Opticks (1721, 3rd Edition) — part 6 of 18

FOR determining this Problem,! made the following Experiment. Exper. 7. When I had caufed the redili- near fides A F, G M, [in Fig. 4.] of the Spe- drum of Colours made by the Prifm to be di- ftindly defined , as in the fifth Experiment of the firft Part is defcribed, there were found in it all the homogeneal Colours in the fame or- der and fituation one among another as in the Spe6lrum of fimple Light, defcribed in the fourth Propofition of that Part. For the Cir- cles of which the Spedrum of compound Light

PT

[ no ]

PTis compofed, and which in the middle parts of the Speftrum interfere and are intermixed with one another, are not intermixed in their GUtmolt parts where they touch thofe redili- near fides A F and G M. And therefore in thofe re6^ilinear fides when dillindly defined, there is no new Colour generated by Refra- dion. I obferved alfo , that if any where be- tween the two outmod Circles T M F and P G A a right Line, as y ^, was crofs to the Spedrum, fo as at both ends to fall perpendi- cularly upon its redlilinear fides, there appear- ed one and the fame Colour and degree of Co- lour from one end of this Line to the other. I delineated therefore in a Paper the perimeter of the Spedrum F A P G M T, and in trying the third Experiment of the firlt Book, I held the Paper fo that the Spedrum might fall upon this delineated Figure, and agree wdth it exact- ly, whillt an Aiiiltant whofe Eyes for diilinguifh- ing Colours were more critical than mine, did by right Lines a, 13, y^^e t, &c. drawn crofs the Spec^trum, note the Confines of the Colours,* that is of the red M ci/3 F, of the orange cty ^(3, of the yellow y s^^, of the green iv^ ^y of the blue }] iic9, of the indigo * a /* z, and of the vio- let A G A=/tA. And this Operation being divers times repeated both in the fame and in feveral Papers, I found that the Oblervations agreed well ehough with one another, and that the redilinear fides M G and F A were by the faid crofs Lines divided after the manner of a mu- fical Chord. Let G M be produced to X, that M X may be equal to G M, and conceive

GX,

C HI ]

GX, aX, ;X, ^X, eX, yX, ^X, MX, to be in proportion to one another, as the numbers, I, V, 4, 4, T> T. tI, 4, and fo to reprefent the Chords of the Key, and of a Tone, a third Mi- nor, a fourth, a fifth, a fixth Major , a feventh and an eighth above that Key: And the inter- vals Met, ay, ye, sf}-, >? *. ^ ^y and A G, will be the Species which the feveral Colours (red, o- range, yellow, green, blue, indigo, violet) take up.

Now thefe Intervals or Spaces fubtending the differences of the Refradions of the Rays go- ing to the Umits of thofe Colours, that is, to the Points M, ct, y, £, tj, ;, a, G, may without any fenfible Error be accounted proportional to the differences of the Sines of Refradion of thofe Rays having one common Sine of Inci- dence, and therefore lince the common Sine of Incidence of the moil and lead refrangible Rays out of Glafs into Air was (by a method df^fcri- bed above) found in proportion to their Sines of Refradion, as 50 to yy and 78, divide the dif- •ference between the Sines of RefracStion 77 and 78, as the Line G M is divided by thofe Inter- vals, and you will have 77, 774, 77t, 77i^ n-\y 77t» 774, 7^? the Sines of Refradion of thofe Rays out of Glafs into Air, their common Sine of Incidence being 5-0. So then the Sines of the Incidences of all the red- making Rays out of Glafs into Air, were to the Sines of their Re- fradions, not greater than- 5-0 to 77^ nor lefs than 50 to 77^^ but they varied from one ano- ther according to all intermediate proportions. And the Sines of the Incidences of the green-

making

[ ii^ 3

Jiiaking Rays were, to the Sines of their Refra° flions in all proportions from that of 50 to 77-f, unto that of 50 to 'jj^. And by the like limits abovementioned were the Refradions of the Rays belonging to the reit of the Colours de- fined, the Sines of the red-making Rays extend- ing from "jj to 'j'j^'i thofe of the orange- making from ^-j^ to yj^^ thofe of the yellow-making from -j-j^ to 'jj^^ , thofe of the green making from y-j^ to yy^^ thofe of the blue making from 774 to 77t, thofe of the indigo-making from 774 to yyi , and thofe of the violet from yy~ to 78.

Thefe are the Laws of the Refradiohs made out of Glafs into Air, and thence by the third Axiom of the firft part of this Book, the Laws of the Refradions made out of Air into Glafs are eafily derived.

Expr. 8. I found moreover that when Light goes out of Air through feVeral contiguous re- frading Mediums as through Water and Glafs, and thence goes out again into Air, whether the refracting Superficies be parallel or inclin'd to one another, that Light as often as by con- trary Refradions 'tis fo corrected, that it emer- geth in Lines parallel to thofe in which it was incident, continues ever after to be white. But if the emergent Rays be inclined to the inci- dent, the whitenefs of the emerging Light will by degrees in palling on from the place of E- mergence, become • tinged in its edges with Colours. This I tryed by refrading Light with Prifms of Glafs placed within a prifmatick Vef- fel of Water. Now thofe Colours argue a di- verging

r "3 ]

verging and feparation of the heterogeneous Rays from one another by means of their une- qual Refradions, as in what follows will more fully appear. And on the contrary, the per- manent whitenefs argues, that in like Inciden- ces of the Rays there is no fuch feparation of the emerging Rays, and by confequence no in- equality of their whole Refraftions. Whence I feem to gather the two following Theorems.

I. The ExcelTes of the Sines of Refraftion of feveral forts of Rays above their common Sine of Incidence when the Refractions are made out of divers denier Mediums immedi- ately into one and the fame rarer Medium, fup- pofe of Air, are to one another in a given Pro- portion.

^. The Proportion of the Sine of Incidence to the Sine of Refradion of one and the fame fort of Rays out of one Medium into another, is compofed of the Proportion of the Sine of Incidence to the Sine of Refradion out of the firlt Medium into any third Medium, and of the .Proportion of the Sine of Incidence to the Sine of Refradion out of that third Medium into the fecond Medium.

By the firft Theorem the Refractions of the Rays of every fort made out of any Medium in- to Air are known by having the Refraction of the Rays of any one fort. As for inftance, if the Refractions of the Rays of every fort out of Rain-water into Air be defired, let the com- mon Sine of Incidence out of Glafs into Air be

I fubduCted

4

C "4 ]

fubdufled from the Sines of Refradlion, and the ExcelTes will' be x/, 274, x74, 2,7-7, 274, . 2-77* '^lif 2-8. Suppofe now that the Sine of Incidence of the leail refrangible Rays be to their Sine of Refradion out of Rain-water in- to Air as 3 to 4, and fay as i the difference of thofe Sines is to 3 the Sine of Incidence, fo is 17 the leaft of the ExcefTes above-mentioned to a fourth number 81 ; and 81 will be the common. Sine of Incidence out of Rainwater into Air, to which Sine if you add all the a- bovementioned ExcefTes you will have the de- fired Sines of the Refradions 108, 1084, 1084, 1084, 1084, 1084, 1084, 109.

By the latter Theorem the Refradion out of one Medium into another is gathered as often as you have the Refradions out of them both into any third Medium. As if the Sine of In- cidence of any Ray out of Glafs into Air be to its Sine of Rerradion as 1.0 to 31, and the Sine of Incidence of the fame Ray out of Air into Water, be to its Sine of Refradion as 4 to 3 ; the Sine of Incidence of that Ray out of Glafs into Water will be to its Sine of Refradion as io to 31 and 4 to 3 jointly, that is, as the Fa- ftum of 20 and 4 to the Fadrum of 31 and 3, or as 80 to 93.

And thefe Theorems being admitted into Op- ticks, there would be fcope enough of hand- ling that Science voluminoufly after a new man- ner; not only by teaching thofe things which tend ro the perfedion of Vifion, but alfo by determining mathematically all kinds of Phae- nomena of Colours which could be produced Jl

by I

[ 115 ]

by Refra61ions. For to do this, there is tio» thing elfe requifite than to find out the Separa- tions of heterogeneous Rays, and their various Mixtures and Proportions in every Mixture. By this way of arguing I invented almoft all the Phsenomena defcribed in thefe Books, befide fome others lefs necefTary to the Argument; and by the fuccelTes I met with in the Trials, I dare promife, that to him who fhall argue tru- ly, and then try all things with good Glafles and fuflRcient Circumfpcdion, the expefted E- vent will not be wanting. But he is firlt to know what Colours will arife from any others mix'd in any ailigned Proportion.

TROT. IV. Theor.III.

Colours may be produced by Compojition 'which jhall be like to the Colours of homogeneal Light as to the Appearance of Colour^ but not as to the Immutability of Colour and Conftitution of Light. And thofe Colours by how much they are more compounded by fo much are they lefs fill and intenfe^ and by too much Compo- Jition they may be diluted and weaken d till they ceafe^ and the Mixture becomes white or grey. There may be alfo Colours produced by Compojition, which are not fully like any of the Colours of homogeneal Light,

FOR a Mixture of homogeneal red and yel- low compounds an orange, like in appear- ance of Colour to that orange which in the

I ^. feries

[ iiO

feries of unmixed prifmatick Colours lies be- tween them ; but the Light of one orange is homogeneal as to Refrangibility , that of the other is heterogeneal , and the Colour of the one, if viewed through a Prifm, remains un- changed , that of the other is changed and re- folved into its component Colours red and yel- low. And after the fame manner other neigh- bouring homogeneal Colours may compound new Colours, like the intermediate homoge- neal ones, as yellow and green, the Colour be- tween them both, and afterwards, if blue be ad- ded, there will be made a green the middle Co- lour of the three which enter the Compofition : For the yellow and blue on either hand, if they are equal in quantity they draw the intermedi- ate green equally towards themfelves in Com- pofition, and fo keep it as it were in i5iquilibrio, that it verge not more to the yellow on the one hand, than to the blue on the other, but by their mix'd Adions remain ftill a middle Colour. To this mix'd green there may be farther ad- ded fome red and violet, and yet the green will not prefently ceafe but only grow lefs full and vivid , and by increafing the red and vio- let it will grow more and more dilute, until by the prevalence of the added Colours it be over- come and turned into whitenefs, or fome other Colour. So if to the Colour of any homoge- neal Light, the Sun's white Light compofed of all forts of Rays be added, that Colour will not vanidi, or change its Species but be diluted, and by adding more and more white it will be diluted more and more perpetually. Laltly, if

red

[ 117 ]

red and violet be mingled, there will be gene- rated according to their various Proportions various Purples, fach as are not like in appear- ance to the Colour of any homogeneal Light, and of thefe Purples mix'd with yellow and blue may be made other new Colours.

• "PRGT.V. Theor. IV.

IFh'ttenefs and all grey Colours between white and blacky ?nay be compounded of Colours^ and the whitenefs of the Sun's Light is compound- ed of all the primary Colours mix'd in a due proportion.

The Proof by Experiments.

Exper.^. 'I ^ H E Sun lliining into a dark \ Chamber through a Uttle round hole in the Window-lhut, and his Light being there refraded by a Prifm to call his coloured Image PT [in Fig. 5.] upon the oppofite Wall : I held a white Paper V to that Image in fuch man- ner that it might be illuminated by the colour'd Light refledled from thence, and yet not inter- cept any part of that Light in its palTage from the Prifm to the Speflrum. And I found that when the Paper was held nearer to any Colour than to the reft, it appeared of that Colour to which it approached neareft ; but when it was equally or almoft equally diftant from all the Colours, fo that it might be equally illumina- ted by them all it appeared white. And in this laft fituation of the Paper, if fome Colours wer^

I 3 inte^-

[ ii8 ]

intercepted, the Paper loft its white Colour, and appeared of the Colour of the rell of the Light which was not intercepted. So then the Pa- per was illuminated with Lights of various Co- lours , namely , red , yellow , green , blue and violet, and every part of the Light retained its proper Colour, until it was incident on the Pa- per, and became reflefted thence to the Eye; lb that if it had been either alone (the reft of the Light being intercepted) or if it had a- hounded moft and been predominant in the Light reflefted from the Paper, it would have tinged the Paper with its own Colour ; and yet being mixed with the reft of the Colours in a due proportion, it made the Paper look white, and therefore by a Compofition with the reft produced that Colour. The feveral parts of the coloured Light refleded from the Spedrum, whilft they ar^ propagated from thence through the Air, do perpetually retain their proper Co- lours, becauie wherever they fall upon the Eyes of an)rSpedator, they make the feveral parts of the SpeArum to appear under their proper Co- lours. They retain therefore their proper Co- lours when they fall upon the Paper V, and fo by the confufion and perfeft mixture of thofe Colours compound the whitenefs of the Light reflefted from thence.

Exper. TO. Let that Speftrum or folar Image P T. [in Fig. 6.] fall now upon the Lens M N above four Inches broad, and about fix Feet di- ftant from the Prifm ABC, and fo figured that it may caufe the coloured Light which diverg- eth from the Prifm to converge and meet again

at

[ II? 3

at its Focus G, about fix or eight Feet diftant from the Lens, and there to fall perpendicular- ly upon a white Paper D E. And if you move this Paper to and fro, you will perceive that near the Lens, as at d e, the whole folar Image (fuppofe at / t J will appear upon it intenfely coloured after the manner above-explained, and that by receding from the Lens thofe Colours will perpetually come towards one another, and by mixing more and more dilute one another continually, until at length the Paper come to the Focus G, where by a perfeft mixture they will wholly vanifli and be converted into white- nefs, the whole Light appearing now upon the. Paper like a little white Circle. And after- wards by receding farther from the Lens, the Rays which before converged will now crofs one another in the Focus G, and diverge from thence, and thereby make the Colours to ap- pear again, but yet in a contrary order ; fup- pofe at ^i^ where the red t is now above which before was below, and the violet / is below which before was above.

Let us now Hop the Paper at the Focus G where the Light appears totally white and cir- cular, and let us confider its whitenefs. I fay, that this is compofed of the converging Colours. For if any of thofe Colours be intercepted at the Lens, the whitenefs will ceafe and degene- rate into that Colour which arifeth from the compofition of the other Colours which are not intercepted. And then if the intercepted Co- lours be let pafs and fall upon that compound Colour, they mix with it, and by their mixture

I ^ reftpre

[ I20 ]

redore the vvhitenefs. So if the violet, blue and green be intercepted, the remaining yel- low, orange and red will compound upon the Paper an orange, and then if the intercepted Colours be let pafs they will fall upon this com- pounded orange, and together with it decom- pound a white. So alfo if the red and violet be intercepted, the remaining yellow, green and blue, will compound a green upon the Pa- per, and then the red and violet being let pafs will fall upon this green, and together with it decompound a white. And that in this Com- pofition of white the feveral Rays do not fufFer any Change in their colorific qualities by ading upon one another, but are only mixed, and by a mixture of their Colours produce white, may farther appear by thefe Arguments.

If the Paper be placed beyond the Focus G, fuppofe at ^£y and then the red Colour at the Lens be alternately intercepted, and let pafs a- gain, the violet Colour on the Paper will not fuffer any Change thereby, as it ought to do if the feveral forts of Rays afted upon one ano- ther in the Focus G, where they crofs Nei- ther will the red upon the Paper be changed by any alternate {lopping, and letting pafs the violet which crofleth it.

And if the Paper be placed at the Focus G, and the white round Image at G be viewed through the Prifm HIK, and by the Refradion of that Prifm be tranflated to the place r v, and there appear tinged with various Colours, name- ly, the violet at v and red at r, and others be- tween, and then the red Colour at the Lens

be

[ 121 ]

be often ftopp'd and let pafs by turns, the red at r will accordingly disappear and return as often, but the violet at v will not thereby fuf- fer any Change. And fo by Hopping and letting pafs alternately the blue at the Lens, the blue at r will accordingly difappear and return, with- out any change made in the red at r. The red therefore depends on one fort of Rays, and the blue on another fort, which in the Focus G where they are commix'd do not a6l on one another. And there is the fame reafon of the other Colours.

I confidered farther, that when the moil re- frangible Rays P/, and the lead refrangible ones T^, are by converging inclined to one another, the Paper, if held very oblique to thofe Rays in the Focus G, might refled one fort of them more copioully than the other fort, and by that means the refleded Light would be tinged in that Focus with the Colour of the pre- dominant Rays, provided thofe Rays feverally retained their Colours or colorific Qualities in the Compofition of white made by them in that Focus. But if they did not retain them in that w^hite, but became all of them feverally endued there with a difpoficion to itrike the Senfe with the perception of white, then they could never lofe their whitenefs by fuch Reflexions. I in- clined therefore the Paper to the Rays very ob- liquely, as in the fecond Experiment of this Book, that the molt refrangible Rays might be more copioufly refleded than the reft, and the whitenefs at length changed fucceffively into blue, indigo and violet. Then I inclined it

the

[ 122 ]

the contrary way, that the lead refrangible Rays might be more copious in • the refleded Light than the refi:, and the whitenefs turned fuc- ceffively to yellow, orange and red.

Laftly, I made an Inftrument X Y in fafhion of a Comb, whofe Teeth being in number fix- teen were about an Inch and an half broad, and the Intervals of the Teeth about two Inches wide. Then by interpofmg fucceflively the Teeth of this Initrument near the Lens, I in- tercepted part of the Colours by the interpofed Tooth, whilft the reft of them went on through the interval of the Teeth to the Paper DE, and there painted a round folar Image. But the Paper I had tirft placed fo, that the Image might appear white as often as the Comb was taken away ; and then the Comb being as was faid in- terpofed, that whitenefs by realbn of the inter- cepted part of the Colours at the Lens did al- ways change into the Colour compounded of thofe Colours which were not intercepted, and that Colour was by the motion of the Comb perpetually varied fo, that in the paffing of every Tooth over the Lens all thefe Colours, red, yellow, green, blue and purple, did always fuc- ceed one another. I caufed therefore all the Teeth to pafs fucceflively over the Lens, and when the Motion was flow, there appeared a perpetual fucceflion of the Colours upon the Paper : But if I fo much accelerated the Mo- tion, that the Colours by reafon of their quick fucceflion could not be diftinguifhed from one another , the appearance of the fingle Colours ceafed. There was no red, no yellow, no

green.

[ 123 ]

green, no blue, nor purple to be feen any lon- ger, but from a confufion of them all there a- rofe one uniform white Colour. Of the Light which now by the mixture of all the Colours ap- peared white, there was no part really white. One part was red, another yellow, a third green, a fourth blue, a fifth purple, and every part re- tains its proper Colour till it llrike the Senfori- um. If the Impreilions follow one another flowly, fo that they may be feverally perceived, there is nlade a dilHnft Senfation of all the Co- lours one after another in a continual fuccef- fion. But if the Impreflions follow one ano- ther fo quickly that they cannot be feverally perceived, there arifeth out of them all one common Senfation, which is neither of this Colour alone nor of that alone, but hath it felf indifferently to 'em all, and this is a Senfation of whitenefs. By the quicknefs of the Succef- fions the Impreffions of the feveral Colours are confounded in the Senforium, and out of that confufion arifeth a mix'd Senfation. If a burn- ing Coal be nimbly moved round in a Circle with Gyrations continually repeated, the whole Circle will appear like Fire ; the reafon of which is, that the Senfation of the Coal in the feveral places of that Circle remains imprefs'd on the Senforium, until the Coal return again to the fame place. And fo in a quick confecu- tion of the Colours the ImprefTion of every Co- lour remains in the Senforium, until a revolu- tion of all the Colours be compleated, and that firft Colour return again. The Impreffions there- fore of all the fucceffive Colours are at once in

the

C '24 ]

the Senforium, and jointly ftir up a Senfation of them all ; and fo it is manifelt by this Expe- riment, that the commix'd ImprefKons of all the Colours do flir up and beget a Senfation of white, that is, that whitenefs is compounded of all the Colours.

And if the Comb be now taken away, that aU the Colours may at once pafs from the Lens to the Paper, and be there intermixed, and to- gether reflected thence to the Spedators Eyes ; their Impreflions on the Senforium being now more fubtilly and perfectly commixed there, ought much more to Itir up a Senfation of whitenefs.

You may inflead of the Lens ufe two Prifms HIK and LMN, which by refrafting the co- loured Light the contrary way to that of the firft Refradion, may make the diverging Rays converge and meet again in G; as you lee re- prefented in the feventb Figure. For where they meet and mix they will cpmpofe a white Light, as when a Lens is ufed.

Exper. II. Let the Sun's coloured Image PT [ii)/%.8.] fall upon the Wall of a dark Cham- ber, as in the third Experiment of the firft Book, and let the fame be viewed through a Prifm abc^ held parallel to the Prifm ABC, by whofe Refradion that Image was made, and let it now appear lower than before, fuppofe in the place S over againft the red Colour T. And if you go near to the Image P T, the Speftrum S will appear oblong and coloured Hke the Image PT; but if you recede from it, the Colours of the Spedrum S will be contraded more and more,

and

[ 125 ]

and at length vanifh, that Speflrum S becoming perfedly round and white ; and if you recede yet farther, the Colours will emerge again, but in a contrary order. Now that Spedrum S ap- pears white in that cafe when the Rays of fe- veral forts which converge from the feveral parts of the Image P T, to the Prifm abc^ are fo refrafted unequally by it, that in their paf- fage from the Prifm to the Eye, they may di- verge from one and the fame point of the Spe- drum S, and fo fall afterwards upon one and the fame point in the bottom of the Eye, and there be mingled,

And farther, if the Comb be here made ufe of, by whofe Teeth the Colours at the Image P T may be fucceflively intercepted ; the Spe- drum S when the Comb is moved flowly will be perpetually tinged with fucceflive Colours : But when by accelerating the motion of the Comb, the fucceflion of the Colours is fo quick that they cannot be feverally feen, that Spe- drum S, by a confufed and mix'd Senfation of them all, will appear white.

Exper. -L. The Sun fhining through a large Prifm ABC [in Fig. 9] upon a Comb XY, placed immediately behind the Prifm, his Light which palTed through the Interllices of the Teeth fell upon a white Paper DE. The breadths of the Teeth were equal to their In- terftices, and feven Teeth together with their Interftices took up an Inch in breadth. Now when the Paper was about two or three Inches diftant from the Comb, the Light which paf- fed through its feveral Interltices painted fo

many

[ »2^ ]

many ranges of Colours, kl^ m ;/, of^ q r, &c. which were parallel to one another and conti- guous, and without any mixture of white. And thefe ranges of Colours, if the Comb was mo- ved continually up and down with a receiprocal motion, afcended and defcended in the Paper, and when the motion of the Comb was fo quick, that the Colours could not be dillinguiflied from one another, the whole Paper by their con- fufion and mixture in the Senforium appeared white.

Let the Comb now reft, and let the Paper be removed farther from the Prifm, and the feveral ranges of Colours will be dilated and expanded into one another more and more, and by mixing their Colours will dilute one another , and at length , when the diilance of the Paper from the Comb is about a Foot, or a little more (fuppofe in the place ^DzE) they will fo far dilute one another as to become white.

With any obilacle let all the Light be now ftopp'd which pafles through any one interval of the Teeth, fo that the range of Colours which comes from thence may be taken away, and you will fee the Light of the reft of the ranges to be expanded into the place of the range ta- ken away, and there to be coloured. Let the intercepted range pafs on as before, and its Colours falling upon the Colours of the other ranges, and mixing with them, will reftore the whitenefs.

Let the Paper i D i E be now very much in- clined to the Rays, fo that the moft refrangible

Rays

[ 127 3

Rays may be more copioufly refleded than the reil, and the white Colour of the Paper through the Excefs of thofe Rays will be changed into blue and violet. Let the Paper be as much in- clined the contrary way, that the leaft refran- gible Rays may be now more copioufly refleded than the reil, and by their Excefs the white- nefs will be changed into yellow and red. The leveral Rays therefore in that white Light do retain their colorific quaUties, by which thofe of any fort, when-ever they become more co- pious than the reit, do by their Excefs and Pre- dominance caufe their proper Colour to ap- pear.

And by the fame way of arguing, applied to the third Experiment of this Book, it may be concluded, that the white Colour of all refra- fted Light at its very firft Emergence, where it appears as white as before its Incidence, is compounded of various Colours.

Exper. 13. In the foregoing Experiment the feveral intervals of the Teeth of the Comb do the office of fo many Prifms, every interval pro- ducing the Phsenomenon of one Prifm. Whence inftead of thofe intervals ufing feveral Prifms, I try'd to compound whitenefs by mixing their Colours, and did it by ufmg only three Prifms, as alfo by ufing only two as follows. Let two Prifms ABC and abc^ [in Fig. 10. ] whofe re- frading Angles B and b are equal, be fo placed parallel to one another, that the refrading An- gle B of the one may touch the Angle c at the Bafe of the other, and their Planes CB and € b^ at which the Rays emerge, may lie in Di-

redum.

C 128 ]

redum. Then let the Light trajeded through them fall upon the Paper MN, diflant about 8 or II Inches from the Prifms. And the Co- lours generated by the interior limits B and c of the two Prifms, will be mingled at P T, and there compound white. For if either Prifm be taken away, the Colours made by the other will appear in that place P T, and when the Prifm is rellored to its place again, fo that its Colours may there fall upon the Colours of the other, the mixture of them both will rellore the whitenefs.

This Experiment fucceeds alfo, as I have tri- ed, when the Angle b of the lower Prifm, is a little greater than the Angle B of the upper, and between the interior Angles B and r, there intercedes fome fpace B c^ as is reprefented in the Figure, aud the refrading Planes B C and bcy are neither in diredum nor parallel to one another. For there is nothing more requifite to the fuccefs of this F,xperiment, than that, the Rays of all forts may be uniformly mixed upon the Paper in the place P T. If the molt refrangible Rays coming from the fuperior Prifm take up all the fpace from M to P, the Rays of the fame fort which come from the in- ferior Prifm ought to begin at P, and take up all the reft of the fpace from thence towards N. If the leait refrangible Rays coming from the fuperior Prifm take up the fpace MT, the Rays of the fame kind which come from the other Prifm ought to begin at T, and take up the remaining fpace T N. If one fort of the Rays which have intermediate degrees of Re-

frangibility.

frahgibility, and come from the fuperior Prifrri be extended through the fpace MQ, and an- other fort of thofe Rays through the fpace MR^ and a third fort of them through the fpace M Sj the fame forts of Rays coming from the Idwer Prifm, ought to illuminate the remaining fpaces Q N, R N, S N, refpeftively. And the fame is to be underitood of all the othei* forts of Rays. For thus the Rays of every fort will be fcattered uniformly and eavenly thro' the whole fpace M N, and fo being every where mix'd ill the fame proportion, they mud every where produce the fame Colours. And therefore iince by this mixture they produce white in the ex- terior fpaces M P and T N, they muft alfo pro- duce white in the interior fpace P T. This is the reafon of the compofition by which white- nefs was produced in this Experiment, and by what other way foever I made the like compofi- tion the refult was whitenefs.

Laftly, If with the Teeth of a Comb of a due fize, the colour'd Lights of the two Prifms which fall upon the fpace P T be alternately intercepted, that fpace P T, when the motion of the Comb is flow, will always appear coloured, but by accelerating the motion of the Comb fo much, that the fucceflive Colours calnnot be diftinguifli'd from one another, it will appear while

Exper. 14. Hitherto I have produced white^ nefs by mixing the Colours of Prifms. If now the Colours of natural Bodies are to be min- gled, let Water a little thicken'd with Soap be; . agitiited to raife a Froth, and after that Froth

K has

[ no ]

has flood a little, there will appear to one that iliall view it intently various Colours every where in the Surfaces of the feveral Bubbles ; but to one that fliall go fo far off that he cannot di- ftinguilh the Colours from one another, the whole Froth will grow white with a perfed whiten efs.

Exper. If. Laitly, in attempting to com- pound a white by mixing the coloured Powders w^iich Painters ufe, I confider'd that all co- lour'd Powders do fupprefs and Hop in them a very confiderable part of the Light by which they are illuminated. For they become colour'd by refleding the Light of their own Colours more copiouily, and that of all other Colours more fparingly, and yet they do not reflect the Light of their own Colours fo copiouily as white Bodies do. If red Lead, for initance, and a white Paper, be placed in the red Light of the colour'd Spedrum made in a dark Cham- ber by the Refradion of a Prifm, as is defcri- bed in the third Experiment of the firil Book ; the Paper will appear more lucid than the red Lead, and therefore refleds the red-making Rays more copiouily than red Lead doth. And if they be held in the Light of any other Co- lour, the Light refle61ed by the Paper will ex- ceed the Light refleded by the red Lead in a much greater proportion. And the like hap- pens in Powders of other Colours. And there- fore by mixing fuch Powders we are not to ex- ped a ib'ong and full white, fuch as is that of Paper, but fome dusky obfcure one, fuch as might arife from a mixture of light and dark-

nefs,

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