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popular Opinion, is that of the Ariftotc linns ; who maintain Colour to be a Quality f eliding in the coloured Body ; and to exift, independently of Light. See Quality.
T'heCarteJzass come nearer the Matter: They own, that as the colour d Body is not immediately applied to the Or- gan, to occafion the Senfation, and that as no Body can af- fect the Seme but by immediate contact $ the colour' d Body docs not excite the Senfation of it felf, or contribute anything to it, otherwife than by moving fome interpos'd Medium, and by that the Organ of Sight.
They add, that, as we find that Bodies don't affect the Senfe in the dark, Light only occafions the Senfe of Colour, by moving the Organ ; and that coloured Bodies are no further concern 'd than as they reflect the Light with a certain Modification : the differences in their Colours arifing from a difference in the Texture of their Parts , whereby they are difpos'd to reflect the Light with this or that Modi- fication.
But 'tis to Sir Iptnc Ne-zvtcn we arc owing for a folid and confident Theory of Colours; built on fure Experiments, and folving all the Phenomena thereof; His Doctrine is as fol- lows.
- Tis found by Experience, that Rays, or Beams of Light,
are compos'd of Particles very heterogeneous, or difllmilar to each other : i.e. fome of them, as 'tis highly probable, are larger, and others le!s. For a Ray of Light, as T E, (Tab. Optics, Fig- 5-) being receiv'd on a refracting Surface, as AD, in a dark Place, is not wholly refracted to L ; but fplit, as it were, and diffufed into feveral little Rays, fome whereof are refracted to L, a^nd others to the other interme- diate Points between L and G ; i. e. thofe Particles of the Light which are the molt minute, are of all others the moil: eaiily and molt considerably diverted, by the Aclion of the refracting Surface, out of their rectilinear Courfe towards L: and the reft, as each exceeds another in Mignirude, fo is it with more difficulty, and" lefs considerably turn'd out of its right Line to the Points between L and G. See Refran- gibility.
Now, each Ray of Light, as it differs from another m its' degree of Refrangibility, fo does it differ from it in Colour : this is warranted by numerous Experiments. Thofe Parti- cles, v.g. which are molt refracted, are found to conftitutc a Ray of a violet Colour ; ;". e. in all probability, the moft minute Particles of Light, thus, feparately impell'd, excite the fhorteft Vibration in the Retina; which are thence pro- pagated by the folid Fibres of the Optic Nerves into the Brain, there to excite the Senfation of violet Colour; as be- ing the moft dufky and languid of all Colours. See Violet.
Again, thofe Particles which are the leaft refracted, confti- tutea Radhlus, or Ray of a red Colour ; i. c. the largeft Particles of Light excite the long^ft Vibrations in the Re- tina ; Co as to excite the Senfation of red Colour, the bright- eft and moll vivid of all others. See Red.
The other Particles being in like manner feparated, ac- cording to their rclpective Magnitudes, into little Rays ; ex- cite the intermediate Vibrations, and thus occasion the Senfa- tions of the intermediate Colours ; much in the fame man- ner as the feveral Vibrations of the Air, according to their rclpective Magnitudes, excite the Senfations of different Sounds. See Vibration.
To this it may be added, that not only the more diftinct and notable Colours of red, yellow, blue, $$e, have thus their rife from the different Magnitude and Refrangibility of the Ravs ; but alfo the intermediate Degrees or Teints of the fame Colour; as of yellow up to green, of red down to yel- low, &c.
Further, the Colours of thefe little Rays, not being any adventitious Modifications thereof, but connate, primitive, and neceffiry Properties ; as confifting, in all probability, in the Magnitudes of their Parts, muft be perpetual and immu- table ; i. e. cannot be chang'd by any future Refraction or Reflection, or any Modification whatfoever-
This is confirra'd by abundance of Experiments ; all en- deavours having been us'd, after feparatmg a colour'* d Ray from thofe of other Kinds, to change it into fome other Co- lour by repeated Refractions, but to no effect. Apparent Tranfmutarions of Colours, indeed, may be effected ; viz. where there is an Aflemblage, or Mixture of Rays of diffe- rent Kinds ; the component Colours never appearing in their natural Hue in fuch Mixtures, but always aliay'd and tem- pered with each other: whence refults a middling Kind of Colour, which, by Refraction, may be feparated into the component ones : and thofe, after Separation, being remix'd, return to their former Colour.
Hence, the Tranfmutations of Colours, by mixing thofe of different Kinds, are not real ; but mere Appearances or De- ceptions of the Sight : For the Rays being again fevered, exhibit the fame Colours as at firft. Thus, blue and yellow Pouders, well mix'd, appear to the naked Eye green ; yet, without having pafs'd any Alteration, when view'd thro a Microfcope, the blue and yellow Particles Hill appear dif- tinct.
Hence there arife two Kinds of 'Colours; the one oftgtoM* and Jimple, produced by homogeneal Light; or by "Rays that have the fame degree of Refrangibility, and the fame Magnitude of their Parts : Such are red, yellow, green blue a violet purple, orange, and indigo; with all their interme- diate Teints and Gradations.
The other Kind of Colour is fecundary, or heterogeneous ; compounded of the primary ones, or of a Mixture of Rajs differently refrangible, %£c.
There may alio be fecondary Colours produced by Compo- fition, like the primary ones, or thofe confliting of homoge- neal Light, as to the Species or Appearance of the Colour; but not as to the Permanency, or Immutability thereof. Thus, yellow and blue make green; red and yellow, orange ; orange and yellowifh green, yellow : And in the general, if any two Colours be mix'd, which in the Series of thofe gene- rated by the Prifm, are not too far apart, from their Mixture refults that Colour, which in the faid Series is found in the mid-way between 'em : but thofe fituatedat too great a dil- tance do not fo.
Indeed, the more any Colour is compounded, the lefs per- fect and vivid it is : by too much Composition they may be diluted and weaken'd till they ceafe. By Compofirion there may likewife be produced Colours, not. like any of thofe of homogeneal Light.
The moll extraordinary Compofition is that of Wbitenefs ; for to this, all the primary Colours abovemention'd are re- ejuir'd ; and thofe to be mix'd in a certain degree. Hence it is that White is the ordinary Colour of Light ; Light being nothing elfe but a confufed Aflemblage of Rays of all Colours. See Whiteness.
It the Rays of different Colours do thus begin to be fepa- . rated by one Refraction of one fingle Surface ; that Sepa- ration is much promoted, fo as even to become fenfible to the Eye by a double Refraction. This is obferv'd in the two Surfaces of any Glafs ; provided thofe Surfaces be not parallel : but, of all others, it is moft fenfible in the two Faces of a triangular Prifm ; the Phenomena whereof, as they are the Touchftone of all Theories of Colours ; and as they contain the Foundation of that here dcliver'd, we mall lay down as follows.
1. The Rays of the Sun, tranfmitted thro' a triangular Prifm, exhibit an Image of various Colours, (the chief where- of are red, yellow, green, blue, and violet) on the oppofite Wall.
The Reafbn is, that the differently coloured Rays are fe- parated by Refraction: For the blue Rays, v. g. mark'd with the dotted Line, (Plate Optics, Fig. 6.) which begin to be feparated from the reft by the firft Refraction in dd, of the Side ca of the Prifm ah c, (as alfo in the firft Sur- face of the Globe of Water ah c, Fig. 7.) are ftill further fe- parated in the other Side of the Prifm he, (as alfo in their egrefs out of the Globe a h c) by a fecond Refraction in ce t in the fame Direction as the former : whereas, on the con- trary, in the plane Glafs a he f, Fig. 9. (as alfo in the Prifm glo, Fig. 8. now placed in another Situation) thofe blue Rays which begin to be feparated from the reft in the firft Surface, in d d, by a fecond Refraction, the contrary way, pafs out parallel, i. c. remix'd with all the Colours of the other Rays.
a. That Image is nor round, but oblong; its length, when the Prifm is an Angle of (Jo or 65 Degrees, being five times its breadth.
The Reafon is, that fome of the Rays arc refracted more than others ; and by that means, exhibit feveral Images of the Sun, extended lengthwile, inftead of one.
3. Thofe Rays which exhibit yellow, are turn'd further from the rectilinear Courfe, than thofe which exhibit red j thofe which exhibit green, than thofe which exhibit yellow: but, of all others, thofe which exhibit violet the moft : Ac- cordingly, if the Prifm thro' which the Light is tranfmitted be turn'd about its Axis, fo as the red, yellow, green, &c. Rays be projected, in order, thro' a narrow Aperture into another Prifm, placed at the Diftance of about 12 Feet ; the yellow, green, $£!c. Rays, tho falling thro' the fame Aper- ture, in the fame manner, and on the fame Point of the fe- cond Prifm, will not be refracted to the fame Place as the red, but to a Point at fome diftance from it ; on that fide to which the Rotation is made.
This is what Sir 7. Neivton calls the Exper'nncntum Cru- ets : being that which led him out of the Difficulties into which the firft Phenomenon, &c. had thrown him ; and plainly fhew'd a different Degree of Refrangibility, and a different Colour correfponding thereto in the Rays of Light : and that yellow Rays, v. g. are more refracted than red ones, green ones more than yellow ones ; and blue and violet ones moft of all.
4. The Colours of coloured Rays, well feparated by the Prifm, are not at all chang'd or deftroy'd by patting an illu- minated Medium, nor by their mutual Decuffation ; their bordering on a deep Shadow ; nor their being refleaed from
any
