the Groſſeſt Pigments. For I took at once two Triangular Glaſſes, and one of them being kept fixt in the ſame Poſture, that the Iris it projected on the Floor might not Waver, I caſt on the ſame Floor another Iris with the other Priſm, and Moving it too and fro to bring what part of the ſecond Iris I pleas'd, to fall upon what part of the firſt I thought fit, we did ſometimes (for a ſmall Errour ſuffices to hinder the Succeſs) obtain by this means a Green Colour in that part of the more Stable Iris, that before was Yellow, or Blew, and frequently by caſting thoſe Beams that in one of the Iris's made the Blew upon the Red parts of the other Iris, we were able to produce a lovely Purple, which we can Deſtroy or Recompoſe at pleaſure, by Severing and Reapproaching the Edges of the two Iris's.
EXPERIMENT XV.
On this occaſion, Pyrophilus, I ſhall add, that finding the Glaſs-priſm to be the uſefulleſt Inſtrument Men have yet imploy'd about the Contemplation of Colours, and conſidering that Priſms hitherto in uſe are made of Glaſs, Tranſparent and Colourleſs, I thought it would not be amiſs to try,
what change the Superinduction of a Colour, without the Deſtruction of the Diaphaneity, would produce in the Colours exhibited by the Priſm. But being unable to procure one to be made of Colour'd Glaſs, and fearing alſo that if it were not carefully made, the Thickneſs of it would render it too Opacous, I endeavoured to ſubſtitute one made of Clarify'd Roſin, or of Turpentine brought (as I elſewhere teach) to the conſiſtence of a Tranſparent Gum. But though theſe Endeavours were not wholly loſt, yet we found it ſo difficult to give theſe Materials their true Shape, that we choſe rather to Varniſh over an ordinary Priſm with ſome of theſe few Pigments that are to be had Tranſparent; as accordingly we did firſt with Yellow, and then with Red, or rather Crimſon, made with Lake temper'd with a convenient Oyl, and the Event was, That for want of good Tranſparent Colours, (of which you know there are but very few) both the Yellow and the Red made the Glaſs ſo Opacous, (though the Pigment were laid on but upon two Sides of the Glaſs, no more being abſolutely neceſſary) that unleſs I look'd upon an Inlightned Window, or the Flame of a Candle, or ſome other Luminous or very Vivid object,
I could ſcarce diſcern any Colours at all, eſpecially when the Glaſs was cover'd with Red. But when I did look on ſuch Objects, it appear'd (as I expected) that the Colour of the Pigment had Vitiated or Drown'd ſome of thoſe which the Priſm would according to its wont have exhibited, and mingling with others, Alter'd them: as I remember, that both to my Eyes, and others to whom I ſhow'd it, when the Priſm was cover'd with Yellow, it made thoſe Parts of bright Objects, where the Blew would elſe have been Conſpicuous, appear of a light Green. But, Pyrophilus, both the Nature of the Colours, and the Degree of Tranſparency, or of Darkneſs in the Pigment, beſides divers other Circumſtances, did ſo vary the Phænomena of theſe Tryals, that till I can procure ſmall Colour'd Priſms, or Hollow ones that may be filled with Tincted Liquor, or obtain Some better Pigments than thoſe I was reduc'd to imploy, I ſhall forbear to Build any thing upon what has been delivered, and ſhall make no other uſe of it, than to invite you to proſecute the Inquiry further.
EXPERIMENT XVI.
And here, Pyrophilus, ſince we are treating of Emphatical Colours, we ſhall add what we think not unworthy your Obſervation, and not unfit to afford ſome Exerciſe to the Speculative. For there are ſome Liquors, which though Colourleſs themſelves, when they come to be Elevated, and Diſpers'd into Exhalations, exhibit a conſpicuous Colour, which they loſe again, when they come to be Reconjoyn'd into a Liquor, as good Spirit of Nitre; or upon its account ſtrong Aqua-fortis, though devoid of all appearance of Redneſs whilſt they continue in the form of a Liquor, if a little Heat chance to turn the Minute parts of them into Vapour, the Steam will appear of a Reddiſh or deep Yellow Colour, which will Vaniſh when thoſe Exhalations come to reſume the form of Liquor.
And not only if you look upon a Glaſs half full of Aqua-fortis, or Spirit of Nitre, and half full of Nitrous ſteams proceeding from it, you will ſee the Upper part of the Glaſs of the Colour freſhly mention'd, if through it you look upon the Light. But which is much more conſiderable, I
have tried, that putting Aqua-fortis in a long clear Glaſs, and adding a little Copper or ſome ſuch open Metall to it, to excite Heat and Fumes, the Light trajected through thoſe Fumes, and caſt upon a ſheet of White Paper, did upon that appear of the Colour that the Fumes did, when directly Look'd upon, as if the Light were as well Ting'd in its paſſage through theſe Fumes, as it would have been by paſſing through ſome Glaſs or Liquor in which the ſame Colour was Inherent.