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( 6ioB ) 

An Bxtrao^ ofMr.lfaac Newton's Letter, written to the Ptthli. 
/her from Cambridge April 9. 1673. concerning theUumber 
ofColorS) and the Uecejjity of mixing them all for the pro- 
duGion <?f White § as alfi touching the Caufe why a PiUure cafi 
by GlaJJes into a darned room appear sfo difiinB notrnthjiand- 
ing its Irregular refraUton : (Which Letter^ being an Imme- 
diat anfwer to that from Paris, printed N*.96,p,6o86. ofthefi 
Tra(3:s;, fhould alfo, iftt had not been mif-laid, havs immedi- 
ately foliovpedtbe fame.") 

IT Teems to tne,that N. takes an improper way of examining 
the nature of Cobrs^whitd he proceeds upon compounding 
thofe that are already compounded ; as he doth in the former 
part of his Letter* Perhaps he "would fooncr (atisfie himfelf 
by refolving Light into Colors, as far as may be done by Art, 
and then by examining the properties of thoft; colors apart, 
and afterwards by trying thseffsfis of rc-conjoinng two or 
morcorall ofthofej andlaftly, by feparating them ag^;n to 
examine, what changes that re-conjunfl:ion had wrought »n 
them. Thisjl conftfs, will prove a tedious and diffijult ta^k 
to do it as it ought to be done 5 but I could notbe (att'fijd, 
till I had gone through it. However,l only propound Jit,and 
leave every roan to his own method. 

As to the Contents of his Letter,! conceive,my former An- 
fwer to the^w^rc about the Number oi Colon is fufficienr, which 
was to this ctfcd: 5 That all Colors cannot praftically be deri- 
ved out of the Teilotv and Blerv, and confequcntly that thofe 
Hypothefes are groundltfs wiiich imply they may. If you ask. 
What colors cannot be derived out o^ yeUow and blevp> \ an- 
fwer,none of all thofe which I defio'd to be Original and if 
hecanfliew by experiment,how they may.I will acknowledge 
wy felf in an error. Nor is it eafier to frame an Hjpi^thejis by 
aiTumingoniy tw(5 Original colors rather than ao indtfiuit 
varicy ; uolcfsi! be eafier to fuppofe, that there arc but two 
figurcfjlizws and degrees of velocity or torce of the iEihereal 
corpuicles or puires,rdthcr than indefioii variety ^ which cer- 
tainly would be a harftifuppofitioo. No man wonders at the 
iodefiait variety oi Waves of theSeajor of fands on the fliarc^ 

bur. 



( ^»09 ) 
bnf, were they all but two fizcs, it would be a very puzling 
phenomenon. And I (hould think it as unaccouotabie, if the 
feveral parts or corpufclcs, of which a (hining body confifts, 
which tnuft be fuppob'd of various figures, fizes and Biotions, 
(hould imprcls but two forts of motion on the adjiccot A)the- 
XGiX medium^ or any other way beget but two forts of Kays. 
But to cxaaiinc.how Colors may be c-^'pUm'd hypotBeticall^ ^'i% 
bcfides my purpofe. I never intended to fbew, wherein con- 
fifts the Nature and Difference of colorSjbut only to fbew, that 
defa&o they are Original and Immutable qualities ot the Hays 
which exhibit them 5 and to leave it to others to explicate by 
Mechanical Hypothefet the Nature and Difference of thofe 
qualities: which I take to be no difficult matter. But I would 
not be underftood, as if their Difference confiftcd in the Dif- 
ferent Refrangibiiity of thofe rays 5 for.that different Refrao- 
gibility conduces to their produftion no ctherwife, than by 
feparating th*^ Rays whofe qualities they are. Whence it is, 
that the fame Rays exhibit the fame Colors when fcparattd by 
any other means 5 as by their different Ke/^i^x;^;///)', a quality 
not yet difcourfcd of. 

Id the next particular, where N» would fhew, that it is not 
ncccflary to mix all Colors for the produftion of tFhite j the 
mhturcof Tel/ffw, Green a^nd Blew ^ without Red and Fiolet^ 
which he propounds for that end, will not produce iF^?/e,but 
Creen , and thebrighteft part of the Yellow will afford no o- 
ther colour but Yellow, if the Experiment be made in a room 
well darkn'djas it ought -^ becaufe the Colour'd light is much 
weaken'd by the Reflexion, and fo apt to be diluted by the 
mixing of any <?iher fcattering light. But yet there is an Ex- 
periment or tvo mcntion'd in my Letter in the Tranfa&ions 
Nui»h4,tBy by which! have produced W^/&;Ve out of two colors 
alooe,aad tha" variouflyi as out oiOrange and a full Blevp, and 
cut of Red and pale Blew, and oui oiYellow and Violet, as aUo 
out ■ i other p-^irs of Intermediat colors. The moft conveni- 
ent Experimt r r for performing this,was that of cafting the co- 
lors of oneP?'*rie up< n thofe of another, after a due manner. 
But what N. c * 1 deduc*: from hence, I lee not. For the two 
colors were c mpounded of all others, and fo the refulting 
"White, (to I'peak properly,^ was compounded of them alJ, 

Qjq q q q q 2 and 



( 6iio ) 

and only de-conipounded of thofe two. For infiance, the 
Orange was compounded of Red, Orauge, Yellow and fome 
Green ^ and the B/^WjofViolctjfull Blewjlight RleWjand fome 
Green,with all their Intcrmediat degrees ^ and confcquently 
the Orange and Blew together made an Aggregate of all co- 
lors to conftitutc the White. Thus^if one mix red, orange and 
yellow Powders to make an Orange j and grcenjbltw and vi- 
olet colors to make a Blew j and iaftly, the two mixtures, to 
make a Grey; that Grey ^thoughde- compounded of no more 
than two Mixtures, is yet compounded of all the fix Powders, 
as truly as if the powders had been all mixtat once. 

This is fo plain, that I conceive there can be no further 
fcruple ; efpecially to them who know how to cxaraine,whc- 
ther a colour be firaple or coinpoundedjand of what colors it 
is compounded 3 which having explained in another place, I 
need not now repeat. If tberefortN, would conclude any thing, 
he niufl: fhew, how White may be produced out of two Vn. 
compounded colors i, which when he hath done, 1 willfuither 
tell him,why he can conclude nothing from that.Bui I believe, 
there cannot be found an Experiment of that kind j beciufe, 
as I remember,l once trycd,by gradual fucceffioDjthe mixmre 
of all pairs of Uo-compounded colors 5 and, though fume of 
them were paler, and nearer to White, than others, yet none 
could be truly call'd White. But it being fome years fince this 
tryal was msde, I remember not well the circumftances, and 
thcrefcre recommend it to others to be tryed again. 

In the laft place, had I thought, the Diilindnefs of the Pi- 
(fture,which (for ttijiance) a Twelffoot Objed glalscafts ioto 
a darken'd room,to be fo contrary to me as N. is pleaf-d to af- 
firm,! ihould have waved ray Theuiy in that point before I 
propounded it. For, that I hud thought on that difficulty, 
youmay eafiiy gucjs by an expreffioo, fomc- 
* y^'Numb. So. where in my firft Letftr *. ro 'his purpofe;That 
^''^° 1 wond i'd,huw Tehflcopes could be brought 

to fo great perfcftionbv Refr ifti >ns which wen-* fo Irregular. 
Butjto take away thedifficuhy,! muit acquaint y 'U,^ri?jThar, 
thougjh I put 'he greatcrt Lateral error of the rays from one 
anothtr to be abouf '- of the G^afK s di.«merer s y^^ '^^^^ grea- 
ter error from the Points on which thty ought to fall, wiH be 

but 



( 6ui ) 

but f^^ of that diameter : hnd then, thaVtht t&ys, wbofe error 
is fo great,are but very few in coatparifoa tothofe, which are 
refrafted more Juftly , for, the rays which fall upon the mid- 
dle-parts of the Glafs, are refraftcd with fufficieut exadoefi, 
as alfo are thofe that fall near the perimeter Avidhive a meaft de- 
gree of Rtlrangibility j So that there remain only the rays, 
which fall near the perimeter and are moji or leaji refrangible 
to caufe any feofible confufion in the Pifture. And thefe are 
yet (o much further weaken'd by the greater fpace, through 
which they are fcattt r'd, that the Light which falls on the due 
poiot, is infinitely mire denle than that which falls on any 
other poiHf round ab'^ut it. Which though it may feem a 
Paradox^ yet is certainly dcmooftrable. Yea, although the 
Light, which pafTs through the middle parts ot the Glafs, 
were wholly intercepted, yet would the remaining light con- 
vene iiifioiteiy more denfc at the due points,than atother pla- 
ces. And by this excefs of Dcnfityjthe Light, which falls in 
or invifibly near the juft point ,may »I conceive, ftrike the fen- 
forium fo vigorouflyjthat the imprefs of the weak light, which 
errs round about ir,fhall,in comparifon, not be ftrong enough 
to be animad*verted,or to caufe any more fenfible confufioa io 
the Pidure than is found by Experience. 

Thibjl C3nceive,is enough to fliewjWhy the Pidure appears 
fo diftinftjUotwithftandiDg the Irregular refraftion* But, if 
this fatisfie not,N. may try,if he pleafe,how diftinft the Pifture 
will appear.whcn all the Lens is cover'd excepting a little hole 
next its edge on one fide only : And if in this cafe ha pleafe to 
meafure tbe breadth of the colors thus made at the edge of the 
Suns pi^ure, he will perhaps find it to approach nearer to my 
proportion than he expedls. 



ji» 



( 6113 ; 

Jft Anjber {to the former Letter y written to the Puhliffer June 
10.157:?. />y the ame Parifian rh'Ufophrr.thatwoi lati/jjaidto 
havevpritteu the Letter already extant in N', 96.P.6086. 



1 



'Ouchir.g t^.c Solutions,givtn by M Newton u.. the 
(crupies by mt propos'd about hibTheory of Co- 
lorSjthere were matter tc Jiifwer tht.'m, and to form new diffi- 
culties ; but feeing ihjt h? maintains his opinion with fo much 
concern,! lift not to difpute. But what means it,! pray, that 
b,c fi'ith '-y Though IJIiouldpen) him,that the Wkue couldheprodn- 
cedofi^nly (wo'Un'Con/poHndcd colon ^yit I could conclude nothing 
from that. And yet he hath sffirra'd in/>, 9083. ot the Tranf 
a&ioas, that to compofe the White, all primitive colors arc 
cectffiry* 

As to the manner,wherf by he reconciles the efftft of Con- 
vex glaflesfor fo well aflembling tht rays, with what he tfta- 
blifties concerning the difTerent rcfrangibility, lam fatiified 
with it 5 but then he is alfo to acknowlcdge,that this aberrati- 
on of the rays is not fbdifadvantagious to Optic glafies as he 
fccms to have been willing to make us believe, when he pro- 
pofed Concave fpecuhms as the only hopes of perfefting Tele- 
fcopes. His invention certainly was very good j but.as far as 
I could perceive by experience, the deftd of the Matter ren- 
ders it as iropofliblc to execute, as the difficulty of the Form 
obftruds the ufe of the Hyperbole oiM^Dei-Cartes : So that, in 
my opinioojwe rouft (tick to our Spheric Glafies, whom we are 
already (b much obliged tOjand that are yet capable of great- 
er perfeftionjas well by increafing the length of Teltlcopes,a8 
by corrcd^ing the nature of Glafs it felf So far He. 

To this Letter is to be referred that, ivbich is already extant i» 
N.95.P.6C87. as being an Anfvoer thereto^ 



A Letter