Opticks (1721, 3rd Edition) — part 8 of 18

Degree, and fo the breadth of the interior Iris will be X Degr. 1$ Min. that of the exterior 3 begr. 40 Min. their diitance 8 Degr. 15- Min. the greateft Semi-diameter of the interior Bow 42 Degr. 17 Min. and the lealt of the exterior 50 Degr. 41 Min. And fuch are the Dimenfions of the Bows in the Heavens found to be very nearly, when their Colours appear ftrong and perfect. For once, by fuch means as I then had, I meafuredthe greateit Semi-diameter of the interior Iris about 41 Degrees, and the breadth of the red, yellow and green in that Iris 6} or 64 Minutes, befides the outmoft faint red ob- fcured by the brightnefs of the Clouds, for which we may allow 3 or 4 Minutes more. The breadth of the blue was about 40 Minutes more befides the violet, which was fo much obfcu- red by the brightnefs of the Clouds, that I could not meafure its breadth. But fuppofmg the breadth of the blue and violet together to equal that of the red, yellow and green together, the whole breadth of this Iris will be about z^ De- grees, as above. The leaft diitance between this Iris and the exterior Iris was about 8 De-. grees and 30 Minutes. The exterior Iris was broader than the interior, but fo faint, efpeci- ally on the blue fide, that I could not meafure its breadth dillindly. At another time when both Bows appeared more diiHn(51, I meafured the breadth of the interior Iris x Gr. 10, and the breadth of the red, yellow and green in the exterior Iris, was to the breadth of the fame ^ Colours in the interior as 3 to 2,.

This

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This Explication of the Rain-bow is yet far- ther confirmed by the known Experiment (made hyAntonius de 'Domt7iis 'iiwd IDes-Cartes ) of hanging up any where in the Sun fliine a Glafs Globe filled with Water, and viewing it in fuch- a pollure that the Rays which come from the Globe to the Eye may contain with the Sun's Rays an Angle of either 41 or 5*0 Degrees. For if the Angle be about 41 or 43 Degrees, the Spedator (fuppofe at O) fliall fee a full red Colour in that fide of the Globe oppofed to the Sun as 'tis reprefented at F, and if that An- gle become lefs (fuppofe by deprefling the Globe to E) there will appear other Colours, yellow, green and blue fucceffively in the fame fide of the Globe. But if the Angle be made about $0 Degrees (fuppofe by lifting up the Globe to G) there will appear a red Colour in that fide of the Globe towards the Sun, and if the An- gle be made greater (fuppofe by lifting up the Globe to H) the red will turn fucceiiively to the other Colours, yellow, green and blue. The fame thing I have tried by letting a Globe reft, and raifing or depreffiug the Eye, or o- therwife mo^ing it to make the Angle of a jail magnitude.

I have heard it reprefented, that if the Light of a Candle be refraded by a Prifm to the Eye ; when the blue Colour falls upon the Eye the Spectator iliall fee red in the Prifm, and when the red falls upon the Eye he fliall fee blue; and if this were certain, the Colours of the Globe and Rain-bow ought to appear in a con- trary order to what we find. But the Colours.

of

[ '55 ]

of the Candle being very faint, the miilake feems to arife from the difficulty of difcerning what Colours fall on the Eye. For, on the con- trary, I have fomtimes had occafion to ob- ferve in the Sun's Light refraded by a Prifm, that the Spectator always fees that Colour in the Prifm which falls upon his Eye. And the fame I have found true alfo in Candle-light. For when the Prifm is moved flowly from the Line which is drawn diredly from the Candle to the Eye, the red appears firft in the Prifm and then the blue, and therefore each of them is i^een when it falls upon the Eye. For the red palTes over the Eye firlt, and then the blue.

The Light which comes through drops of Rain by two Refradions without any Reflexion, ought to appear llrongefl at the diilance of a- bout 26. Degrees from the Sun, and to decay gradually both ways as the diilance from him increafes and decreafes. And the fame is to be underltood of Light tranfmitted through fpherical Hail-flones. And if the Hail be a lit« tie flatted, as it often is, the Light tranfmitted may grow fo ftrong at a little lefs diilance than that of 26 Degrees, as to form a Halo about the Sun or Moon ; which Flalo, as often as the Hail-itones are duly figured may be colour'd, and then it mult be red within by the leail re- frangible Rays , and blue without by the moft refrangible ones, efpecially if the Hnil-ilones have opake Globules of Snow in their center to intercept the Light within the Halo (as ////- gefji7is has obferv'd) and make the infide there- of more diltindly, defined than it would other- ' wife

C 15^ ]

wife be. For fuch Hail-llones, though fphe- rical, by terminating the Light by the Snow, may make a Halo red within and colourlefs without, and darker in the red than without, as Halos ufe to be. For of thofe Rays which pafs clofe by the Snow the Rubriform will be lead refraded, and fo come to the Eye in the diredelt Lines.

The Light which paiTes through a drop of Rain after two Refradions, and three or more Reflexions , is fcarce Itrong enough to caufe a fenfible Bow ; but in thofe Cylinders of Ice by which Hugenms explains the Tarhelia^ it may perhaps be fenfible.

T R O T.X. Prob. V.

By the d'tfcovered ^roferties of Light to ex- plain the permanent Colours of Natural BO' dies.

'THHESE Colours arife from hence, that jl fome natural Bodies refled fome forts of Jtays, others other forts more copioufly than the reft. Minium refle6ls the leall refrangible or red making Rays moft copioufly, and thence appears red. Violets refled: the moft refrangi- ble, moft copioufly, and thence have their Co- lour, and fo of other Bodies. Every Body re- fle(^ts the Rays of its own Colour more copi- oufly than the reft, and from their excefs and predominance in the refleded Light has its

Colour.

Ex^er^

[ 157 ]

Exfer. 17. For if in the homogeneal Lights obtained by the folution of the Problem pro- pofed in the fourth Proportion of the firft Part you place Bodies of feveral Colours, you will find, as I have done, that every Body looks moft fplendid and luminous in the Light of its own Colour. Cinnaber in the homogeneal red Light is moft refplendent, in the green Light it is manifeftly lefs refplendent, and in the blue Light ftill lefs. Indigo in the violet blue Light is moft refplendent, and its fplendor is gradu- ally diminilh'd as it is removed thence by de- grees through the green and yellow Light to the red. By a Leek the green Light, and next that the blue and yellow which compound green^ are more ftrongly reflefted than the other Co- lours red and violet, and fo of the reft. But to make thefe Experiments the more manifeit, fuch Bodies ought to be chofen as have the fuUeft and moft vivid Colours, and two of thofe Bodies are to be compared together. Thus, for in- ftance, if Cinnaber and ultra-mznne blue, or fome other full blue be held together in the red homogeneal Light, they will both appear red, but the Cinnaber will appear of a ftrongly lu- minous and refplendent red, and the ftUra-m^L" rine blue of a faint obfcure and dark red ; and if they be held together in the blue homogeneal Light they will both appear blue, but the tiUra- marine will appear of a ftrongly luminous and refplendent blue, and the Cinnaber of a faint and dark blue. Which puts it out of difpute, that the Cinnaber refledls the red Light much more copioufly than the ultra m^r'me doth, and

the

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the uUra-mmne reflefts the blue Light much more copioufly than the Cinnaber doth. The fame Experiment may be tried fuccefsfully with red Lead and Indigo, or with any other two colour'd Bodies, if due allowance be made for the different itrength or weaknefs of their Co- lour and Light.

And as the reafon of the Colours of natural Bodies is evident by thefe Experiments , fo it is farther confirmed and put palt difpute by the two firfl Experiments of the firlt Part, where- by 'twas proved in fuch Bodies that the reflect- ed Lights which differ in Colours do differ alfo in degrees of Refrangibility. , For thence it's certain, that fome Bodies refleft the more re- frangible, others the lefs refrangible Rays more copioufly.

And that this is not only a triie reafon of thefe Colours, but even the only reafon may ap- pear farther from this coniideration , that the Colour of homogeneal Light cannot be chan- ged by the Reflexion of natural Bodies.

For if Bodies by Reflexion cannot in the leaft change the Colour of any one fort of Rays, they cannot appear colour'd by any other means than by refleding thofe which either are of their own Colour, or which by mixture muft pro- duce it.

But in trying Experiments of this kind care muft be had that the Light be fufficiently ho- mogeneal. For if Bodies be illuminated by the ordinary prifmatick Colours, they will appear neither of their own Day-light Colours, nor of the Colour of the Light call: on them, but of

fome

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fome middle Colour between both, as I have found by Experience. Thus red Lead (for in- ftance) illuminated with the ordinary prifma- tick green will not appear either red or green, but orange or yellow, or between yellow and green, accordingly as the green Light by which 'tis illuminated is more or lefs compounded. For becaufe red Lead appears red when illu- minated with white Light, wherein all forts of Rays are equally mix'd, and in the green Light all forts of Rays are not equally mix'd, the Ex- cefs of the yellow-making, green-making and blue-making Rays in the incident green Light, will caufe thofe Rays to abound fo much in the retleded Light as to draw the Colour from red towards their Colour. And becaufe the red Lead reflefts the red-making Rays moft copioufly in proportion to their number, and next after them the orange-making and yellow- making Rays ; thefe Rays in the refleded Light will be more in proportion to the Light than they were in the incident green Light, and there- by will draw the retleded Light from green to- wards their Colour. And therefore the red Lead will appear neither red nor green, but of a Colour between both.

In tranfparently colour'd Liquors 'tis obfer- vable, that their Colour ufes to vary with their thicknefs. Thus, for inftance, a red Liquor in a conical Glafs held between the Light and the Eye , looks of a pale and dilute yellow at the bottom w^here 'tis thin, and a little higher where 'tis thicker grows orange, and where 'tis Hill thicker becomes red, and where 'tis thickeft

the

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the red is deepell and darkeft. For it is to be conceiv'd that fuch a Liquor flops the indigo- making and violet-making Rays molt eafily, the blue-making Rays more dii^cultly, the green- Jnaking Rays Hill more difficultly, and the red- making moil difficultly : And that if the thick- nefs of the Liquor be only fo much as fuffices to Hop a competent number of the violet-ma- king and indigo-making Rays, without dimi- nifliing much the number of the reft, the reft muft (by Trop. 6. Tart i.) compound a pale yellow. But if the Liquor be fo much thicker as to ftop alfo a great number of the blue-ma- king Rays^ and fome of the green-making, the reft muft compound an orange ; and where it is fo thick as to ftop alfo a great number of the green-making and a confiderable number of the yellow-making, the reft muft begin to com- pound a red, and this red muft grow deeper and darker as the yellow-making and orange- making Rays are more and more ftopp'd by in- creafmg the thicknefs of the Liquor, fo that i^"^ Rays befides the red- making can get .through.

Of this kind is an Experiment lately related to me by Mr. Halley^ who, in diving deep into the Sea in a diving Veflel, found in a clear Sun-fliine Day^ that when he was funk many Fathoms deep into the Water, the upper part of his Hand on which the Sun ilione diredly through the Water and through a fmallGlafs Window in the VeiTelj appeared of a red Colour like that of a Damask Rofe, and the Water below and the under part of his Hand illuminated by Light refleded from the

Water

Water below look'd green. For thence it may be gather'd, that the Sea Water retleCh back the violet and blue-making Rays molt eailly, and lets the red-making Rays pafs moll freely and copioufly to great depths. For thereby the Sun's dired: Light at all great depths, by reafon of the predominating red-making Rays, mud ap- pear red ; and the greater the depth is, the ful- ler and intenfer mult that red be. And at fuch depths as the violet-making Rays fcarce pene- trate unto, the blue-making, green-making and yellow- making Rays being refleded from be- low more copioully than the red-making ones, mull compound a green.

Now if there be two Liquors of full Colours, ftippofe a red iind a blue, and both of them fo thick as fuffices to make their Colours fuffici- ently full ; though either Liquor be fufficient- ly tranfparent apart, yet will you not be able to fee through both together. For if only the red-making Rays pafs through one Liquor, and only the blue-making through the other, no Rays can pais through both. This Mr. Hook tried eafually with Glafs Wedges filled with red and blue Liquors, and was furprized at the unex- pedted event, the reafon of it being then un- known ; which makes me trull the riiore to his Experiment, though I have not tried it my lelf. But he that would repeat it mull take care the Liquors be of very good and full Colours.

Now whilil Bodies become coloured by re Heal- ing or tranfmitting this or that fort of Rays more copioully than the refl, it is to be conceived that they ilop and flifle in themfelves the Rays

M which

[ 1^2 ]

which they do not refled or tranfmit. For if Gold be foliated and held between your Eye and the Light, the Light looks of a greenifli blue, and therefore mafly Gold lets into its Body the blue making Rays to be refleded to and fro within it till they be flopp'd and flifled, whilit it refleds the yellow-making outwards, and thereby looks yellow. And much after the fame manner that Leaf Gold is yellow by re- fledted, and blue by tranfmitted Light, and maf- fy Gold is yellow in all Pofitions of the Eye ; there are fome Liquors , as the Tinfture of Lignum Nephriticum^ and fome forts ofGlafs which tranfmit one fort of Light moll copiouf- ly, and refled another fort, and thereby look 6f feveral Colours, according to the pofition of the Eye to the Light. But if thefe Liquors or Glades were fo thick and mafly that no Light could get through them , I queltion not but they would like all other opake Bodies appear of one and the fame Colour in all Pofitions of the Eye, though this I cannot yet affirm by ex- perience. For all colour'd Bodies, fo far as my Obfervation reaches, may be feen through if made fuiiiciently thin, and therefore are in fome meafure tranfparent, and differ only in degrees of Tranfparency from tinged tranfparent Li- quors ; thefe Liquors, as well as thole Bodies, by a fuliicient thicknefs becoming opake. A tranfparent Body which looks of any Colour by tranfmitted Light, may alfo look of the fame ( -olour by refleded Light, the Light of that Colour being reflet^led by the farther Surface of the Body, or by the Air beyond it. And

then

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then the refle<fted Colour will be diminifhed, and perhaps ceafe, by making the Body very thick, and pitching it on the backfide to dimi- nifh the Reflexion of its farther Surface, fo that the Light retleded from the tinging Particles may predominate. In fuch cafes, the Colour of the reflected Light will be apt to vary from that of the Light tranfmitted. But whence it is that tinged Bodies and Liquors refleft fome fort of Rays , and intromit or tranfmit other forts, fliall be faid in the next Book. In this Propofition I content my felf to have put it pafl difpute, that Bodies have fuch Properties, and thence appear colour'd.

TROT. XL Prob. VL

By mixing colour'd Lights to compound a beam of Light of the fame Colour and Nature with a beam of the Sun's dire^ Light, and therein to experience the Truth of the foregoing Tro- ^ofitions,

LET ABC ^^f [in F/^. i^.] reprefeot a Prifm by which the Sun's Light let into a dark Chamber through the Hole F, may be refraded towards the Lens MN, and paint up- on it at /, q, r, s and ?, the uiual Colours viO' let, blue, green, yellow and red, and let the diverging Rays by the Refradtion of this Lens converge again towards X, and there, by the mixture of all thofe their Colours, compound a wliite according to what was fhewn above.

M ^ Thep

[ 1^4-]

Tlien let another Prifm D E G deg, parallel to the former, be placed at X, to refrad that white Light upwards towards Y. Let the re- frading Angles of the Prifms, and their diltances from the Lens be equal, fo that the Rays which converged from the Lens towards X, and with- out Refradion, would there have croiTed and diverged again, may by the Refraction of the fecond Prifm be reduced into Parallelifm and diverge no more. For then thofe Rays will re- compofe a beam of white Light X Y. If the refrafting Angle of either Prifm be the bigger, that Prifm muft be fo much the nearer to the Lens. You will know when the Prifms and the Lens are well fet together, by obferving if the beam of Light XY which comes out of the fe- cond Prifm be perfedly white to the very edges of the Light, and at alldillanccs from the Prifm continue perfectly and totally white like a beam of the Sun's Light. For till this happens, the pofition of the Prifms and Lens to one another muft be correded, and then if by the help of a long beam of Wood, as is repiefented in the Figure, or by a Tube, or fome other fuch In- frument made for that purpofe, they be made faft in that fituation, you may try all the fame Fxperiments in this compounded beam of Light X Y, which have been made in the Suns dired Light. For this compounded beam of Light has the fame appearance, and is endow'd with rjl the fame Properties with a direft beam of 4 jie Suns Light, fo far as my Obfervation reaches. And in trying Experiments in this beam you may by ftopping any of the Colours /, ^, r, s

and

[ 1^5]

and ty at the Lens, fee how the Colours pro- duced in the Experiments are no other than thofe which the Rays had at the Lens before they enter'd the compofition of this Beam : And by confequence that they arife not from any new modifications of the Light by Refradions and Reflexions, but from the various Separations and Mixtures of the Rays originally endow'd with their colour-making qualities.

So for inftance, having with a Lens 4^ Inches broad, and two Prifms on either hand 6\ Feet diftant from the Lens, made fuch a beam of compounded Light : to examin the reafon of the Colours made by Prifins, I refraded this compounded beam of Light X Y with another Prifm H I K >^ /:>, and thereby caft the ufual prif- matick Colours PQ R S T upon the Paper LV placed behind. And then by Hopping any of the Colours/, q, r, s, r, at the Lens, I found that the fame Colour would vanifh at the Pa- per. So if the purple / was llopp'd at the Lens, the purple P upon the Paper would va- ni(h, and the rell of the Colours would remain unalter'd, unlefs perhaps the blue, fo far as fome purple latent in it at the Lens might be feparated from it by the following Refradions. And fo by intercepting the green upon the Lens, the green R upon the Paper would vanifh, and fo of the relt ; which plainly fliews, that as the white beam of Light X Y was compounded of feveral Lights varioufly colour'd at the Lens, fo the Colours which afterwards emerge out of it by new Refradions are no other than thofe of which it§ whitenefs was compounded. The

M 3 Refra-

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Reflation of the Prifm HlKkh generates the Colours P Q R S T upon the Paper , not by changing the colorific qualities of the Rays, but by feparating the Rays which had the very fame colorific quaUties before they enter'd the Com- pofition of the refrafted beam of white Light X Y. For otherwife the Rays which were of one Colour at the Lens might be of another upon the Paper, contrary to what we find.

So again, to examin the reafon of the Co- lours of natural Bodies, I placed fuch Bodies in the Beam of Light X Y, and found that they all appeared there of thofe their own Colours which they have in Day light, and that thofe Colours depend upon theRays which had the fame Colours at the Lens before they enter'd the jGompofition of that beam. Thus, for in- fiance, Cinilaber illuminated by this beam ap- pears of the fame red Colour as in Day-light ; and if at the Lens you intercept the green-ma- king and blue-making Rays, its rednefs will be- come more full and lively : But if you there in- tercept the red-making Rays, it will not any longer appear red, but become yellow or green, or of fome other Colour, according to the forts of Rays which you do not intercept. So Gold in this Light X Y appears of the fame yellow Colour as in Day- light, but by intercepting at the Lens a due quantity of the yellow-making Rays it will appear white like Silver (as I have tried) which fhews that its yellownefs arifes from the Excefs of the intercepted Rays tinging that whitenefs with their Colour when they are let pafs. So the infufion of Lignum Nephritz-

cum

cum (as I have alb tried) when held in this beam of Light X Y, looks blue by the refleft' ed part of the Light, and red by the tranimit. ted part of it, as when 'tis view'd in Day- light, but if you intercept the blue at the Lens the infufion will lofe its refleded blue Colour, whillt its tranfmitted red remains perfeft and by the lofs of fome blue-making Rays wherewith it was allay'd becomes more intenfe and full. And, on the contrary, if the red and orange- making Rays be intercepted at the Lens, the Infufion will lofe its tranfmitted red, whilft its blue will re- main and become more full and perfed. Which fhews, that the Infufion does not tinge the Rays with blue and red, but only tranfmit thofe moft copioufly which were red-making before, and reflects thofe moft copioully which were blue- making before. And after the fame man- ner may the Reafons of other Phaenomena be examined, by trying them in this artificial beam of Light XY.

M

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T H E

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O F

o

CKS.

PART I. ^

Obfervatioiis 'concerning the Reflexions^ Refror Bionsy and Colours of thin tranjparent Bo- dies,

IT has been obferved by others, that p traiifparent Subftances, as Glafs, Wa- I ter, Air, ^c. when made very thin by ^"- being blown into Bubbles., or other wife formed into Plates, do exhibit various Colours according to their various thinnefs, although at

a greater-

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a greater thicknefs they appear very clear and colourlefs. In the former Book I forbore to treat of thefe Colours, becaufe they feemed of a more difficult Confideration, and were not neceflaryfor eltabliihing the Properties of Light there difcourfed of But becaufe they may con- duce to farther Difcoveries for compleating the Theory of Light, efpecially as to the conftitu- tion of the parts of natural Bodies, on which their Colours or Tranfparency depend ; I have here fet down an account of them. To render this Difcourfe fhort and diltinft, I have firfl de- fcribed the principal of my Obfervations , and then confider'd and made ufe of them. The Obfervations are thefe.

Obf.x. Comprefling two Prifms hard toge- ther that their fides (which by chance were a very little convex) might fomewhere touch one another : I found the place in which they touch- ed to become abfolutely tranfparent, as if they had there been one continued piece of Glafs. For when the Light fell fo obliquely on the Air, which in other places was between them, as to be all reflefted ; it feemed in that place of contad: to be wholly tranfmitted, infomuch that when look'd upon, it appeared like a black or dark fpot, by reafon that Uttle or no fenfible Light was refleded from thence, as from other places ; and when looked through it feemed (as it were) a hole in that Air which was formed into a thin Plate, by being comprefs'd between the GlalTes. And through this hole Objeds that were beyond might be feen dittiiidly, which could not at all be feen through other parrs of

th.j

[ '70 ] . ^

the GlafTes where the Air was interjacent. AW , though the Glaffes were a little convex, yet this- ' tranfparent fpot was of a confiderable breadth^ " which breadth feemed principally to proceed from the yielding inwards of the parts of the GlafTes, by reafon of their mutual prelTure. For they prefling them very hard together it wouM^ - become much broader than otherwife.

0^/2. When the Plate of Air, by turning the Prifms about their common Axis, became fo little inclined to the incident Rays, that fome of them began to be tranfmitted , there arofe in it many (lender Arcs of Colours which at firfl were fhaped almoft like the Conchoid , as you fee them delineated in the firit Figure. And by continuing the Motion of the Prifms, thefe Arcs increafed and bended more and more a- bout the faid tranfparent fpot , till they were completed into Circles or Rings incompaffing it, and afterwards continually grew more aM more con traded. ' .•^.

Thefe Arcs at their firlt appearance were of ' a violet and blue Colour, and between therri were white Arcs of Circles, which prefentt^ by continuing the Motion of the Prifms becanfe 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 thefe Colours from the central dark fpot, was at that time white, blue, violet ; black, red, orange, yellow, white, blue, violet, ^c. But the yel- low and red were much fainter than the blue and violet.

' ■ ■ The

[ '71 ]

The Motion of the Prifms about their Axis being continued, thefe Colours contraded more and more, Shrinking towards the whitenefs on either fide of it, until they totally vaniflied in- to it. And then the Circles in thofe parts ap- pear'd black and white, without any other Co^ lours intermix'd. But by farther moving the Prifms about, the Colours again emerged out of the whitenefs, the violet and blue at its inward Limb, and at its outward Limb the red and yellow. So that now their order from the cen- tral Spot was white, yellow, red, black, vio- let, blue, white, yellow, red, &c. contrary to what it was before.

O^f 3. When the Rings or fome parts of them appeared only black and white, they were very dillindt and well defined, and the black-^ nefs feemed as intenfe as that of the central Spot. Alfo in the Borders of the Rings, where the Colours began to emerge out of the white- nefs, they were pretty diiiinft, which made them vifible to a very great multitude. I have fometimes number'd above thirty Succeflions (reckoning every black and white Ring for one Succeflion) and feen more of them, which by reafon of their fmalnefs I could not number. But in other Pofitions of the Prifms, at which the Rings appeared of m^any Colours, I could not diftinguilh above eight or nine of them, and the Exterior of thofe were very confufed and dilute.

In thefe two Obfervations to fee the Rings di- flinft, and without any other Colour than black and white, I found it necelfary to hold my Eye

at

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^t a good diftance from them. For by ap- proaching nearer, although in the fame inclina- tion of my Eye to the Plane of the Rings, there emerged a blueifti Colour out of the white, which by dilating it felf more and more into the black, render'd the Circles lefs diilind:, and left the white a little tinged with red and yel- low. I found alfo by looking through a flit or oblong bole, which was narrower than the Pu- pil of my Eye, and held clofe to it parallel to the Prifms, 1 could fee the Circles much di- itinfter and vifible to a far greater number than otherwife,

Obf. 4. To obferve more nicely the order of the Colours which arofe out of the white Circles as the Rays became lefs and lefs incli- ned to the Plate of Air ; I took two Objeft- glafles, the one a Plano-convex for a fourteen Foot Telefcope, and the other a large double Convex for one of about fifty Foot ; and upon this, laying the other with its plane fide down- wards, I prelTed them (lowly together, to make the Colours fucceffively emerge in the middle of the Circles, and then flowly lifted the upper Glafs from the lower to make them fucceflive- ly vanifli again in the fame place. The Colour, w^hich by prefling the GlalTes together emerged laft in the middle of the other Colours, would upon its firit appearance look like a Circle of a Colour almoft uniform from the circumference to the center, and by comprefling the Glalles Hill more, grow continually broader until a new Co- lour emerged in its center, and thereby it became a Ring encoinpalTuig that new Colour. And by

com-

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comprefling the GlafTes ftill more, the diameter' of this Ring would increafe, and the breadth of its Orbit or Perimeter decreafe until another new Colour emerged in the center of the laft: And fo on until a third; a fourth, a fifth,. and other following new Colours fucceffively emer- ged there, and became Rings encompailing the innermoft Colour , the laft of which was the black Spot. And, on the contrary, by lifting up the upper Glafs from the lower, the diame- ter of the Rings would decreafe, and the breadth of their Orbit increafe, until their Colours reach- ed fucceffively to the center; and then they being of a confiderable breadth, I could more eafily difcern and diftinguifli their Species than before. And by this means I obferv'd their Suc- ceffion and Quantity to be as followeth.

Next to the pellucid central Spot made by the contaft of tne Glafles fucceeded blue, white, yellow, and red. The blue was fo little in quan- tity that I could not difcern it in the Circles made by the Prifms, nor could I well diftinguifli any violet in it, but the yellow and red were pretty copious, and feemed 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 encompaffing thefe were violet, blue, green, yellow, and red : and thefe were all of them copious and vivid, excepting the green, which was very little in quantity, and feemed much more faint and dilute than the other Colours. Of the other four, the vio- let was the leaft in extent, and the blue lefs than the yellow or red. The third Circuit or

Order

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^rder was purple, blue, green, yellow, and red ; in which the purple feemed more reddifli than the violet in the former Circuit, and the green was much more confpicuousj 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. Af- ter this fucceeded the fourth Circuit of green and red. The green was very copious and live-^ ly, inclining on the one fide to blue, and on the other fide to yellow. But in this fourth Circuit there was neither violet, blue, nor yel- low, and the red was very imperfeft and dir- ty. Alfo the fucceeding Colours became more and more imperfed and dilute, till after three or four revolutions they ended in perfeft whitenefs. Their form, when the Glafles were molt comprefs'd fo as to make the black Spot appear in the center , is delineated in the fe- . cond Figure ; where a^ b, c, d^ e: f^ g, h, /, k : /, my fty Oy f : q, r : s, t : v, x : j, z de- note the Colours reckon'd in order from the center , black, blue , white , yellow, red : vio- let, blue, green, yellow, red: purple, blue, green, yellow, red: green, red: greeniih blue, red: greeniih blue, pale red: greenifli blue, reddiHi white.

Obf. 5". To determine the interval of the Glafles, or thicknefs of the interjacent Air, by which each Colour was produced, I meafured the Diameters of the firit fix Rings at the moft lucid part of their Orbits, and fquaring them, I found their Squares to be in the arithmetical ProgrefTion of the odd Numbers, i, 3, 5-, 7, 9, 11.

And

C 175 ]

And fmce one of thefe GlafTes was plane, and the other fpherical, their Intervals at thofe Rings mull be in the fame Progreflion. I meafured alfo the Diameters of the dark or faint Rings between the more lucid Colours , and found their Squares to be in the arithmetical Progref- lion of the even Numbers, x, 4, 6, 8, lo, iz. And it being very niCe and difficult to take thefe meafures exadly ; I repeated them divers times at divers parts of the GlaiTes, that by their Agreement I might be confirmed in them. And the fame method I ufed in determining fome others of the following Obfervations.

Olff. 6. The Diameter of the fixth Ring at

the molt lucid part of its Orbit was ^ parts of

an Inch, and the Diameter of the Sphere on which the double convex Objeft-glafs was ground was about lox Feet, and hence I ga* thered the thicknefs of the Air or Aereal Inter- val of the GlafTes at that Ring. But fome time after, fufpefting that in making this Obfervation I had not determined the Diameter of the Sphere with fufficient accuratenefs, and being uncertain whether the Piano convex Glafs was truly plane, and not fomething concave or con- vex on that lide which I accounted plane ; and whether I had not prelTed the GlaiTes together, as I often did, to make them touch ; (For by prelling fuch GlaiTes together their parts eafily vield inwards , and the Rmgs thereby become ienfibly broader than they would be, did the GlaiTes keep their Figures.) I repeated the Experiment , and found the Diameter of the

fixth

t n^ ]

fixth lucid Ring about ^ parts of an Inch. I