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Show in refleéting the Light in a certain Modification; the Difference in Colours, according to them, arifes in a Difference in the Texture oftheir Parts, by which they are difpos’d to reflect the Light with this or that Modification. 16. Dr. Hook, inhis Micograpbia, fays, The Phantafm of Colours is caus’d by the Senfation of the oblique or uneven Pulfe of Light, and that this is capable of no more Varieties than two, VA VA VA which arife from the two Sides of the oblique Pulfe ; fo that there is in reality but two fimple Colours, Yellow and Blue ; from the Mixture of which, and a due Proportion of Black and White, (that is, Darknefs and Light) all Colours may be produc’d. 17. But this Phenomenonof Nature, Colour, having long perplex’d Philofophers to account for the Difcoveries relating thereto, the incomparable Sir [aac Newton tound, by two Experiments on Prifms, that there is a great Deformity in the Rays of Light, and that hereby x of Colours may be unfolded: The ‘s therefore, according to his iments, are contain’d in the following Propofitions : That Light confifts of an infinite Number right-lin’d and parallel; but of difes of Refrangibility, when meeting rent Medium. Light being receiv’d on a refraéting Surface in a dark Place, is not wholly refracted, bur fplit, as it were, anddiffus’d into feveral little Rays, fome of whichare refraéted to the extreme Points, and others to the intermediate Points ; i. e. thofe Particles of the Light which are moft minute, are diverted the moft eafily and moft confiderably ofall others, by the Action ofthe refracting Surface, out oftheir rectilineal Courfe ; andthe reft, as each exceeds another in Magnitude, fo it is turn’dout ofits Right Line with more confiderably. Difficulty, and lefs Noweach Rayof Light, as it differs from another in its Degree of Refrangibility, fo likewife it differs from it in Colour: This is warranted by numerous Experiments. Thofe Particles which are moft refracted, are found to conflitute a Ray of a Violet Colour ; i.e. in all Probability, the moft minute Particles of Light, thus feparately impell’d, excite the fhorteft Vibration in the Retina, which are thence propagated by the folid Fibres of the Optick Nerves into the Brain, there to excite the Senfation of Violet Colour, as being the moft dufky and languid ofall Colours. Again : Thofe Particles whichare the leaft refracted, conftitute a Radiolus, orlittle Ray, of a Red Colour; i. ¢. The largeft Particles of h Ray, according to its Degree of Light excite the longeft Vibrations in the Retina, fo as to excite the Senfation of Red Colour ; the moft refrangible aplet Blue: Thei ate pro- from Scarlet to Yellowi/h, then to light and fo to Bl 4. The Colours arifing from the different Colour, the brighteft and moft vivid of all Colours. Authors diftinguifh three general Kinds of Red ; one bordering on the Blue, as Columbine or Dove Colour, Purple and Crimfon, &c. another bordering on Yellow, as Flame Colour, and Orange. Degrees of Refrangibility of Light, are not only the more noted Colours of Red, Yellow, Green, Blue; but alfo all the intermediate Colours of Red to Yellow, of Yellow to partakes neither of the one northe other, and (fuch as the Sun’s Light aps), containing all thofe Degrees of Refranity, is confequently made up of all the pale Yellow. al’s change Red into Violet, or Purple ntion’d Colours. - or LHomogeneal Colours, are fuch as are produc’d by Homogeneal Lightsor Rays, lour. Terreftrial and Sulphureous Matters become which have the fame Degree of Refrangibility; and mix’d Colours are fuch as are produc’d by Rays ofdifferent Reftangibility. 7. Rays ofthe fame Refrangibility produce the fame Colour ; which Colour is not alterable by repeated Refractions, but only made more firong or faint, as the Between thefe Extremes isa Medium, which this is what we properly call Red. into R Acids turn Black, Violet, and and Red into Yellow; and Yellowintoa very and Yellow into Feuillemort or Dead-leaf Co- Red by extreme Heat ; and fome at length Black, as we fee in Brick, Red Bole, Red Chalk, Slate, Pumice, &c. which, whenvitrify’d by J a Burning-glafs, become Black. Lobjters become Red by a moderate Fires and by violent one, Black Rays are united or ty than the reft. hefe Thi farther : by Experience, that Rays or compos’d ofParticles very diffimilar to each other; mM, as ‘it is highly probable, r, and others lefs: For a Ray of ificial 5 An Acid Spirit, as Le 4é Solution of beautiful Red; Alkal; Blue; and fi trating the reddeft W from it its Red Colour. Monf. dela Hire has obfery That a very Luminous Body, feen thre always appears Red ; as the Sun fhining through a black Clc 5 2 : That manyother People who fe Colours perfectly well, yet have no Idea of VA or thofe confifting of Homogeneal Light, as to the Species or Appearance of the Colour, other Particles being in like manner feparated, according to their refpective Mages, into little Rays, excite the intermeVibrations, and by this means occafion fations of the intermediate Colours ; much in the fame manner as the feveral Vibrations of the Air, according to their refpective Magnitudes, excite the Senfations of different Sounds. We may add to this, That not only the more diftinét and notable Colours of Red, Yello Blue, &c. do thus take their Rife from the different Magnitude and Refrangi- bility of the Rays; but alfo the intermediate but notas to the Premanencyand Immutability of it. Thus Yllow and Blue mix’d, make Green ; Red and Yellow being mix’d, make Orange ; Ora and yellowifh Green mix’d, make And in the general, any two Colours being mix’d, which in the Series of thofe generated by the Prifmare not too far apart, that Colour will refult from this Mixture, which is found in the Mid-way between themin the faidSeries : but thofe will not do fo that are fituated at too great a Diftance. Indeed, the more any Colour is compounded, the lefs perfect andvividit is; they may be egrees or Teints of the fame Colour ; as of diluted and weaken’d, till they ceafe, by too Yellow up to Green, of Red down to Yéllow, &c. Farther : The Colours of thefe little Rays, not being anyadventitious Modification thereof, but connate, primitive and neceffary Properties, as confiiting, in all Probability, in the Magnitudes of their Parts, muft be perpetual and immutable, 7. e. cannot be changed by any fraction or Reflection, or any Moition whatfoever. is is confirm’d by abundance of Experiments ; all Endeavours having been us’d, after feparating a colour’d Ray trom thofe of other Kinds, to change it into fome other Colour by repeated Refraétions; but to no Effect. Indeed, it is poffible to effect apparent Tranfions of Colours, . where there is an iblage or Mixture of Rays of different 3 the component Colours never aparing in their natural Hue in fuch Mi tures, but always allay’d and temper’d with each other : whence a middling KindofColour does refult ; which may be feparated into the component ones by Refraction ; andthofe, after Separation, being remix’d, returnto theirformer Colour. Hence the Tranfmutations of Colours, by ixing thofe ofdifferent Kinds, are not real, Appearances or Deceptions of the Sight ; for the Rays being again fevered, exhibit the fame Colours as they did ac firft: is Blue and Yellow Powders being mix’d, t Green to the naked Eye; yet hav ° y Alteration, when view’d through a Microfcope, the Blue and Yellow Particles will appear d much Compofition. Alfo, Colours not like to any of thofe of Homogeneal Light, may be preduc’d by Compofition. The moft extraordinary Compofition is that of Whitenefi: for all the primary Colours abovemention’d, are required to this; and thofe muft be mix’d in a certain Degree. Hence it is that White is the ordinary Colour of Light; Light being nothing elfe but a confus’d Affemblage of Rays of all Colours. If the Rays of different Colours do thus begin to be feparated by one Refraction of one fingle Subftance 5 that Separation is much promoted, fo as even to become fenfible to the Eyeby a double Refraction. This is obfervable in the two Surfaces of any Glafs ; provided thofe Surfaces are not parallel : but of all others, it is moft fenfible in the two Faces of a triangular Prifm 5 the Phenomena whereof, as they are the Touch-ftone of all Theories of Colours, and as they contain the Foundation of what is here deliver’d, I fhall lay downas follows : 1. The Rays of the Sum tranfmitted through a triangular Prifm, exhibit an Image of va~ rious Colours (the chief whereof are Red, Yellow, Green, Blue, and Violet) on the opfite Wall. The Reafon is, that the differently-colour’d Rays are feparated by Refraction: Forthe Blue Rays, v.g. mark’d with the dotted Line, which begins to be feparated from the reft 1 Hence two Kinds of Colours do arife; the e Orig 1 Si » produc’d by home y Rays that have e of Refrangibility, and the fame of their Parts; fuch are Red, Blue, a F by the firft Refraétion dd (in the Figure) of the Side ¢ a of the above Prifm abe. 2. That Image is not round, but oblong ; its Length, when the Prifm is an 60 or 65 Degrees, being t |