- in the Glafs before its Incidence on the reflect- ing Surface. And therefore fmce all the Rays of all forts at their entrance into the Glafs were in a Fit of Tranfmiffion, they were alfo in a Fit of TranfmifTion at their returning to the fame Surface after Reflexion ; and by confequence were tranfmitted and went out to the white Ring on the Chart. This is the reafon why that Ring was of the fame bignefs in all the Co- lours, and why in a mixture of all it appears white. But in Rays which are reflefted in o- ther Angles , the Intervals of the Fits of the leail refrangible being greateit, make the Rings of their Colour in their progrefs from this white Ring, either outwards or inwards, increafe or decreafe by the greatefl Iteps; fo that the Rings of this Colour without are greatell, and within lead. And this is the reafon why in the lall Obfervation, when the. Speculum was illumina- ted with white Light, the exterior Rings made by all Colours appeared red without and blue within, and the interior blue without and red within.

Thefe are the Phaenomena of thick convexo- concave Plates of Glafs, which are every where of the fame thicknefs. There are yet other Phaenomena when thefe Plates are a little thick- er on one fide than on the other, and others

when

[ 288 ]

when . the Plates are more or lefs concave than convex, or plano-convex, or double- convex. For in all theie cafes the Plates make Rings of Colours, but after various manners ; all which, fo far as I have yet obferved, follow from the Propofitions in the end of the third part of this Book, and fo confpire to confirm the truth of thofe Propofitions. But the Phaenomena are too various, and the Calculations whereby they follow from thofe Propofitions too intricate to be here profecuted. I content my felf with ha- ving profecuted this kind of Phaenomena fo far as to difcover their Caufe , and by difcovering it to ratify the Propofitions in the third Part of this Book.

Obf. 13. As Light refle(^ed by a Lens quick- filver'd on the backfide makes the Rings of Co- lours above defcribed, fo it ought to make the like Rings of Colours in pafling through a drop of Water. At the fiiil Reflexion of the Rays within the drop, fome Colours ought to be tranfmitted, as in the cafe of a Lens, and others to be refleded back to the Eye. For inftancej if the Diameter of a fmall drop or globule of Water be about the 5'ooth part of an Inch, fo that a red-making Ray in pafTing through the •middle of this globule has 15-0 Fits of eafy Tranfmiffion within the globule, and that all the red- making Rays which are at a certain di^ fiance from this middle Ray round about it have 249 Fits within the globule, and all the Hke Rays at a certain farther diitance round a- bout it have 248 Fits, and all thofe at a cer- tain farther diftance 247 Fits^ and fo on ; thefef

conceii-

[ 28^ ]

concentrick Circles of Rays after their tranf- miflion, falling on a white Paper, will make concentrick Rings of red upon the Paper, fup- pofing the Light which palTes through one An- gle globule, ftrong enough to be fenfible. And, in like manner, the Rays of other Colours will make Rings of other Colours. Suppofe now that in a fair Day the Sun Ihines through a thin Cloud of fuch globules of Water or Hail, and that the globules are all of the fame bignefs; and the Sun feen through this Cloud ihall ap- pear encompalled with the Hke concentrick Rings of Colours, and the Diameter of the iiril Ring of red ihall be 7-^ Degrees, that of the fe- cond 10^ Degrees, that of the third iz Degrees 33 Minutes. And accordingly as the Globules of Water are bigger or lefs, the Rings lliall be lefs or bigger. This is the Theory, and Expe- rience anlwers it. For in Jm^e 1692. I faw by reflexion in a VeiTel of itagnating Water three Halos, Crowns, or Rings of Colours about the Sun, like three little Rain-bows, concentrick to his Body. The Colours of the firlt or in- nermort Crown were blue next the Sun, red without, and white in the middle between the blue and red. Thofe of the fecond Crown were purple and blue within, and pale red with- out, and green in the middle. And thoie of the third were pale blue within, and pale red without ; thefe Crowns enclos'd one another immediately j fo that their Colours proceeded in this continual order from the Sun outward : blue, white, red; purple, blue, green, pale

U yelldw

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yellow and red ; pale blue, pale red. The Di- ameter of the fecond Crown meafured from the middle of the yellow and red on one fide of the Sun, to the middle of the fame Colour on the other fide was 94- Degrees, or therea- bouts. The Diameters of the firft and third I had not time to meafure, but that of the firft feemed to be about five or fix Degrees, and that of the third about twelve. The like Crowns appear fometimes about the Moon: for in the beginning of the Year 1664, Feb, 19th at Night, I faw two fuch Crowns about her. The Diameter of the firft or innermolt was about three Degrees, and that of the fe- cond about five Degrees and an half. Next a- bout the Moon was a Circle of white, and next about that the inner Crown which was of a bluifli green within next the white, and of a yellow and red without, and next about thefe Colours were blue and green on the infide of the outward Crown, and red on the outfide of it. At the fame time there appear'd a Halo a- bout 21 Degrees" 35^ diftant from the center of the Moon. It was elliptical, and its long Dia- meter was perpendicular to the Horizon, verg- ing below fartheft from the Moon. I am told that the Moon has fometimes three or more eoncentrick Crowns of Colours encompaffing one another next about her Body. The more equal the globules of Water or Ice are to one another, the more Crowns of Colours will ap- pear, and the Colours will be the more lively. The Halo at the diftance of 114 Degrees from

the

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the Moon is of another fort. By its being oval and remoter from the Moon below than above, I conclude, that it was made by Refraftioq in fome fort of Hail or Snow floating in the y\ir in an horizontal polture, the refrading Angle being about 5-8 or 60 Degrees.

y *.

THE

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THE

THIRD BOOK

O F

PART I.

W

^ -^ "i^W^ ^ -^^ ^ -^^ ^:$' ^ ^ ^ ^ ^ «» ^ * ^ # *• "^^ ^

Ohfervations CGucemhig the Inflexions of the Rays of Lights and the Colours made thereby.

S^^^^R\IMALTiO has infbrm'd us, that K(p|:§i if a beam of the Sun's Light be let in- K^r^l ^^ ^ ^^i"k Room through a very fmall ^^^^! hole, the Shadows of thinss in this Light will be larger than they ought to be if the Rays went on by the Bodies in llrait Lines,

'and

[ 2^3 ]

and that thefe Shadows have three parallel Fringes, Bands or Ranks of colour'd Light ad- jacent to them. But if the Hole be enlarged the Fringes grow broad and run into one ano- ther, fo that they cannot be diftinguidi'd. Thefe broad Shadows and Fringes have been reckon'd by fome to proceed from the ordinary refra- ttion of the Air, but without due examination of the Matter. For the circumftances of the Phaenomenon, fo far as I have obferved them, are as follows.

Oi^f I. I made in a piece of Lead a fmall Hole with a Pin, whofe breadth was the 4zd part of an Inch. For xi of thofe Pins laid to- gether took up the breadth of half an Inch. Through this Hole I let into my darken'd Chamber a beam of the Sun's Light, and found that the Shadows of Hairs, Threds, Pins, Straws, and fuch like flender Subitances placed in this beam of Light, were conliderably broader than they ought to be, if the Rays of Light pafTed on by thefe Bodies in right Lines. And parti- cularly a Hair of a Man's Head, whofe breadth was but the xSoth part of an Inch, being held in this Light, at the diftance of about twelve Feet from the Hole, did call a Shadow which at the diftance of four Inches from the Hair was the fixtieth part of an Inch broad, that is, above four times broader than the Hair, and at the diftance of two Feet from the Hair was a- bout the eight and twentieth part of an Inch broad, that is, ten times broader than the Hair, and at the diftance of ten P'ect was the eighth part of an Inch broad, that is 3 ^r times broader.

U 3 Nor

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