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