Experiments and considerations touching colours first occasionally written, among some other essays to a friend, and now suffer'd to come abroad as the beginning of an experimental history of colours / by the Honourable Robert Boyle ...

CHAP. II.

1 TO come then in the first place to our more general Considerations, I shall begin with saying something as to the Importance of examining the Colours of Bodies. For there are some, especially Chymists, who think, that a considerable diversity of Colours does constantly argue an equal diversity of Nature, in the Bodies wherein it is conspicuous; but I confess I am not altogether of their mind; for not to mention changeable Taffaties, the blew and golden necks of Pidgeons, and divers Wa∣ter-fowl, Rainbows Natural and Artificial, and other Bodies, whose Colours the Philo∣sophers have been pleased to call not Real, but Apparent and Phantastical; not to insist on these, I say, (for fear of needlesly en∣gaging in a Controversie) we see in Parrots, Goldfinches, and divers other Birds, not only that the contiguous feathers which are probably as near in properties as place, are some of them Red, and others White, some of them Blew, & others Yellow, &c. but that in the several parts of the self-same feather there may often be seen the greatest dispari∣ty of Colours; and so in the leaves of Tulips, July-flowers, and some other Vegetables

the several leaves, and even the several parts of the same leaf, although no difference have been observed in their other proper∣ties, are frequently found painted with very different Colours. And such a variety we have much more admired in that lovely plant which is commonly, and not unjustly call'd the Marvayl of Peru; for of divers scores of fine Flowers, which in its season that gau∣dy Plant does almost daily produce, I have scarce taken notice of any two that were dyed perfectly alike. But though Pyro: such things as these, among others, keep mee from daring to affirm, that the Diversity and change of Colours does alwaies argue any great difference or alteration, betwixt, or in, the Bodies, wherein it is to be discerned, yet that oftentimes the Alteration of Co∣lours does signifie considerable Alterations in the disposition of parts of Bodies, may ap∣pear in the Extraction of Tinctures, and di∣vers other Chymical Operations, wherein the change of Colours is the chief, and sometimes the only thing, by which the Artist regulates his proceeding, and is taught to know when 'tis seasonable for him to leave off. Instances of this sort are more obvious in divers sorts of fruits, as Cher∣ries, Plums, &c. wherein, according a•• the Vegetable sap is sweetned, or otherwise

ripened, by passing from one degree to a∣nother of Maturation, the external part of the fruit passes likewise from one to ano∣thor Colour. But one of the noblest In∣stances I have met with of this kind, is not so obvious; and that is the way of temper∣ing Steel to make Gravers, Drills, Springs, and other Mechanical Instruments, which we have divers times both made Artificers practise in our presence, and tryed our selves, after the following manner. First, the slender Steel to be tempered is to be hardened by heating as much of it as is re∣quisite among glowing Coals, till it be glowing hot, but it must not be quenched assoon as it is taken from the fire (for that would make it too brittle, and spoil it) but must be held over a bason of water, till it descend from a White heat to a Red one, which assoon as ever you perceive, you must immediacely quench as much as you desire to harden in the cold water. The Steel thus hardened, will, if it be good, look some∣what White and must be made bright at the end, that its change of Colours may be there conspicuous; and then holding it so in the flame of a Candle, that the bright end may be, for about half an inch, or more, out of the flame, that the smoak do not stain or fully the brightness of it, you shall after a

while see that clean end, which is almost contiguous to the flame, pass very nimbly from one Colour to another, as from a brighter Yellow, to a deeper and reddish Yellow, which Artificers call a sanguine, and from that to a fainter first, and then a a deeper Blew. And to bring home this Ex∣periment to our present purpose, it is found by daily Experience, that each of these suc∣ceeding Colours argue such a change made in the texture of the Steel, that if it be taken from the flame, and immediately quenched in the tallow (whereby it is setled in what∣ever temper it had before) when it is Yel∣low, it is of such a hardness as makes it fit for Gravers Drills, and such like tools; but if it be kept a few minutes longer in the flame till it grow Blew, it becomes much softer, and unfit to make Gravers for Me∣talls, but fit to make Springs for Watches, and such like Instruments, which are there∣fore commonly of that Colour; and if the Steel be kept in the flame, after that this deep Blew hath disclosed it self, it will grow so soft, as to need to be new hardened again, before it can be brought to a temper, fit for Drills or Penknives. And I confess Pyro. I have taken much pleasure to see the Colours run along from the parts of the Steel con∣tiguous to the flame, to the end of the In∣strument,

and succeed one another so fast, that if a man be not vigilant, to thrust the Steel into the tallow at the very nick of time, at which it has attain'd its due Colour, he shall miss of giving his tool the right temper. But because the flame of a Candle is offensive to my weak eyes, and because it is apt to either black or sully the contiguous part of the Steel which is held in it, and thereby hinder the change of Colours from being so long and clearly discern'd, I have sometimes made this Experiment by laying the Steel to be tempered upon a heated bar of Iron, which we finde also to be employ'd by some Artificers in the tempering of such great Instruments, as are too big to be soon heated sufficiently by the flame of a Candle. And you may easily satisfie your self Pyro: of the differing hardness and toughness, which is ascribed to Steel temper'd at dif∣ferent Colours, if you break but some slen∣der wires of Steel so temper'd, and ob∣serve how they differ in brittleness, and if with a file you also make tryal of their va∣rious degrees of hardness.

2 But Pyrophilus, I must not at present any further prosecute the Consideration of the importance of Experiments about Colours, not only because you will in the following papers finde some Instances, that would here

be presented you out of their due place, of the use that may be made of such Experi∣ments, in discovering in divers bodies, what kind the salt is, that is predominant in them; but also because a speculative Naturalist might justly enough allege, that as Light is so pleasing an object, as to be well worth our looking on, though it discover'd to us nothing but its self; so modifi'd Light cal∣led Colour, were worth our contempla∣tion, though by understanding its Nature we should be taught nothing else. And however, I need not make either you or my self excuses for entertaining you on the sub∣ject I am now about to treat of, since the pleasure Pyro: takes in mixing and laying on of Colours, will I presume keep him, and will (I am sure) keep mee from think∣ing it troublesome to set down, especially after the tedious processes (about other mat∣ters) wherewith I fear I may have tyr▪d him, some easie, and not unpleasant Expe∣riments relating to that subject.

3 But, before we descend to the more particular considerations, we are to present you concerning Colours, I presume it will be seasonable to propose at the very entrance a Distinction; the ignorance or neglect of which, seems to mee to have frequently e∣nough occasioned either mistakes or confu∣sion

in the Writings of divers Modern Philosophers; for Colour may be consider∣ed, either as it is a quality residing in the body that is said to be coloured, or to mo∣difie the light after such or such a manner; or else as the Light it self, which so modifi'd, strikes upon the organ of sight, and so causes that Sensation which we call Colour; and that this latter may be look'd upon as the more proper, though not the usual ac∣ception of the word Colour, will be made probable by divers passages in the insuing part of our discourse; and indeed it is the Light it self, which after a certain manner, either mingled with shades, or some other waies troubled, strikes our eyes, that does more immediately produce that motion in the organ, upon whose account men say they see such or such a Colour in the object; yet, because there is in the body that is said to be coloured, a certain disposition of the superficial particles, whereby it sends the Light reflected, or refracted, to our eyes thus and thus alter'd, and not otherwise, it may also in some sense be said, that Colour de∣pends upon the visible body; and therefore we shall not be against that way of speak∣ing of Colours that is most us'd among the Modern Naturalists, provided we be al∣lowed to have recourse when occasion shall

require to the premis'd distinction, and to take the more immediate cause of Colour to be the modifi'd Light it self, as it af∣fects the Sensory; though the disposition al∣so of the colour'd body, as that modifies the Light, may be call'd by that name Metonimi∣cally (to borrow a School term) or Effici∣ently, that is in regard of its turning the Light, that rebounds from it, or passes thorow it, into this or that particular Co∣lour.

4 I know not whether I may not on this occasion add, that Colour is so far from be∣ing an Inherent quality of the object in the sense that is wont to be declar'd by the Schools, or even in the sense of some Mo∣dern Atomists, that, if we consider the mat∣ter more attentively, we shall see cause to suspect, if not to conclude, that though Light do more immediately affect the organ of sight, than do the bodies that send it thither, yet Light it self produces the sensa∣tion of a Colour, but as it produces such a determinate kind of local motion in some part of the brain; which, though it happen most commonly from the motion where∣into the slender strings of the Retina are put, by the appulse of Light, yet if the like mo∣tion happen to be produc'd by any other cause, wherein the Light concurrs not at

all, a man shall think he sees the same Co∣lour. For proof of this, I might put you in mind, that 'tis usual for dreaming men to think they see the Images that appear to them in their sleep, adorn'd some with this, and some with that lively Colour, whilst yet, both the curtains of their bed, and those of their eyes are close drawn. And I might add the confidence with which di∣stracted persons do oftentimes, when they are awake, think, they see black fiends in places, where there is no black object in sight without them. But I will rather ob∣serve, that not only when a man receives a great stroak upon his eye, or a very great one upon some other part of his head, he is wont to see, as it were, flashes of lightning, and little vivid, but vanishing flames, though perhaps his eyes be shut: But the like ap∣paritions may happen, when the motion proceeds not from something without, but from something within the body, provided the unwonted fumes that wander up and down in the head, or the propagated con∣cussion of any internal part in the body, do cause about the inward extremities of the Optick Nerve, such a motion as is wont to be there produc'd, when the stroak of the Light upon the Retina makes us conclude, that we see either Light, or such and such a

Colour: This the most ingenious Des Car∣tes hath very well observ'd, but because he seems not to have exemplifi'd it by any un∣obvious or peculiar observation, I shall in∣deavour to illustrate this doctrine by a few Instances.

5 And first, I remember, that having, through Gods goodness, been free for seve∣ral years, from troublesome Coughs, being afterwards, by an accident, suddenly cast into a violent one, I did often, when I was awaked in the night by my distempers, ob∣serve, that upon coughing strongly, it would seem to mee, that I saw very vivid, but immediately disappearing flames, which I took particular notice of, because of the conjecture I am now mentioning.

6 An excellent and very discreet person, very near ally'd both to you and mee, was relating to mee, that some time since, whilst she was talking with some other Ladies, upon a sudden, all the objects, she looked upon, appeared to her dyed with unusual Colours, some of one kind, and some of a∣nother, but all so bright and vivid, that she should have been as much delighted, as sur∣priz'd with them, but that finding the ap∣parition to continue, she fear'd it portended some very great alteration as to her health: As indeed the day after she was assaulted

with such violence by Hysterical and Hypo∣condrical Distempers, as both made her rave for some daies, and gave her, during that time, a Bastard Palsey.

7 Being a while since in a Town, where the Plague had made great havock, and in∣quiring of an ingenious man, that was so bold, as without much scruple to visit those that were sick of it, about the odd symp∣tomes of a Disease that had swept away so many there; he told mee, among other things, that he was able to tell divers Pa∣tients, to whom he was called, before they took their beds, or had any evident symp∣tomes of the Plague, that they were indeed infected, upon peculiar observations, that be∣ing asked, they would tell him that the neighbouring objects, and particularly his cloths, appear'd to them beautifi'd with most glorious Colours, like those of the Rain-bow, oftentimes succeeding one another; and this he affirm'd to be one of the most usual, as well as the most early symptomes, by which this odd Pestilence disclos'd it self: And when I asked how long the Patients were wont to be thus affected, he answered, that it was most commonly for about a day; and when I further inquired whether or no Vomits, which in that Pestilence were usu∣ally given, did not remove this symptome

(For some used the taking of a Vomit, when they came ashore, to cure themselves of the obstinate and troublesome giddiness caus'd by the motion of the ship) reply'd, that ge∣nerally, upon the evacuation made by the Vomit, that strange apparition of Colours ceased, though the other symptomes were not so soon abated, yet he added (to take no∣tice of that upon the by, because the obser∣vation may perchance do good) that an ex∣cellent Physician, in whose company he was wont to visit the sick, did give to almost all those to whom he was called, in the be∣ginning before Nature was much weakened, a pretty odd Vomit consisting of eight or ten dramms of Infusion of Crocus Metallorum, and about half a dramm, or much more, of White Vitriol, with such success, that scarce one of ten to whom it was seasonably admi∣nistred, miscarried.

8 But to return to the consideration of Colours: As an apparition of them may be produced by motions from within, without the assistance of an outward object, so I have observed, that 'tis sometimes possible that the Colour that would otherwise be produced by an outward object, may be chang'd by some motion, or new texture al∣ready produced in the Sensory, as long as that unusual motion, or new disposition

lasts; for I have divers times try'd, that af∣ter I have through a Telescope look'd upon the Sun, though thorow a thick, red, or blew glass, to make its splendor suppor∣table to the eye, the impression upon the Retina, would be not only so vivid, but so permanent, that if afterwards I turned my eye towards a flame, it would appear to mee of a Colour very differing from its usu∣al one. And if I did divers times successively shut and open the same eye, I should see the adventitious Colour (if I may so call it) changed or impair'd by degrees, till at length (for this unusual motion of the eye would not presently cease) the flame would appear to mee, of the same hew that it did to other beholders; a not unlike effect I found by looking upon the Moon, when she was near full, thorow an excellent Telescope, without colour'd Glass to screen my eye with; But that which I desire may be taken notice of, because we may elsewhere have occasion to reflect upon it, and because it seems not agreeable to what Anatomists and Optical Writers deliver, touching the rela∣tion of the two eyes to each other, is this circumstance, that though my Right eye, with which I looked thorow the Telescope, were thus affected by the over-strong im∣pression of the light, yet when the flame

of a Candle, or some other bright object ap∣pear'd to me of a very unusual Colour, whilst look'd upon with the Discompos'd Eye, or (though not so notably) with both eyes at once; yet if I shut that Eye, and look∣ed upon the same object with the other, it would appear with no other than its usual Colour, though if I again opened, and made use of the Dazled eye, the vivid adventitious Colour would again appear. And on this occasion I must not pretermit an Observa∣tion which may perswade us, that an over∣vehement stroak upon the Sensory, especi∣ally if it be naturally of a weak constitution, may make a more lasting impression than one would imagine, which impression may in some cases, as it were, mingle with, and vitiate the action of vivid objects for a long time after.

For I know a Lady of unquestionable Ve∣racity, who having lately, by a desperate fall, receiv'd several hurts, and particularly a considerable one upon a part of her face near her Eye, had her sight so troubl'd and disorder'd, that, as she hath more than once related to me, not only when the next morning one of her servants came to her bed side, to ask how she did, his cloaths ap∣pear'd adorn'd with such variety of dazling Colours, that she was fain presently to

command him to withdraw, but the Images in her Hangings, did, for many daies after, appear to her, if the Room were not extra∣ordinarily darken'd, embellish'd with seve∣ral offensively vivid Colours, which no bo∣dy else could see in them; And when I en∣quir'd whether or no White Objects did not appear to her adorn'd with more lumi∣nous Colours than others, and whether she saw not some which she could not now well describe to any, whose eyes had never been distemper'd, she answer'd mee, that some∣times she thought she saw Colours so new and glorious, that they were of a peculiat kind, and such as she could not describe by their likeness to any she had beheld either before or since, and that White Objects did so much disorder her sight, that if several daies after her fall, she look'd upon the in-side of a Book, she fanci'd she saw there Colours like those of the Rain-bow, and even when she thought her self pretty well recover'd, and made bold to leave her Chamber, the coming into a place where the Walls and Ceeling were whited over, made those Objects appear to her cloath'd with such glorious and dazling Colours, as much offended her sight, and made her re∣pent her venturousness, and she added, that this Distemper of her Eyes lasted no less

than five or six weeks, though, since that, she hath been able to read and write much without finding the least Inconvenience in doing so. I would gladly have known, whe∣ther if she had shut the Injur'd Eye, the Phaenomena would have been the same, when she employ'd only the other, but I heard not of this accident early enough to satis∣fie that Enquiry.

9 Wherefore, I shall now add, that some years before, a person exceedingly eminent for his profound Skil in almost all kinds of Philological Learning, coming to advise with mee about a Distemper in his Eyes, told me, among other Circum∣stances of it, that, having upon a time looked too fixedly upon the Sun, thorow a Telescope, without any coloured Glass, to take off from the dazling splendour of the Object, the excess of Light did so strongly affect his Eye, that ever since, when he turns it towards a Window, or any White Object, he fancies, he seeth a Globe of Light, of about the bigness the Sun then appeared of to him, to pass before his Eyes: And having inquir'd of him, how long he had been troubled with this Indispo∣sition, he reply'd, that it was already nine or ten years, since the Accident, that occa∣sioned it, first befel him.

10 I could here subjoyn, Pyrophilus, some memorable Relations that I have met with in the Account given us by the experienc'd Epiphanius Ferdinandus, of the Symptomes he observ'd to be incident to those that are bitten with the Tarantula, by which (Re∣lations) I could probably shew, that with∣out any change in the Object, a change in the Instruments of Vision may for a great while make some Colours appear Charm∣ing, and make others Provoking, and both to a high degree, though neither of them produc'd any such Effects before. These things, I say, I could here subjoyn in confir∣mation of what I have been saying, to shew, that the Disposition of the Organ is of great Importance in the Dijudications we make of Colours, were it not that these strange Stories belonging more properly to another Discourse, I had rather, (content∣ing my self to have given you an Intimation of them here) that you should meet with them fully deliver'd there.