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 Phaenomena 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.