Opticks (1721, 3rd Edition) — part 5 of 18

three " quarters without , and that which falls without will be fpread through about four or five times more fpace than that which falls with- in;, and fo in the grofs be rarer, and if com- pared with the whole Light within it, will be about 15 times rarer than all that taken in the grofs ; or rather more than 30 or 40 times ra- rer, becaufe the deep red in the end of the Speftrum of Colours made by a Prifm is very- thin and rare, and the willow green is fome- thing rarer than the orange and yellow. The Light of thefe Colours therefore being fo very much rarer than that within the Circle, will fcarce affect the Senfe, efpecially fmce the deep red and willow green of this Light, are much darker Colours than the refl. And for the fame reafon the blue and violet being much darker Colours than thefe, and much more rarified, may be neglefted. For the denfe and bright Light of the Circle, will obfcure the rare and weak Light of thefe dark Colours round about it , and render them almoft infenfible. The fenfible Image of a lucid point is therefore fcarce broader than a Circle whofe diameter is the a^oth part of the diameter of the Aperture of the Objeft-glafs of a good Telefcope, or not much broader, if you except a faint and dark mif- ty Light round about it, which a Spectator will fcarce regard. And therefore in a Telefcope whofe aperture is four Inches, and length an hundred Feet, it exceeds notx^'45"'^',or i'^ . And in a Telefcope whofe aperture is two Inches, and length lo or 30 Feet, it may be 5-^^ or 6" and fcarce above. And this aiifwers well 10

G 4 expe-

[ 88 ]

experience : For fome Aftronomers have found the Diameters of the fix'd Stars, in Telefcopes of between lo and 60 'Feet in length, to be about 5" or 6'\ or at moll %'^ or \o'^ in diame- ter. But if the Eye-Glafs be tinfted faintly with the fmoke of a Lamp or Torch , t o ob- fcure the Light of the Star, the fainter Light in the circumference of the Star ceafes to be vilible, and the Star (if the Glafs be fufficient- ly foiled with fmoke ) appears fomething more like a mathematical Point. And for the fame reafon, the enormous part of thfe Light in the circumference of every lucid Point ought to be lefs difcernible in iliorter Telefcopes than in longer, becaufe the fliorter tranfmit lefs Light to the Eye.

Now that the fix'd Stars, by reafon of their immenfe diftance, appear like Points, unlefs fo far as their Light is dilated by Refradion, may appear from hence ; that when the Moon paf- fes over them and Qclipfes them, their Light vanilhes, not gradually like that of the Planets, but all at once ; and in the end of the EcHpfe it returns into Sight all at once, or certainly in . lefs time than the fecond of a Minute ; the Re- fradion of the Moon's Atmofphere a little pro- trading the time in which the Light of the Star firlt vanifhes, and afterwards returns into Sight.

Now if we fuppofe the fenfible Image of a lu- cid point, to be even x5"o times narrower than the aperture of the Glafs : yet this Image would be ftill much greater than if it were only from the fpherical Figure of the Glafs. For were it not for tlie different Refrangibility of the

Rays,

C 85 ]

Rays, its breadth in an 100 Foot Telefcope whofe aperture is 4 Inches would be but-

961

72.0C0000

parts of an Inch, as is manifeil by the foregoing computation. And therefore in this cafe the greateft Errors arifing from the fpherical Figure of the Glafs, would be to the greateft fenfible Errors arifing from the different Refrangibility

of the Rays as —^-^^^ — to— at mofl, that is only

•' 710C0000 250 J

as I to 12-00. And this fufKciently Hiews that it is not the fpherical Figures of GlafFes but the different RefrangibiUty of the Rays which hin- ders the perfedion of Telefcopes.

There is another Argument by which it may appear that the different Refrangibility of Rays, is the true caufe of the iraperfedion of Tele- fcopes. For the Errors of the Rays arifmg from the fpherical Figures of Objeft-glafTes, are as the Cubes of the Apertures of the Obje6l- glalTes ; and thence to make Telefcopes of va- rious lengths, magnify with equal diflindnefs, the Apertures of the Objed-glafTes , and the Charges or magnifying Powers, ought to be as the Cubes of the fquare Roots of their lengths ; which doth not anlwer to experience. But the Errors of the Rays arifmg from the different Refrangibilty, are as the Apertures of the Ob- jedt-glafTes, and thence to make Telefcopes of various lengths, magnify with equal diftindnefs, their Apertures and Charges ought to be as the fquare Roots of their lengths ; and this anfwers to experience, as is well known. For inflance, a Telefcope of 64 Feet in length, within Aper- ture

C 50 ]

ture of i-f Inches, magnifies about 120 times, with as much diftinftnefs as one of a Foot in length, with 7 of an Inch aperture, magnifies 15: times.

Now were it not for this different Refrangi- bility of Rays, Telefcopes might be brought to a greater perfedion than we have yet defcrib'd, by compofmg the Objeft-Glafs of two GlafTes with Water between them. Let ADFC [in Fig, ^8.] reprefent the Objed-glafs compofed of two GlalTes ABED and BEFC, alilie convex on the outfides AGD and CHF, and alike concave on the infides B M E, BNE, with Water in the concavity B MEN. Let the Sine of Inci- dence out of Glafs into Air be as I to R, and out of Water into Air as K to R, and by con- fequence out of Glafs into Water, as T to K : and let the diameter of the Sphere to which the convex fides AGD and CHF are ground be D, and the diameter of the Sphere to which the concave fides BME and BNE are ground be to D, as the Cube Root of KK— KI to the Cube Root of RK— RI: and the Refraftions on the concave fides of the GlafTes, will very much corred the Errors of the Refradions on the convex fides, fo far as they arife from the fphericalnefs of the Figure. And by this means might Telefcopes be brought to fufficient per- fedion, were it not for the different Refrangi- bility of feveral forts of Rays. But by reafon of this different Refrangibility, I do not yet fee any other means of improving Telefcopes by Refraftions alone than that of increanng their lengths, for which end the late Contrivance of

Hugeniits

[ 91 ]

Hugenins feems well accommodated. For ve- ry long Tubes are cumberfome, and fcarce to be readily managed, and by reafon of their length are very apt to bend, and Ihake by bend- ing fo as to caule a continual trembling in the Objeds, whereby it becomes difficult to fee them dillindly : whereas by his contrivance the Glafles are readily manageable, and the Objedt- glafs being fix'd upon a Itrong upright Pole- be- comes more ileady.

Seeing therefore the Improvement of Tele- fcopes of given lengths by Refradions is defpe- rate ; I contrived heretofore a Perfpec^ive by Reflexion, ufmg initead of an Objed-glafs a concave Metal. The diameter of the Sphere- to which the Metal was ground concave was a- bout 25- Englifli Inches, and by confequence the length of the Inilrument about fix Inches and a quarter. The Eye-glafs was Plano-con- vex, and the diameter of the Sphere to which the convex fide was ground was about 4 of an Inch, or a little lefs, and by confequence it magnified between 30 and 40 times. By ano- ther way of meafuring I found that it magnified about 35- times. The concave Metal bore an Aperture of an Inch and a third part ; but the Aperture was hmited not by an opake Circle, covering the Limb of the Metal round about, but by an opake Circle placed between the Eye- glafs and the .'Eye, and perforated in the mid- dle with a little round hole for the Rays to pafs through to the Eye. For this Circle by being placed here , llopp'd much of the erroneous Light, which otherwife would have diilurbed

the

C 92 ]

the Vifion. By comparing it with a pretty good Perfpedive of four Feet in length, made with a concave Eye-glafs, I could read at a greater diftance with my own Inflrument than with the Glafs. Yet Objedls appeared much darker in it than in the Glafs, and that partly becaufe more Light was loft by Reflexion in the Metal, than by Refradion in the GJafs, and partly be- caufe my Inllrument was overcharged. Had it magnified but 30 or 25 times it would have made the Objeft appear more brisk and plea» fant. Two of thefe I made about 16 Years a- go, and have one of them Hill by me by which I can prove the truth of what I write. Yet it .is not fo good as at the firlt. For the concave has been divers times tarnillied and cleared a- gain, by rubbing it with very fofc Leather. When I made thefe, an Artifl in London undertook to imitate it ; but ufmg another way of polifliing them than I did, he fell much ihort of what I had attained to, as I afterwards underflood by difcourfing the under Workman he had em- ployed. The Polilli I ufed was in this man- ner. I had two round Copper Plates each fix Inches in diameter, the one convex the o- ther concave, ground very true to one another. On the convex I ground the Objeft-Metal or Concave which was to be polifh'd, till it had taken the Figure of the Convex and was ready for a Polifh. Then I pitched over the convex very thinly , by dropping melted Pitch upon it and warming it to keep the Pitch foft, whilft I ground it with the concave Copper whetted to make it fpread eavenly all over the convex.

Thus

[ 93 ]

Thus by working it well I made it as thin as a Groat, and after the convex was cold I ground it again to give it as true a Figure as I could. Then I took Putty which I had made very fine by wafliing it from all its groffer Particles, and laying a little of this upon the Pitch, I ground it upon the Pitch with the concave Copper till it had done making a noife ; and then upon the Pitch I ground the Objed-Metal with a brisk motion, for' about two or three Minutes of time , leaning hard upon it. Then I put frefli Putty upon the Pitch and ground it again till it dad done making a noife, and afterwards ground the Objeft-Metal upon it as before. And this Work I repeated till the Metal was poHilied, grinding it the lail time with all my Itrength for a good while together, and. frequently breathing upon the Pitch to keep it moitt with- out laying on any more frefli Putty. The Ob- jedt-Metal was two Inches broad and about one third part of an Inch thick, to keep it from bending. I had two of thefe Metals, and when I had polifhed them both I tried which was bed, and ground the other again to fee if I could make it better than that which I kept. And thus by many Trials I learn'd the way of po- lifliing, till I made thofe two refleding Perfpe- ftives I fpake of above. For this Art of po- hfliing will be better learn'd by repeated Pra- ftice than by my defcription. Before I ground the Obje6t-Metal on the Pitch, I always ground the Putty on it with the concave Copper till it had done making a noife, becaufe if the Parti- cles of the Putty were not by this means made

to

[94 ]

to flick fafl in the Pitch, they would by rolling up and down grate and fret the Objeft-Metal and fill it full of little holes.

But becaufe Metal is more difficult to polifh than Glafs , and is afterwards very apt to be fpoiled by tarnifliing and refleds not fo much Light as Glafs quick-filver'd over does : I would propound to ufe inltead of the Metal, a Glafs ground concave on the forefide, and as much convex on the back-fide, and quick filver'd o- ver on the convex fide. The Glafs muft be every where of the fame thicknefs exadly. O- therwife it will make Objefts look colour'd and indiftindl. By fuch a Glafs I tried about five or fix Years ago to make a refleding Telefcope of four Feet in length to magnify about 15-0 times, and I fatisfied my felf that there wants nothing but a good Artiit to bring the Defign to perfe- ftion. For the Glafs being wrought by one of our London Artiits after fuch a manner as they grind GlalTes for Telefcopes, tho' it feemed as well wrought as the obje^t-glalTes ufe to be, yet when it was quick- filver'd, the Reflexion dif- covered innumerable Inequalities all over the Glafs. And by reafon of thefe Inequalities, Ob- jedfs appeared indiftindl in this Inftrument. For the Errors of reflecSted Rays caufed by any In- equality of the Glafs, are about fix times great- er than the Errors of refraded Rays caul'ed by the like Inequalities. Yet by this Experiment I fatisfied my felf that the Reflexion on the concave fide of the Glafs, which I feared would diilurb the Vifion, did no fenfible prejudice to it, and by confequence that nothing is wanting

to

[95]

to perfeft thefe Telefcopes, but good Work- men who can grind and polifti Glafles truly fpherical. An Objed-gla.fs of a fourteen Foot Telefcope, made by an Artificer at London^ I once mended confiderably, by grinding it on Pitch with Putty, and leaning very eafily on it in the grinding, left the Putty fliould fcratch it. Whether this way may not do well enough for poliihing thefe refleding GlafTes, I have not yet tried. But he that fliall try either this or any other way of pohfliing which he may think bet- ter, may do well to make his GlalTes ready for polifhing by grinding them without that vio- lence, wherewith our London Workmen prefs their GlafTes in grinding. For by fuch violent prefTure, GlafTes are apt to bend a Httle in the grinding, and fuch bending will certainly fpoil their Figure. To recommend therefore the confideration of thefe reflecting Glaffes, to fuch Artifts as are curious in figuring GlafTes, I fhall defcribe this optical Inllrument in the following Propofition.

TROT, WW. Prob.il To jhorte7t Telefcopes.

LET A B D C [in Fig. 29.] reprefent a Glafs fpherically concave on the forefide AB, and as much convex on the backfide CD, fo that it be every where of an equal thicknefs. Let it not be thicker on one fide than on the other, left it make Objeds appear colour'd and

indi-

C 96 ]

ihdiflin(3:, and let it be very truly wrought and quick filver'd over on the back- fide ; and fet in the Tube V X Y Z which mufl be very black within. Let E F G reprefent a Prifm of Glafs or Cryftal placed near the other end of the Tube, in the middle of it, by means of a han- dle of Brafs or Iron F GK, to the end of which made flat it is cemented. Let this Prifrn be redangular at E, and let the other two Angles at F and G be accurately equal to each other, and by confequence equal to half right ones, and let the plane lides F E and G E be fquare, and by confequence the third fide F G a re- ftangular Parallelogram, whofe length is to its breadth in a fubduplicate proportion of two to one. Let it be fo placed in the Tube, that the Axis of the Speculum may pafs through the middle of the fquare fide E F perpendicularly, and by confequence through the middle of the fide F G at an Angle of 45 Degrees, and let the fide EF be turned towards the Speculum, and the diftance of this Prifm from the Speculum be fuch that the Rays of the Light PQ, RS, ^c. which are incident upon the Speculum in Lines parallel to the Axis thereof, may enter the Prifm at the fide E F, and be reflefted by the fide F G, and thence go out of it through the fide G E, to the point T which mult be the com- mon Focus of the Speculum ABDC, and of a Plano-convex Eye-glafs H, through which thofe Rays muft pafs to the Eye. And let the Rays at their coming out of the Glafs pafs through a fmall round hole, or aperture made in a lit- tle plate of Lead, Brafs or Silver, wherewith

the

(911

the Glafs is to be covered, which hole mufl be ho bigger than is necefTary for Light enough to pais through. For fo it will render the Ob- jeft diflinft, the Plate in which 'tis made inter- cepting all the erroneous part of the Light which comes from the verges of the Speculum AB. Such an Inilrument well made, if it be fix Foot long, (reckoning the length from the Speculum- to the Prifm, and thence to the Fo- cus T) will bear an aperture of fix Inches at the Speculum, and magnify between two and three hundred times. But the hole H here limits the aperture with more advantage, than if the aperture was placed at the Speculum. If the Inilrument be made longer or morter, the aper- ture mull be in proportion as the Cube of the fquare-fquare Root of the length, and the mag- nifying as the aperture. But it's convenient that the Speculum be an Inch or two broader than the aperture at the leatl, and that the Glafs of the Speculum be thick, that it bend not in the working. The Prifm EFG mull be no bigger than is neceifary , and its back fide F G rhuft not be quick-filver'd over. For without quick- filver it will refleft all the Light incident on it ft'om the Speculum.

In this Inftrument the Objedl will be invert* ed, but may be ereded by making the fquare fides EF and E G bf the Prifm E F G not plane but fpherically convex, that the RayS may crofs as well before they come at it as afterwards between it and the Eye- glafs. If it- be defired that the Inilrument bear a larger aperture, that

H may

[98 ]

may be alfo done by compofing the Speculum of two GlaiTes with Water between them.

If the Theory of making Telefcopes could at lengrh be fully brought into pradice, yet there would be certain Bounds beyond which Tele- fcopes could not perform. For the Air through which we look upon the Stars, is in a perpe- tual Tremor ; as may be feen by the tremulous Motion of Shadows call from high. Towers, and by the twinkling of the fix'd Stars. But thefe Stars do not twinkle when view'd through Telefcopes which have large apertures. For the Rays of Light which pafs through divers parts of the aperture, tremble each of them a- part, and by means of their various and fome- times contrary Tremors, fall at one and the fame time upon different points in the bottom of the Eye, and their trembling Motions are too quick and confufed to be perceived feverally. And all thefe illuminated Points conftitute one broad lucid Point, compofed of thofe many trembling Points confufedly and infenfibly mix- ed with one another by very fliort and fwift Tremors , and thereby caufe the Star to appear broader than it is, and without any trembling of the whole. Long Telefcopes may caufe Ob- jeds to appear brighter and larger than ibort ones can do, but they cannot be fo formed as to take away that confufion of the Rays which arifes from the Tremors of the Atmofphere. The only remedy is a molt-ferene and quiet Air, fuch as may perhaps be found on the tops of the higheil Mountains above the grolTer. Clouds.

THE

THE

FIRST BOOK

O F

TICKS.

P A R T II.

TROT. I. Theor. I.

The Thanomena of Colours in refraBed or re* JleBed Light are not caufed by new Modifi- cations of the Light varioujly imprejs'd, ac* cording to the various Terminations of ths Light and Shadow.

The Proof by Experiments.

Exfer.i. ^^^|OR if the Sun iliine into a ^^tf?rTr^I| ^gj.y j^j.|, (^}-,^j^-5[^(^j^. through

g an oblong hole F, [in Fig. i.] ^ whofe breadth is the lixth or eigth part of an Inch, or fomething lefs ; and his beam F H do afterwards pals firit through a

H 2. very

C too ]

very large Prifm ABC, diftant about ^o Feet from the hole, and parallel to it, and then (with its white part) through an oblong hole H, whofe breadth is about the fortieth or fixtieth part of an Inch, and which is made in a black opake ^ Body G I, and placed at the diilance of two or three Feet from the Prifm, in a parallel Situa- tion both to the Prifm and to the former hole,: and if this white Light thus tranfmitted through' the hole H, fall afterwards upon a white Paper / 1, placed after that hole H, at the diftance of three. or four Feet from it, and there paint the ufual Colours of the Prifm , fuppofe red at ?, yellow at s, green at r, blue at ^, and violet ^t/; you may with an Iron Wire, or any fuch like {lender opake Body, whofe breadth is a- bout the tenth part of an Inch , by intercepting the Rays at k, /, m, n or o^ take away any one of the Colours at t^ s, r, q or/, whilfl the other Colours remain upon the Paper as before ; or with an Obftacle fomething bigger you may take away any two, or three , or four Colours together, the reft remaining: So that any one of the Colours as well as violet may become outmolt in the Confine of the Shadow towards /, and any one of them as w^ell as red may be- come outmoll in the Confine of the Shadow towards t, and any one of them may alfo bor- der upon the Shadow made within the Colours by the Obftacle R intercepting fome interme- diate part of the Light ; and, laftly, any one of them by being left alone may border upon the Shadow on either hand. All the Colours have themfelves indifferently toany Confines of Sha- dow,

[ ,o, ]

dow, and therefore the differences of thefe Co- lours from one another , do not arife from the different Confines of Shadow , whereby Light is varioufly modified, as has hitherto been the Opinion of Philofophers. In trying thefe things 'tis to be obferved, that by how much the holes F and H are narrower, and the Intervals be- tween them, and the Prifm greater, and the Chamber darker, by fo much the better doth the Experiment fucceed ; provided the Light be not fo far diminilhed, but that the Colours at /> ^ be fufliciently vifible. To procure a Prifm of folid Glafs large enough for this Experiment will be" difificult, and therefore a prifmatick VeiTel mult be made of poliili'd Glafs Plates cemented together, and filled with fait Water or clear Oil.

Exper. z. The Sun's Light let into a dark Chamber through the round hole F, [in Fig. z.] half an Inch wide, paffed firft through the Prifm ABC placed at the hole, and then through a Lens PT fomething more than four Inches broad, and about eight Feet diftant from the Prifm, and thence converged to O the Focus of the Lens diftant from it about three Feet, and there fell upon a white Paper DE. If that Paper was perpendicular to that Light incident upon it, as 'tis reprefented in the pofture DE, all the Colours upon it at O appeared white. But if the Paper being turned about an Axis parallel to the Prifm, became very njuch incli- ned to the Light as 'tis reprefented in the Po- rtions de and h\ the fame Light in the one cafe appeared yellow and red, in the other blue.

H 3 Her«

[ I02 ]

Here one and the fame part of the Light in one and the fame place, according to the va- rious Inclinations of the Paper, appeared in one cafe white, in another yellow or red, in a third blue, whillt the Confine of Light and Shadow, and the Refractions of the Priim in all thefe ca- fes remained the fame.

Exper. 3. Such another Experiment may be more eafily tried as follows. Let a broad beam of the Sun*s Light coming into a dark Cham- ber through a hole in the Window-diut be re- fraded by a large Prifm ABC, [in Fig. 3.] whofe refrading Angle C is more than 60 De- grees, and fo foon as it comes out of the Prilm let it fall upon the white Paper D E glewed up- on a ftiff Plane"; and this Light, when the Pa- per is perpendicular to it, as 'tis reprefented in D E., will appear perfectly white upon the Pa7 per, but when the Paper is very much inclin'd to it in fuch a manner as to keep always paral- lel to the Axis of the Prifm, tlie whitenefs of the whole Light upon the Paper will according to the inclination of the Paper this way or that way, change either into yellow and red, as in the pollure de^ or into blue and violet, as in the pofture ^i. And if the Light before it fall upon the Paper be twice refraCled the fame way by two parallel Prifms, thefe Colours will become the more confpicuous. Here all the middle parts of the broad beam of white Light which fell upon the Paper, did without any Confine of Shadow to modify it, become co- iour'd all over with one uniform Colour, the Golgur being always the fame in the middle of

the

I 103 ]

the Paper as at the edges, and this Colour chan- ged according to the various obhquity of the refleding Paper^ without any change in the Re- fradions or Shadow, or in the Light which fell upon the Paper. And therefore thefe Colours are to be derived from fome other Caufe than the new Modifications of Light by Refradions and Shadows.

If it be asked, What then is their Caufe ? I anfwer, That the Paper in the polture de, be- ing more oblique to the more refrangible Rays than to the leis refrangible ones, is more Itrong- ly illuminated by the latter than by the. former, and therefore the lels refrangible Rays are pre- dominant in the refleded Light. And whcre- ever they are predominant in any Light they tinge it with red or yellow, as may in fome mea- fure appear by the tirlt Propofition of the firft Book, and will more fully appear hereafter. And the contrary happens in the poflure of the Paper ^g, the more refrangible Rays being then predominant which always tinge Light with blues and violets.

Exper. 4. The Colours of Bubbles with which Children play are various, and change their Si- tuation varioufly, without any refped to any Confine of Shadow. If fuch a Bubble be co- ver'd with a concave Glafs, to keep it from be- ing agitated by any Wind or Motion of the Air, the Colours will llowly and regularly change their Situation, even whilll the Eye, and the Bubble, and all Bodies which emit any Light, or caft any Shadow, remain unmoved. And therefore their Colours aril'e from fome regular

H 4 caufe

C 104 ]

Caufe which depends not on any Confine of Shadow. What this Caufe is will be iliewed in the next Book.

To thefe Experiments may be added the tenth Experiment of the firll Book, where the Sun's Light in a dark Room being trajeded through the parallel Superficies .of two Prifms tied together in the form of a Parallelopipede, became totally of one uniform yellow or red Colour, at its emerging out of the Prifms. Here, in the produdion of thefe Colours, the Confine of Shadow can have nothing to do. For the Light changes from white to yellow, orange and red fucceffively, without any alte- ration of the Confine of Shadow : And at both edges of the emerging Light where rhe con- trary Confines of Shadow ought to produce different Effe61s, the Colour is one and the fame, whether it be white, yellow, orange or red : And iq the middle of the emerging Light, where there is no Confine of Shadow at all, th^ Colour is the very fame as at the edges, the whole Light at its very firfl Emergence being of one uniform Colour, whether white, yellow, orange or red, and going on thence perpetual- ly without any change of Colour, fuch as the Confine of Shadow is vulgarly fuppofed to work in refrafted Light after its Emergence. Nei- ther can thefe Colours arife from any new Mo- difications of the Light by Refradions, becaufe they change fucceffively from white to .yellow, orange and red, while the Refradions remain the fame, and alfo becaufe the Refradions are ip^ade contr^ary v>'ays by parallel Superficies which

deflroy

C I05]

deftroy one anothers EfFeds. They arife not therefore from any Modifications of Light made by Refradions and Shadows, but have fome other Caufe. What that Cajife is we fhewed 'above in this tenth Experiment, and need not here repeat it.

There is yet another material circumftance of this Experiment. For this emerging Light be^ ing by a third Prifm HIK [in Fig. tl-l. Tart i.] refracted towards the Paper FT, and there paint- ing the ufual Colours of the Prifm, red, yellow, green, blue, violet : If thefe Colours arole from the Refradions of that Prifm modifying the Light , they would not be in the L ight before its Incidence on that Prifm. And yet in that Experiment we found that when by turning the two firfl Prifms about their common Axis all the Colours were made to vaniili but the red ; the Light which makes that red being left alone, appeared of the very fame red Co^ lour before its Incidence on the third Prifm. And in general we find by other Experiments that when the Rays which differ in Refrangibi- lity are feparated from one another, and any one fort of them is conCdered apart, the Co- lour of the Light which they compofe cannot be changed by any Refradion or Reflexion whatever, as it ought to be were Colours no- thing elfe than Modifications of Light caufed by Refraftions, and Reflexions, and Shadows. This unchangeablenefs of Colour I am now to defcribe in the following Propofition.

TROT,

C 106 ]

T R OTAl. Theor. II.

^11 homogeneal Light has its f roper Colour an- fwering to its degree of Refrangibility^ and. that Colour cannot be changed by Reflexions and Refr anions.

IN the Experiments of the fourth Propofition of the firfl: Book, when I had feparated the heterogeneous Rays from one another, the Spe- ftrum / 1 formed by the feparated Rays, did in the progrefs from its end />, on which the mofl refrangible Rays fell, unto its other end ty on which the leafl reh'angible Rays fell, appear tinged vv^ith this feries of Colours, violet, indi- go, blue, green, yellow, orange, red, together with all their intermediate degrees in a con- tinual Succeflion perpetually varying. So that there appeared as many degrees of Colours, as there were forts of Rays differing in Refran- gibility.

Exper. 5". Now that thefe Colours could not be changed by Refra^ion, I knew by refrading with a Prifm fometimes one very little part of this Light, fometimes another very little part, as is defcribed in the twelfth Experiment of the firft Book. For by this Refradion the Co- lour of the Light was never changed in the Icaft. If any part of the red Light was refradkd, it remained totally of the fame red Colour as be- fore. No orange, no yellow, no green or blue, , no other new Colour was produced by that Refradion. Neither did the Colour any ways change by repeated Refradions, but continued

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always the fame red entirely as at firft. The like conibncy and immutability I found alfo in the blue, green, and other Colours. So alfo if I looked through a Prifm upon any Body illu- minated with any part of this homogeneal Light, as in the fourteenth Experiment of the firft Book is delcribed ; I could not perceive any new Co- lour generated this "way. All Bodies illumina- ted vvith compound Light appear through Prifms confufed (as was faid above) and tinged with various new Colours, but thofe illuminated with homogeneal Light appeared through Prifms neither iefs diitin^l:, nor otherwife coloured, than when viewed with the naked Eyes. Their Colours were not in the leatl changed by the Refradion of the interpofed Prifm. I fpeak here ol a fenfible change of Colour : For the Light whicli I here call homogeneal, being not ablolutely homogeneal, there ought to arife fome httle change of Colour from its hetero- geneity. But if that heterogeneity was fo lit- tle as it might be made by the faid Experiments of the fourth Propofition, that change was not fenfible, and therefore in Experiments, where Senfe is judge, ought to be accounted none at all.

• Exper. 6. And as thefe Colours were not changeable by RefraCdons, fo neither were they by Reflexions. For all white, grey, red, yel- low, green, blue, violet Bodies, as Paper, Aflies, red Lead, Orpiment, Indigo, Bife, Gold, Sil- ver, Copper, Grafs, blue Flowers, Violets, Bubbles of Water tinged wixh various Colours, peacock's Feathers, the Tindure of Lignum

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Nephriticum^ and fuch like, in red homogeneal Light appeared totally red, in blue Light to- tally blue, in green Light totally green , and fo of other Colours. In the homogeneal Light of any Colour they all appeared totally of that fame Colour, with this only difference, that fome of them refleded that Light more ftrong- ly, others more faintly. I never yet found any fiody which by refleding homogeneal Light could fenfibly change its Colour.

From all which it is manifelt, that if the Sun's Light confifted of but one fort of Rays, there would be but one Colour in the whole World, nor would it be pofTible to produce any new Colour by Reflexions and Refracfiions, and by confequence that the variety of Colours depends upon the compofition of Light.

T>EFINlTtON.

TH E homogeneal Light and Rays which appear red, or rather make Objefts ap- pear fo, I call Rubrific or Red- making ; thofe which make Objeds appear yellow, green, blue and violet, I call Yellow-making, Green-ma- king, Blue-making, Violet-making, and fo of the reft. And if at any time I fpeak of Light and Rays as coloured or endued with Colours, I woijld be underftood to fpeak not philofo- phicaliy^id properly, but grollly, and accor- dingly to fuch Conceptions as vulgar People in feeing all thefe Experiments would be apt to frame. For the Rays to fpeak properly are not coloured. In them there is nQthing elfe than a

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certain power and difpofition to flir up a Sen- fation of this or that Colour. For as Sound iti a Bell or mufical String or other founding Bo- dy, is nothing but a trembling Motion, and in the Air nothing but that Motion propagated from the ObjeS, and in the Senforium 'tis a Senfe of that Motion under the form of Sound; fo Colours in the Objed: are nothing but a Dif- pofition to refledl this or that fort of Rays more copioufly than the reft; in the Rays they are nothing but their Difpofitions to propagate this or that Motion into the Senforium, and in the Senforium they are Senfations of thofe Motions under the forms of Colours.

TROT. III. Prob. I.

To define the Refrangib'tUty of the fever al forts of homogeneal Light anfwermg to the feve^ ral Colours.