PHYSICS. green and puple made the purple appear more dirty. The blue and purple made the purple more delightful, and far more deep. The red parts of the marbled paper view'd with the yellow, appear'd of a red, far more like fcarlet, than without it. But the fineness or coarfenefs of the paper, its being carefully or flightly colour'd, with many other circumftances, may fo vary the events of fuch experiments, that they ought to be carefully repeated, before any conclufion is drawn from them. Fig. 1. 4. The triangular prifmatic glafs being a very commodious inftrument to fhew the nature of colours, we thought proper to obferve the feveral reflections and refractions which the rays of light undergo in falling upon or paffing through it. And this we imagined might be beft done, not in an ordinary enlighten'd room, where even the curious have left particulars unregarded; but in a darken'd one, where, by placing the glass in a convenient pofture, the various reflections and refractions may be distinctly observ'd; and where it may appear what rays are untinged, and which they are that paint upon the bodies whereby they are ftopt, either the primary or fecondary iris. In pursuance hereof, we obferv'd in a darken'd room four reflections and three refractions, that were afforded us by the fame prifin. And notwithstanding the rules of catoptrics and dioptrics, we thought it would not be amifs to find, by covering fometimes one part of the prifm, and fometimes another, and obferving where the light or colour vanished thereon, by which reflection and by which refraction each of the feveral places where the light falling, or paffing through the prism, and appearing either pure or ting'd, gave the phenomena. But because these and other particulars which we obferv'd, would be tedious, and not fo intelligible to deliver in words, I refer to the scheme, where all of them may be taken in at one view. PPP an equilatero-triangular cryftalline prifm, one of whofe edges P is placed next the fun. A Band as, two rays from the fun falling on the prifm, at BB; and thence partly reflected towards C and y, and partly refracted towards D and ♪. BC By, thofe reflected rays. part BD and B, thofe refracted rays, which are again partly refracted towards E and, and there reprefent an iris 12345, denoting the five fucceeding colours, red, yellow, green, blue, and purple, and ly reflected towards F and . D and F, and, thofe reflected rays, which are partly refracted towards G and colourlefs, and partly reflected towards H and 0. FH and, thofe reflected rays, which are refracted towards I and 1, and there represent another fainter iris; the colours of which are contrary to the former 54321, fignifying purple, blue, green, yellow, red; fo that the prifin in this pofture reprefents four rain-bows. 5. We obferv'd in a room not darken'd, that the prifmatic iris might be reflected, without lofing any of its feveral colours, not only from a plain looking-glafs, and from the fmooth furface of fair water, but alfo from a PHYSICS. concave looking-glafs; and that refraction did as little deftroy thofe colours, as reflection. For by the help of a large double-convex glafs, thro' which we refracted the fun's rays, we found that tho' one part of the iris might be made to appear either beyond or on this fide of the other parts thereof; yet the fame vivid colours would appear in the disorder'd part as in the other. And by covering that fide of the prifm obverted to the fun, with an opake body, wherein only one fmall hole was left for the light to pass thro', having reduced the prifmatic iris, caft upon white paper, into a very narrow compafs, and view'd it thro' a microfcope, the colours appear'd the fame, as to kind, they did to the naked eye.* 6. It may afford matter of fpeculation, that as the colours of outward objects brought into a darken'd room, fo greatly depend, in their vifibility, upon the dimnefs of the light they are there beheld by, that the ordinary light of the day being freely let in upon them, they immediately difappear; fo our experiments have fhewn, that as to the prifmatic iris, painted on the floor by the beams of the fun paffing thro' a triangular glafs, tho' the colours of it appear very vivid, even at noon-day and in fun-fhiny weather, yet by a more powerful light they may be made to difappear. For having fometimes taken a large metalline concave fpeculum, and therewith caft the converging beams of the fun upon a prifmatic iris, which I had caused to be projected upon the floor; I found that the overpowerful light made the colours of the iris vanifh: and if the light were fo reflected as to cross but the middle of the iris, the colours in that part only The more general phenomena of the prifm, as obferv'd by the illuftrious Sir Isaac Newton, are, in fhort, as follow. 1. The rays of light paffing thro' the prifm, paint an image of various colours on the oppofite wall; the principal whereof are red, yellow, green, blue, and violet. 2. This image is not round, but the angle of the prifm being of 60 or 65 degrees, about five times longer than 'tis broad; the reafon whereof was unknown, till Sir Isaac Newton found it owing to the different refrangibility of the rays. See Philof. Tranfatt. N° 80. 3. The rays which exhibit the yellow colour, are turn'd more out of their ftrait course, than those which make the red; thofe which make the green, more than thofe which make the yellow, &c. and thofe which make the violet, most of all. 4. If the prifm thro' which the rays are tranfmitted, be turn'd about its axis, so that the red rays, the yellow, the green, &c. may fall thro' a fmall hole, in order, upon VOL. II. another prifm, placed about twelve feet 5. The colours of the coloured rays, 6. The colours of the coloured rays remain unchanged in paffing thro' an enlighten'd space, in mutually croffing each other, in the confine of shadow, and in being reflected from any natural bodies in a place otherwise dark. " 7. All the coloured rays collected together by feveral prifms, by a glass lens, I or PHYSICS. only became invifible; thofe parts of the iris that were on the right hand and on the left of the reflected light, continuing to exhibit the fame colours as before. Objects view'd in different kinds of light. 7. I have fometimes thought it worth while to take notice, whether the colours of opake bodies might not appear to the eye fomewhat diverfify'd, as well by the nature of the lucid body that fhines upon them, as by the difpofition of the fuperficial parts of the bodies themselves, and the pofition of the eye with regard to the object and the light: I have obferv'd a manifeft difference in fome kinds of colour'd bodies view'd by day-light, and afterwards by the light of the moon. Several fheets of the colour'd paper that had been view'd in the fun-fhine, being look'd upon at night by the light of a pretty large candle, the changes obferv'd therein were thefe. The yellow feem'd much fainter than in the day, and inclinable to a pale ftraw-colour. The red feem'd little chang'd, but reflected light more ftrongly than the other colours. A fair deep green A fair deep green look'd upon by itself, feem'd to be a dark blue; but view'd together with a dark blue, appear'd greenish; and compared with a yellow, appear'd more blue than at firft. The blue look'd more like a deep purple or murrey, than it had done in the day-light. The purple feem'd very little alter'd. The red compared with the yellow, made the latter look almost like brown cappaper. 8. But to fatisfy ourselves whether the light of the candle were not unfincere, or ting'd with a yellow colour by its fuel; we took a pretty thick cylinder of a deep blue glafs, and viewing the flame of the candle at a convenient diftance through it, we perceived it to look green, which is the colour that emerges from the compofition of opake bodies, one of them feparately blue, and the other yellow. And this perhaps may or a concave fpeculum, or by any other 8. If the fun's rays fall upon the internal 9. All natural bodies, especially white 10. Two prifms being placed together, 11. In like manner, if two powders, a gether, and laid thick upon fome minute body, this body will appear double thro' a prifm, and of two colours, red and blue. See Clark. Annot. in Robault. Ed. 3. p. 199. We are farther to confider, that by other means, light is found to be a body propagated in right lines; and that it spends about feven or eight minutes in paffing from the fun to the earth. "This was firft obferved "by Mr. Roemer, by means of the eclipfes of the fatellites of Jupiter. For thefe eclipfes, when the earth is between "the fun and Jupiter, happen about seven or eight minutes fooner than they ought "to do by the tables; and when the earth "is beyond the fun, they happen about "feven or eight minutes later than they ought to do the light of the fatellites having farther to go in the latter cafe "than in the former, by the diameter of "the earth's orbit." Newton. Optic. p. 252. 66 66 : be the chief reafon why a fheet of very white paper, view'd by candle- PHYSICS. light, is not eafily, at firft, diftinguifhed from a light yellow, or lemoncolour; white bodies, as we formerly obferved, being more of a fpecular nature, because tho', when unpolifhed, they exhibit not the fhape of the Juminary that shines on them; yet they reflect its light more fincere and undisturbed, by either fhades or refractions, than bodies of other colours do. 9. We took a piece of leaf-gold, and with the edge of a knife, lightly moiften'd, laid upon the edge of the gold, we fo faften'd it thereto, that it continued extended, like a little flag; which, being held very near the eye, and obverted to the light, appear'd fo full of pores, that it feem'd to have fuch a kind of tranfparency, as that of a fieve; but the light which paffed thro' thefe pores was in its paffage fo temper'd with fhade, and modify'd, that the eye difcerned no more a golden colour, but a greenish blue. And, for further fatisfaction, we, in the night, look'd upon a candle, thro' fuch a leaf of gold; and, by trying the effect of feveral proportions of distance betwixt the leaf, the eye, and the light, we quickly hit upon fuch a pofition for the gold, wherein the flame, view'd thro' the leaf, appeared of a greenish blue, as in the day-time. But the like experiment, try'd with a leaf of filver, did not fucceed. 66 tincture of 10. Druggifts have a wood they call Lignum Nephriticum; because Experiments the inhabitants of the country where it grows, ufe the infufion of it, with the made in fair water, against the stone in the kidneys: and, indeed, an Lignum Neeminent physician of my acquaintance, who has very particularly en- phriticum. quired into that disease, affures me, he found fuch an infufion one of the most effectual remedies he ever try'd, against that formidable disease. The most ancient account I have met with of this fimple, is given us by Monardes, who fays, "We have a thick, fmooth kind of wood, brought us from New Spain, which has long been used in the diseases "of the kidneys and urinary paffages. To prepare a medicine from "it, we steep its fhavings in clear water, and there fuffer them to "remain, till the whole liquor is drank up; but when it has thus "stood for half an hour, the water appears of a blue colour, which "gradually increases, the longer the infufion is continu'd; tho' the wood "itfelf be white." This wood will afford us an experiment, which, befides the fingularity of it, may greatly affift to difcover the nature of colours. The experiment, as we made it, is this. Take a handful of the thin flices of Lignum Nephriticum, and put them into two, three, or four pound of the pureft fpring-water; let them infuse there for a night, tho' a much shorter time may fuffice; decant the impregnated water into a clear glass vial; and if you then hold it directly between the light, and your eye, you will fee it tinged, (excepting the very top of the liquor, where a sky-coloured circle fometimes appears) of an almoft golden colour, if the infufion be not made too ftrong of the wood; for, in that cafe, it will, against the light, appear fomewhat dark and reddish, and requires to be diluted by the addition of fair water. But if you hold it from the I 2 light, PHYSICS. light, fo that your eye be placed betwixt that and the vial, the liquor will appear of a deep lovely blue colour; as will alfo the drops, if any lie on the outfide of the glafs. And thus far we have try'd the experiment, and found it to fucceed, even by candle-light. But if you hold the vial over-against your eye, fo that it may have a window on one fide, and a dark part of the room before it, on the other fide; you will find the liquor, partly of a bluish, and partly of a golden colour. And if, turning your back towards the window, you pour out fome of the liquor towards the light, and towards your eye; it will feem, upon coming out of the glafs, to be perfectly blue; but when it is fallen down a little way, the drops may appear partycolour'd, according as the rays of light more or lefs fully penetrate, and illumine them. If you take a bafon, about half full of water, and having placed it fo in the fun beams, that one part of the water may be illumined thereby, and the other part of it darken'd by the shadow of the brims of the bafon; and then drop of our tincture, made fomewhat ftrong, both into the fhaded and illuminated parts of the water, you may, by viewing it from feveral places, and by a little agitation of the water, obferve many pleasing phenomena. If a little of this tincture be poured upon a fheet of white paper, fo that the liquor may remain of fome depth upon it, you will perceive the adjacent drops to be partly of one colour, and partly of another, according to the pofition of your eye, with regard to the light: but if you pour off all the liquor, the paper will feem dy'd almost of a yellow colour. And if a fheet of paper, with fome of this liquor on it, be placed in a window, where the fun-beams may come freely to it; then, if you turn your back to the fun, and take a pen, or fome fuch flender body, and hold it a-thwart, betwixt the fun and the liquor, you may perceive, that the fhadow projected by the pen, upon the liquor, will not, all of it, be a common dark fhadow, but, in part, curiously coloured; that edge of it next the body, which makes it, being almost of a lively golden colour; and the remoter verge of a blue one. Thefe, and other phenomena, which I have obferved in this delightful experiment, many of my friends have beheld with wonder. And I remember an excellent oculift, finding, by accident, a vial full of this liquor, and having never heard of the experiment, continued apprehenfive, upon viewing it for a long time, that fome strange new diftemper had invaded his eyes. And, I confefs, the oddnefs of the phenomena made me very follicitous to find out the cause thereof. But tho' I am far from pretending to have difcovered it; yet my inquiries have enabled me to give some confiderable hints about it. In the first place, then, obferving that this tincture, if it were too deep, kept the colours from being fo lively, and their change from being fo difcernible; and finding alfo, that the impregnating virtue of this wood did, by being frequently infufed in fresh water, gradually decay, I conjectured, that the tincture afforded by the wood, muft proceed from fome fubtile parts, drawn out by the water; which, fwimming about therein, fo modify'd |