iminately, without feparating them from their denfity, or the magnitude of their compoother. nent particles. He alfo conftructed a table, whereparticular colour was expreffed in parts of an inch in the thickness of a plate neceffary to reflect any III. Of the SEPARATION of the RAYS of HT by REFLECTION or TRANSMISSION. The foundation of a rational theory of cobeing thus laid, it next became natural to e, by what peculiar mechanifm in the ftrucf each particular body it was fitted to reme kind of rays more than another? This ac Newton attributes to the dentity of thefe Dr Hooke had remarked, that thin tranffubftances, particularly water and foap into bubbles, exhibited various colours acg to the thinnefs; though, when they have fiderable degree of thickness, they appear lefs; and Sir Ifaac himfelf had obferved, s he was compreffing two prifms hard to, in order to make their fides (which hapto be a little convex) to touch one another, place of contact they were both perfectly arent, as if they had been but one continued of glass: But round the point of contact, the glaffes were a little feparated from each rings of different colours appeared. To obferve more nicely, the order of the rs produced in this manner, he placed a glafs whofe convexity was finall, upon a plain and preffing them together, the colours foon emerged, and appeared diftinct to fiderable diftance. Next to the pellucid il pot, made by the contact of the glaffes, ded blue, white, yellow, and red. The vas very little in quantity, nor could he difny violet in it; but the yellow and red were copious, extending about as far as the white, or 5 times as far as the blue. The next circuit diately furrounding thefe, confifted of violet, green, yellow, and red: all thefe were copind vivid, except the green which was very in quantity, and feemed more faint and dihan the other colours. Of the other 4, the was the leaft in extent: and the blue les the yellow or red. The 3d circle of colours urple, blue, green, yellow, and red. In this urple feemed more reddish than the violet in ormer circuit, and the green was more conous; being as brisk and copious as any of ther colours, except the yellow; but the red 1 to be a little faded, inclining much to purThe 4th circle confifted of green and red; of these the green was very copious and liveclining on the one fide to blue, and on the to yellow; but in this 4th circle there was er violet, blue, nor yellow, and the red was imperfect and dirty. All the fucceeding cogrew more and more imperfect and dilute, iter 3 or 4 revolutions they ended in perfect enefs. .) The colours being thus found to vary acng to the different diftances of the glass-plates each other, Sir Ifaac thought that they eeded from the different thickness of the plate r intercepted between the glaffes; this plate being, by the mere circumftance of thinnefs icknefs, difpofed to reflect or tranfmit this or particular colour. From this he concluded, the colours of all natural bodies depended on L. V. PART II. divided into 1,000,000 parts. (11.) Sir Ifaac Newton, purfuing his difcoveries concerning the colours of thin fubftances, found that the fame were alfo produced by plates of a confiderable thickness. There is no glafs or fpeculum, he obferves, how well polifhed foever, but, befides the light which it refracts or reflects regularly, fcatters every way irregularly a faint light; by means of which the polished surface, when illuminated in a dark room, by a beam of the fun's light, may eafily be feen in all positions of the eye. It was with this fcattered light that the colours in the following experiments were produced. (12.) The fun fhining into his darkened chamber through a hole in the fhutter one inch wide, he let the beam of light fall perpendicularly upon a glafs fpeculum concave on one fide and convex on the other, which was quickfilvered over. Then, holding a quire of white paper at the centre of the fphere to which the fpeculum was ground, in fuch a manner as that the beam of light might pafs through à little hole made in the middle of the paper, to the fpeculum, and thence be reflected back to the fame hole, he obferved on the paper 4 or 5 concentric rings of colours, like rainbows furrounding the hole, very much like those which appeared in the thin plates above mentioned, but larger and fainter. Thefe rings, as they grew larger and larger, became more dilute, fo that the fifth was hardly vifible; and yet fometimes, when the fun fhone very clear, there appeared faint traces of a fixth and seventh. (13.) We have obferved above, that the thin plates ufed in the former experiments reflected fome kind of rays in particular parts, and transmitted others in the fame parts. Hence the coloured rings appeared varioully difpofed, according as they were viewed by tranfmitted or reflected light; that is, according as the plates were held up between the light and the eye, or not. For the better understanding of this, we fubjoin the following table, wherein on one fide are mentioned the colours appearing on the plates by reflected light, and on the other fide thofe which were oppofite to them, and which became vifible when the glaffes were held up between the eye and the window. The centre, when the glaffes were in full contact, was perfectly tranfparent. This fpot, therefore, when viewed by reflected light, appeared black, because it tranimitted all the rays; and for the fame reafon it appeared white when viewed by tranfinitted light. (14.) TABLE of COLOURS by REFLECTED LIGHT. Blue Green TRANSMITTED LIGHT. Black Violet Blue White REFLECTED LIGHT. Yellow Red Purple Blue Green Yellow Red Green Red TRANSMITTED LIGHT. Violet Blue Green Yellow Red Bluish green Red Bluish green Greenjin blue Red (15.) In comparing the rings produced by tranf mitted light, with thofe produced by reflected, the white is found oppofed to the black, the red to the blue, the yellow to the violet, and the green to a colour compofed of red and violet; in other words, the parts of the glafs, that, when looked at, are white, appear black on looking through the glass; and, on the contrary, thofe which appear black in the firft inftance, appear white in the fecond; and fo of the other colours, which will be more readily comprehended by confidering this figure, where A B, C D, fig. 1. plate LXXIV. reprefent the glaffes which touch at E; the black lines traced between them are the dif tances between the two furfaces, at different diftances from the centre, each diftance anfwering to a coloured ring: the colours written above are thofe feen by reflected light, thofe underneath, are the colours exhibited by tranfmitted light. (16.) Newton has fhown, that the rays of any particular colour are difpofed to be reflected, when the thickneffes of the plate of air are as the numbers 1, 3, 5, 7, 9, 11, &c, and that the fame rays are difpofed to be tranfmitted at the interme, diate thickneffes, which are as the numbers o, 2, 4, 6, 8, 10, &c. The places of reflection or trarf miflion of the feveral colours in a feries, are fo near each other, that the colours dilute each other by mixture; whence the number of fries, in the open day light, feldom exceeds 7 or 8. But if the fyftem be viewed through a prifin, by which means the rings of various colours are feparated,. pccording to their refrangibility, they may be feen on that fide towards which the refraction is made, fo numerous that it is impoffible to count them. Or, if in a dark chamber the fun's light be feparated into its original rays, by a prifm, and a ray of one uncompounded colour be received upon the two glaffes, the number of circles will become very numerous, and both the reflected and tranfinitted light will remain of the fame colour as the original incident ray. to 8, and confequently the interval fes, at fimilar circles, as caufed by t diums, are as about 3 to 4; that is, of refraction out of water into air. (18.) As the colours produced by buke in foap-water, are commonly too muc by the external air to admit of any cra vation, it is neceffary to cover the bible v clear glafs, in which fituation the fo pearances take place: the colours m the top of the bubble, and as it grows th the fubfidence of the water, they dilate parallel to the horizon, which defcend vanifh fucceffively, at the bottom. T gence continues till the water at the of the bubble becomes too thin to light, at which time a circle of an ite nefs appears at the top, which for fometimes to of an inch in breadth, b bubble breaks. Reckoning from the ha tral pot, the reflected colours are the fucceffion and quality, as thofe product plate of air; and the appearance of the viewed by tranfmitted light, is br the plate of air, in like circumftances (19.) The colours of the rings prod reflection by the thick plates, followed t of thole produced by transmition thin ones, and by the analogy of ther; na with thofe produced from the the pa Ifaac Newton concluded that they ced in a fimilar manner. For he f the quick filver was rubbed off from t the fpeculum, the glafs alone would fame rings, but much more faint the that the phenomenon did not depend -quickfilver, except in as far as, by reflection at the back of the glafs, it light of the coloured rings. He alo fpeculum of metal only, produced rings; which made him conclude, th not arife from one furface only, but a the two furfaces of the plate of glas fpeculum was made, and upon the th the glass between them. (20.) From thefe experiments and it will be eafy to underftand the New ry of colours. Every fubftance in to be tranfparent, provided it is made thin. Gold, the mott denfe fubfta when reduced into thin leaves, train green light through it. If, therefore, any body, gold, for inftance, to be perfectly tranfparent, it is evident a vaft number of plates, fo thin as t reateft part of the rays will pals thro per plates, and when they lule the reflected from the under ones. The (17.) This experiment thews, that in any feries, the circles formed by the lefs refrangible rays exceed, in magnitude, thofe which are formed by the more refrangible; and, confequently, that in any feries, the more refrangible rays are have the fame number of plates to p reflected at lefs thicknefies than thofe which are which they had penetrated before; a jefs refrangible, If we apply water to the edges cording to the number of the p of the glass, it will be attracted between them; which they are obliged to pass, the ch and, filling all the intercedent fpace, it will be of this or that colour, juft as the rings a come a thin plate of the fame dimensions as that appeared different in the experie which before was conftituted of air: in this cafe, plates, according to their difatce fr the cireur rings grow lets, and the colours faint- ther, or the thicknefs of the plate fa er, but not varied in fpecies. They become con- them. 4sted in diameter, nearly in the proportion of 7 (21.) Mr Edward Huffey Delar a le. gory in his Experimental Inquiry into the the changes of colours in opaque and criour. He endeavours to confirm it by a numxperiments on the infufions of flowers of it colours; but his ftrongeft arguments be thofe derived from the different tinges o glafs by metallic fubftances. Here he $, that each metal gives a tinge according pecific denfity; the more denfe metals prothe lefs refrangible colours, and the light sthofe colours which are more easily reGold, which is the denfeft of all metals, sa red colour to glafs, whenever it can be into particles fo minute, that it is capable ig mixed with the materials of which glafs e. Lead, the metal whose density is next er to that of gold, affords a glafs of the cof the hyacinth; a gem whofe diftinguishing teriftic is, that it is red with an admixture ow, the fame colour which is ufually called Silver, next in centy to lead, can only de to communicate a yellow colour to glais. filver laid upon red hot glafs likewife tinges ow. The only colour produced by copper en, and iron the metal next in denfity to T, imparts a blue colour. ) Thefe are the principal of Mr Delaval's ents in favour of Sir Ifaac Newton's theory lours being formed by denfity. Dr PRIESToo hath mentioned fome which deferve at "This great difcovery, (fays he) conng the colours of bodies depending on the nefs of the fine plates which compofe their zes, of whatever denfity thefe plates may be, e been fo happy as to hit upon a method of rating and confirming by means of electrical fions. A number of thefe being received on urface of any piece of metal, change the coof it to a confiderable diftance from the fpot hich they were difcharged; fo that the whole dar fpace is divided into a number of concenrings, each of which confift of all the prifc colours, and perhaps as vived as they can roduced in any method whatever. 4.) Upon fhewing thefe coloured rings to Mr on, I was agreeably furprised to find, that he likewife produced all the prifmatic colours all the metals, but by a different operation. extended fine wires of all the different metals the furfaces of pieces of glass, ivory, wood, and when the wire was exploded, he always d them tinged with all the colours. They not difpofed in fo regular and beautiful a manas in the rings I produced, but they equally onftrate that none of the metals thus exploded overs the leaft preference to one colour more n to another. In what manner thefe colours formed it may not be eafy to conjecture. In Canton's method of producing them, the meor the calcined and vitrified parts of it, feem be difperfed in all directions from the plate of lofion, in the form of spheres of a very great iety of fizes, tinged with all the variety of coirs and fome of them fmaller than can be difetly feen by any magnifier. In my method of king thefe colours, they feem to be produced a manner fimilar to the production of colours on feel and other metals by heat; i. z. the tur face is affected without the parts of it being removed from their places, certain plates or lamine being formed of a thicknets proper to exhibit the refpective colours." SECT. IV. EXPERIMENTS and HYTOTHESES of Mrs MAZEAS and DU TOUR. (15.) However plaufible the Newtonian theory of colours may be, it has been oppofed by the Abbe MAZEAs, and M, Du Tour. The former of thefe gentlemen, endeavouring to give a very higa polish to the flat fide of an object glafs, by rub bing it against another piece of flat and imootir glafs, found that after this friction they adhered firmly together, and that there were produced between them colours fimilar to thofe produced in Newton's experiments, by pretting together a convex and plane glats. (26.) In the Abbe's experiments, red and green curve lines were obferved to arife, and increase in number, in proportion to the adhetion of the glailes, between which they were produced. Sometimes they were mixed without any order, and fometimes difpofed in the manner of concen tric circles, or ovals; in the middle of which, there appeared a small plate of the fame figure, exactly like a plate of gold between the glaffes. In the centre of this plate, there was often a dark fpot, which abforbed all the rays of light except the violet. To produce the preceding appearance, the glaffes required to be well wiped and warmed before the friction: if they were feparated suddenly, and again put together, the colours appeared without friction, but unlefs in this cale, they could not be produced by preffure, the fiction no doubt being neceflary to bring the glaffes near enough each other. (27) The Abbe next procpred two prifms with fmall refracting angles, and joining them to as to form a parallelopiped, by friction the colours appeared with great luftre at the points of contact, owing to the feparation of the rays by the prifm. In this experiment, differently coloured ovals appeared, but the gold coloured plate was much whiter, and only yellow about its edges. It had a black spot on the middle, and was bordered by a deep purple; a fmall quantity of violet could be perceived by the help of a lens with a weak light at the extremity of the purple; next facceeded blue, orange, red tinged with purple, light green, and faint purple. The other rings appeared to the naked eye to confift of nothing but faint red and green, and fo fhaded, that it was not ealy to mark their terminations. (28.) By heating the glaffes while yet in contact, the colours difappeared; but when they were fuffered to cool, the colours returned by des grees to their former place. The Abbe at first believed, that thefe colours were produced by a thin plate of air interpofed between the glafies, ire the manner fuppofed by Newton; but the re markable difference between the circumftances attending thofe produced by the flat plates, and thofe produced by the object glaffes of Newton, convinced him, that the air was not the caufe of the appearance. The colours produced between the flat plates vanifhed at the approach of flame, Lecea but |