CO L
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COL
any natural Body, or refracted thro' any
foever obfcure. „
The reafon is, that Colours are not Modifications anhng from Refraftion, or Rcfleftion, but immutable Properties ; and fuch as belong to the Nature of the Rays.
5. An Affemblage of all the Kinds of colour' A Rays, col- leBed either by feveral Prifms, by a convex Lens, or a con- cave Mirror, or in any other manner, form what we call
Whitenefs ; yet each of thefe, after Decuffation, becoming feparated, again exhibits its proper Colour : For, as the Ray was white before its Parts were feparated by Refraaion ; lo, the Parts being remix'd, it becomes white again : And co- lour'd Rays, when they meet together, don't deftroy one another, but are only intcrfpers'd.
Hence, a red, green, yellow, blue, and violet Colour, be- ing mix'd in a certain Proportion, appear whitiih ; i. e. are of fuch a Colour as arifes from white and black mix'd toge- ther : and if there were not fome Rays abforb'd and loft, wou'd be plainly white. In like manner, if a Paper cut into a Circle, be ftain'd with each of thofe Colours, feparately, and in a certain Proportion ; then fwiftly turn'd round its Centre, fo as the Species of Colours be mix'd together in the Eye, by the briiknefs of the Motion ; the feveral Colours will difappear, and the whole Paper appear of one continued Colour ; which will be a Mean between white and blacky
6. If the Rays of the Sun fall very obliquely on the in- ner Surface of a Prifm, thole that are reflected will be vio- let; thofe tranfmitted, red.
For, the Rays were coloured before any Separation ; and by how much rhcy are the more refrangible, by fo much they are the more eafily reflected ; and by that means are feparated.
7. If two hollow Prifms, the one fill'd with a blue Fluid, the other with a red one, be join'd together, they will be opake ; tho each, apart, be tranfparent.
For, the one tranfmitting none but blue Rays, and the other none but red ones ; the two together will tranfmit none at all. t
8. All natural Bodies, efpecially white ones, view d thro a Prifm held to the Eye, appear fimbriated, or border'd, on one fide with red and yellow, and on the other with blue and violet.
For, thofe Fimbriae are the Extremes of entire Images, which the Rays of any kind, as they are more or lefs refrac- ted, would exhibit, nearer, or at a greater diftance from the real Place of the Object
9. If two Prifms be fo placed, as that the red of the one, and the purple of the other meet together, in a Paper fit for the purpofe, incompafs'd with darknefs 5 the Image will ap- pear pale : and if view'd thro' a third Prifm, held to the Eye at a proper diftance, it will appear double ; the one red, the other purple.
In like manner, if two Pouders, the one perfectly red, the other blue, be mix'd ; any little Body cover'd pretty deeply with this Mixture, and view'd thro' a Prifm held to the Eye, will exhibit a double Image, the one red, and the other blue : in regard, the red and purple, or blue Rays, are feparated by their unequal Refraction.
10. If the Rays tranfmitted thro' a convex Lens, be re- ceiv'd on a Paper ere rhey meet in a Focus ; the Confines or Boundary of Light and Shadow will appear ting'd with a red Colour : but if they be receiv'd beyond the Focus, with a blue one.
Becaufe, in the firft Cafe, the red Rays, being fomewhat more refracted, are the higher ; but, in the fecond, after Decuffation in the Focus, the blue ones.
Laftly, if the Rays about to pafs thro' either fide of the 'Pupilla, be intercepted by the Interpofition of 'any opake Body near the Eye; the Extremes of Bodies, placed as if view'd thro' a Prifm, will appear ting'd with Colours ; tho thofe not very vivid.
For then, the Rays tranfmitted thro' the reft of the Pupil, will be feparated by Refraction into Colours ; without being diluted with the admixture of the intercepted Rays, which would be refracted in a different manner.
And hence it is, that a Body view'd thro' a Paper piere'd with two Holes, appears double, and alio tinged with Colours.
Of the Colours of thin Elates. As Rays of different Colours are feparated by the Refraction of Prifms, and other thick Bodies ; fo are they feparated, tho in a different man- ner, in the thin Lamella, or Plates of any pellucid Matter, v.g. the Bubbles rais'd in Water, thicken'd by Soap, ££?<;. For all Lamella, under a determinate thicknefs, tranfmit Rays of all Colours, without reflecting any at all : but, as they increafe in thicknefs, in arithmetical Proportion, they begin to reflect, firft blue Rays ; then, in order, green, yel- low and red, all pure; then, again, blue, green, yellow, red, more and more mix'd and diluted ; till, at length, ar- riving at a certain thicknefs, they reflect Rays of all Colours perfectly intermix'd, viz,, white.
But, in whatfoever Part a ilendcr Lamella reflects any one
one, in a Place how- Colour, v. g. blue 5 in that Part it always tranfmits the od pofite Colour, v . g, red, or yellow. °~
'Tis found, by Experiment, that the difference of Colour of a Plate does not depend on the Medium that incompafTe it ; but the degree of Vividnefs does : ceteris paribus th Colour will be more vivid, if the denfer Medium be incom pafs'd with the rarer. A Plate, ceteris paribus, refleefs more Light as it is thinner ; as far as a certain Degree of thin. nefs, beyond which it reflects no Light at all. ' In Plates whofe thickneffes increafe in an arithmetical Pro. portion of the natural Numbers 1,2,3,4,5, ££?c. If the firft or thinneft, reflect any homogeneal Ray, the fecond will tranfmit it; the third, again, will reflect it : And thus is the fame Ray alternately reflected and tranfmitted ; /. e, the Plates correfponding to the odd Numbers, 1, 3, 5, 7 ^ c will reflect the fame Rays that thofe correfponding to the even ones, 2, 4, 6, 8, &c. tranfmit.
Hence, an homogeneous Colour in a Plate, is faid to be of the firft Order ; if the Plate reflect all the Rays of that Colour. In a Plate whofe thinnefs is triple the firft, it i 8 faid to be of the fecond Order : in another, whofe thinnefs is five times that of the firft, it is faid to be of the third Order, &c.
A Colour of the firft Order is the moll vivid of any; and fuccefltvely, the Vividnefs of the Colour increafes, as the Quantity of the Order increafes : The more the Thicknefs of the Plate is increas'd, the more Colours it reflects ; and thofe of more different Orders. In fome Plates, the Colour will vary as the Pofition of the Eye varies ; in others it is permanent.
Colours of Natural Bodies. Bodies only appear of dif- ferent Colours, as their Surfaces are difpos'd to reflect Rays of this or that Colour alone ; or of this or that Colour more abundantly than any other : hence Bodies appear of that Co- lour which arifes from the Mixture of the reflected Rays. See Body.
All Natural Bodies confift of very thin, tranfparent Z.a- melUe ; which, if they be fo difpos'd, with regard to each other, as that there happen no Reflections or Refractions in their Interftices, thofe Bodies become pellucid, or tranfpa- rent : but if their Intervals be fo large, and thofe filled with fuch Matter ; or, fo empty, (with regard to the Denfity of the Parts themfelves) as that there happen a Number of Reflections and Refractions within the Body ; the Body, in that Cafe, becomes opake. See Transparency, and 0-
PACITY.
The Rays which are not reflected from an opake Body, penetrate into it ; and there, fuffering innumerable Reflec- tions and Refractions, at length, unite themfelves to the Par- ticles of the Body it felf.
Hence, an opake Body grows hot the fooner, as it reflects Light lefs copioufly : Whence, we fee why a white Body, which reflects almoft all the Rays that ftrike upon ir, heats much more flowly than a black one, which reflects fcarce any. See Heat, Blackness, £f?c.
To determine that Constitution of the Surface of Bodies wherein their Colour depends ; it muft be obferv'd, that the fmalleft Corpufcles, or firft Particles whereof Surfaces are made up, are m oft thin and tranfparent; and feparated by a Medium of a different Denfity from the Particles them- felves. In the Surface, then, of ev,ery coloured Body, are innumerable fmaller thin Plates, correfponding to thofe of Bubbles : wherefore, what has been oblerv'd of thofe, may be underftood of thefe.
Hence we gather, that the Colour of a Body depends on the Denfity and Thicknefs of the Parts of the Body, be- tween the Pores of the Surface : that the Colour is more vivid and homogeneous, as the Parts are thinner ; that cete- ris -paribus, the faid Parts are the thickeft when the Body is red ; and the thinneftwhen violet : That the Parts of Bodies are ufually much denfer, than the Medium contain'd in their Interftices ; but that in the Tails of Peacocks, in fome Silks, and generally, in all Bodies whofe Colour varies according to the Situation of the Eye, it is lefs : and that the Colour of a Body is the lefs vivid to the Eye, as it has a denfer Medium -^thin its Pores.
Now, of the feveral opake Bodies, thofe confifting of the thinneft Lamella are black ; thofe confifting either of the thickeft LamellcS, or of Lamella very different from each other in thicknefs, and on that account fitted to reflect all Co- lours, as the Froth of Water, &c. are white .- Thofe, again, confifting of Lamellce, moft of which are of fome interme- diate thicknefs, are blue, green, yellow, or red ; inafmuch as they reflect the Rays of that particular Colour, much more copioufly than that of any other Colour ; moft ofwhich laft they either abforb and extinguifti, by intercepting them, or elfe tranfmit.
Hence it is, that fome Liquors, v.g. an Infufion of Lig- num Nephriticum, appear red, or yellow, if view'd by reflec- ted Light ; and blue by tranfmitted Light : and gold Leaves yellow in the former Circum fiances, but green or blue in the latter.
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