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 _Phaenomena_