EXPERIMENT I.

Because that, according to the Conjectures I have above propos'd, one of the most General Causes of the Diversity of Colours in Opacous Bodyes, is, that some reflect the Light mingl'd with more, others with less of Shade (either as to Quant.i.ty, or as to Interruption) I hold it not unfit to mention in the first place, the Experiments that I thought upon to examine this Conjecture. And though coming to transcribe them out of some Physiological _Adversaria_ I had written in loose Papers, I cannot find one of the chief Records I had of my Tryals of this Nature, yet the Papers that scap'd miscarrying, will, I presume, suffice to manifest the main thing for which I now allege them; I find then Among my _Adversaria_, the following Narrative.

_October_ the 11. About ten in the Morning in Sun-shiny Weather, (but not without fleeting Clouds) we took several sorts of Paper Stain'd, some of one Colour, and some of another; and in a Darken'd Room whose Window look'd Southward, we cast the Beams that came in at a hole about three Inches and a half in Diameter, upon a White wall that was plac'd on one side, about five foot distance from them.

The White gave much the Brightest Reflection.

The Green, Red, and Blew being Compar'd together, the Red gave much the strongest Reflection, and manifestly enough also threw its _Colour_ upon the Wall; The Green and Blew were scarce Discernable by their Colours, and seem'd to reflect an almost Equal Light.

The Yellow Compar'd with the two last nam'd, Reflected somewhat more Light.

The Red and Purple being Compar'd together, the former manifestly Reflected a good deal more Light.

The Blew and Purple Compar'd together, the former seem'd to Reflect a little more Light, though the Purple Colour were more manifestly seen.

A Sheet of very well fleck'd Marbl'd Paper being Apply'd as the others, did not cast any or its Distinct Colours upon the Wall; nor throw its Light upon it with an Equal Diffusion, but threw the Beams Unstain'd and Bright to this and that part of the Wall, as if it's Polish had given it the Nature of a specular Body. But comparing it with a sheet of White Paper, we found the Reflection of the latter to be much Stronger, it diffusing almost as much Light to a _good Extent_ as the Marble Paper did to _one part_ of the Wall.

The Green and Purple left us somewhat in suspence which Reflected the most Light; only the Purple seem'd to have some little Advantage over the Green, which was Dark in its kind.

Thus much I find in our above mention'd _Collections_, among which there are also some Notes concerning the Production of _Compounded Colours_, _by Reflection_ from Bodyes differingly Colour'd. And these Notes we intended should supply us with what we should mention as our second Experiment: but having lost the Paper that contain'd the Particulars, and remembring onely in General, that if the Objects which Reflected the Light were not Strongly Colour'd and somewhat Glossy, the Reflected Beams would not manifestly make a Compounded Colour upon the Wall, and even then but very Faintly, we shall now say no more of that Matter, only reserving our selves to mention hereafter the Composition of a Green, which we still retain in Memory.

_EXPERIMENT II._

We may add, _Pyrophilus_, on this Occasion, that though a Darken'd Room be Generally thought requisite to make the Colour of a Body appear by Reflection from another Body, that is not one of those that are commonly agreed upon to be Specular (as Polish'd Metall, Quick silver, Gla.s.s, Water, &c.) Yet I have often observ'd that when I wore Doublets Lin'd with some silken Stuff that was very Glossy and Vividly Colour'd, especially Red, I could in an Inlightned Room plainly enough Discern the Colour, upon the Pure White Linnen that came out at my Sleeve and reach'd to my Cufs; as if that Fine White Body were more Specular, than Colour'd and Unpolish'd Bodyes are thought Capable of being.

_EXPERIMENT III._

Whilst we were making the newly mention'd Experiments, we thought fit to try also what Composition of Colours might be made by Altering the Light in its Pa.s.sage to the Eye by the Interposition not of Perfectly Diaphanous Bodies, (that having been already try'd by others as well as by us (as we shall soon have occasion to take notice) but of Semi-opacous Bodyes, and those such as look'd upon in an ordinary Light, and not held betwixt it and the Eye, are not wont to be Discriminated from the rest of Opacous Bodyes; of this Tryal, our mention'd _Adversaria_ present us the following Account.

Holding these Sheets, sometimes one sometimes the other of them, before the Hole betwixt the Sun and the Eye, with the Colour'd sides obverted to the Sun; we found them _single_ to be somewhat Transparent, and appear of the same Colour as before, onely a little alter'd by the great Light they were plac'd in; but laying _two_ of them one over another and applying them so to the Hole, the Colours were compounded as follows.

The Blew and Yellow scarce exhibited any thing but a Darker Yellow, which we ascrib'd to the Coa.r.s.eness of the Blew Papers, and its Darkness in its Kind. For applying the Blew parts of the Marbl'd Paper with the Yellow Paper after the same manner, they exhibited a good Green.

The Yellow and Red look'd upon together gave us but a Dark Red, somewhat (and but a little,) inclining to an Orange Colour.

The Purple and Red look'd on together appear'd more Scarlet.

The Purple and Yellow made an Orange.

The Green and Red made a Dark Orange Tawny.

The Green and Purple made the Purple appear more Dirty.

The Blew and Purple made the Purple more Lovely, and far more Deep.

The Red parts of the Marbl'd Paper look'd upon with the Yellow appear'd of a Red far more like Scarlet than without it.

[Page 191]

But the Fineness or Coa.r.s.eness of the Papers, their being carefully or slightly Colour'd, and divers other Circ.u.mstances, may so vary the Events of such Experiments as these, that if, _Pyrophilus_, you would Build much on them, you must carefully Repeat them.

_EXPERIMENT IV._

The Triangular Prismatical Gla.s.s being the Instrument upon whose Effects we may the most Commodiously speculate the Nature of Emphatical Colours, (and perhaps that of Others too;) we thought it might be usefull to observe the several Reflections and Refractions which the Incident Beams of Light suffer in Rebounding from it, and Pa.s.sing through it. And this we thought might be Best done, not (as is usual,) in an ordinary Inlightn'd Room, where (by reason of the Difficulty of doing otherwise) ev'n the Curious have left Particulars Unheeded, which may in a convenient place be easily taken notice of; but in a Darken'd Room, where by placing the Gla.s.s in a convenient Posture, the Various Reflections and Refractions may be Distinctly observ'd; and where it may appear _what_ Beams are Unting'd; and _which_ they are, that upon the Bodyes that terminate them, do Paint either the Primary or Secondary Iris. In pursuance of this we did in the above mention'd Darken'd Room, make observation of no less than four Reflections, and three Refractions that were afforded us by the same Prism, and thought that notwithstanding what was taught us by the Rules of Catoptricks and Dioptricks, it would not be amiss to find also, by hiding sometimes one part of the Prism, and sometimes another, and observing where the Light or Colour Vanish'd thereupon, by which Reflection and by which Refraction each of the several places whereon the Light rebounding from, or pa.s.sing through, the Prism appear'd either Sincere or Tincted, was produc'd. But because it would be Tedious and not so Intelligible to deliver this in Words, I have thought fit to Referr You to the Annexed Scheme where the Newly mention'd particulars may be at one View taken Notice of.

_EXPERIMENT V._

[Ill.u.s.tration: _The Explication of the Scheme._]

_PPP_. An Aequilaterotriangular Crystalline Prism, one of whose edges _P_.

is placed directly towards the Sun.

_A B_ & [alpha] [beta] Two rays from the Sun falling on the Prism at _B_ [beta]. and thence partly reflected towards _C_ & [gamma]. and partly refracted towards _D_ & [delta].

_B C_ & [beta] [gamma]. Those reflected Rays.

_B D_ & [beta] [delta]. Those refracted Rays which are partly refracted towards _E_ & [epsilon]. and there paint an Iris 1 2 3 4 5. denoting the five consecutions of colours Red, Yellow, Green, Blew, and Purple; and are partly reflected towards _F_ & [zeta].

_D F_ & [delta] [zeta]. Those Reflected Rays which are partly refracted towards _G_ & [eta]. colourless, and partly reflected, towards _H_ & [theta].

_F H_ & [zeta] [theta]. Those reflected Rays which are refracted towards _I_ & [iota]. and there paint an other fainter Iris, the colours of which are contrary to the former 5 4 3 2 1. signifying Purple, Blew, Green, Yellow, Red, so that the Prism in this posture exhibits four Rainbows.

I know not whether you will think it Inconsiderable to annex to this Experiment, That we observ'd in a Room not Darken'd, that the Prismatical Iris (if I may so call it) might be Reflected without losing any of its several _Colours_ (for we now consider not their _Order_) not onely from a plain Looking-gla.s.s and from the calm Surface of Fair Water, but also from a Concave Looking-gla.s.s; and that Refraction did as little Destroy those Colours as Reflection. For by the help of a large (double Convex) Burning-gla.s.s through which we Refracted the Suns Beams, we found that one part of the Iris might be made to appear either beyond, or on this side of the other Parts of the same Iris; but yet the same Vivid Colours would appear in the Displac'd part (if I may so term it) as in the other. To which I shall add, that having, by hiding the side of the Prism, obverted to the Sun with an Opacous Body, wherein only one small hole was left for the Light to pa.s.s through, reduc'd the Prismatical Iris (cast upon White Paper) into a very narrow compa.s.s, and look'd upon it througn a Microscope; the Colours appear'd the same as to kind that they did to the naked Eye.

_EXPERIMENT VI._

It may afford matter of Speculation to the Inquisitive, such as you, _Prophilus_, that as the Colours of outward Objects brought into a Darken'd Room, do so much depend for their Visibility upon the Dimness of the Light they are there beheld by; that the ordinary Light of the day being freely let in upon them, they immediately disappear: so our Tryals have inform'd us, that as to the Prismatical Iris painted on the Floor by the beams of the Sun Trajected through a Triangular-gla.s.s; though the Colours of it appear very Vivid ev'n at Noon-day, and in Sun shiny Weather, yet by a more Powerfull Light they may be made to disappear. For having sometimes, (in prosecution of some Conjectures of mine not now to be Insisted on,) taken a large Metalline Concave _Speculum_, and with it cast the converging Beams of the Sun upon a Prismatical Iris which I had caus'd to be projected upon the Floor, I found that the over-powerfull Light made the Colours of the Iris disappear. And if I so Reflected the Light as that it cross'd but the middle of the Iris, in that part only the Colours vanish'd or were made Invisible; those parts of the Iris that were on the right and left hand of the Reflected Light (which seem'd to divide them, and cut the Iris asunder) continuing to exhibit the same Colours as before. But upon this we must not now stay to Speculate.

_EXPERIMENT VII._

I have sometimes thought it worth while to take notice, whether or no the Colours of Opacous Bodies might not appear to the Eye somewhat Diversify'd, not only by the Disposition of the Superficial parts of the Bodyes themselves and by the Position of the Eye in Reference to the Object and the Light, (for these things are Notorious enough;) but according also to the Nature of the Lucid Body that shines upon them. And I remember that in Prosecution of this Curiosity, I observ'd a manifest Difference in some Kinds of Colour'd Bodyes look'd on by Day-light, and afterwards by the light of the Moon; either directly falling on them or Reflected upon them from a Concave Looking-gla.s.s. But not finding at present in my Collections about Colours any thing set down of this Kind, I shall, till I have opportunity to repeat them, content my self to add what I find Register'd concerning Colours look'd on by Candle-light, in regard that not only the Experiment is more easie to be repeated, but the Objects being the Same Sorts of Colour'd Paper lastly mention'd, the Collation of the two Experiments may help to make the Conjectures they will suggest somewhat the less uncertain.

Within a few dayes of the time above mention'd, divers Sheets of Colour'd Paper that had been look'd upon before in the Sunshine were look'd upon at night by the light of a pretty big Candle, (snuff'd) and the Changes that were observ'd were these.

The Yellow seem'd much fainter than in the Day, and inclinable to a pale Straw Colour.

The Red seem'd little Chang'd; but seem'd to Reflect Light more strongly than any other Colour (for White was none of them.)

A fair Deep Green look'd upon by it self seem'd to be a Dark Blew: But being look'd upon together with a Dark Blew, appear'd Greenish; and beheld together with a Yellow appear'd more Blew than at first.

The Blew look'd more like a Deep Purple or Murray than it had done in the Daylight.

The Purple seem'd very little alter'd.

The Red look'd upon with the Yellow made the Yellow look almost like Brown Cap-paper.

_N_. The Caution Subjoyned to the third Experiments is also Applicable to this.

_EXPERIMENT VIII._