cts those most copiously which were blue-making before. And after
the same manner may the Reasons of other Phaenomena be examined, by
trying them in this artificial beam of Light XY.

FOOTNOTES:

[I] See p. 59.

[J] _See our_ Author's Lect. Optic. _Part_ II. _Sect._ II. _p._ 239.

[K] _As is done in our_ Author's Lect. Optic. _Part_ I. _Sect._ III.
_and_ IV. _and Part_ II. _Sect._ II.

[L] _See our_ Author's Lect. Optic. _Part_ II. _Sect._ II. _pag._ 269,
&c.

[M] _This is demonstrated in our_ Author's Lect. Optic. _Part_ I.
_Sect._ IV. _Prop._ 35 _and_ 36.

THE

SECOND BOOK

OF

OPTICKS

_PART I._

_Observations concerning the Reflexions, Refractions, and Colours of
thin transparent Bodies._

It has been observed by others, that transparent Substances, as Glass,
Water, Air, &c. when made very thin by being blown into Bubbles, or
otherwise formed into Plates, do exhibit various Colours according to
their various thinness, altho' at a greater thickness they appear very
clear and colourless. In the former Book I forbore to treat of these
Colours, because they seemed of a more difficult Consideration, and were
not necessary for establishing the Properties of Light there discoursed
of. But because they may conduce to farther Discoveries for compleating
the Theory of Light, especially as to the constitution of the parts of
natural Bodies, on which their Colours or Transparency depend; I have
here set down an account of them. To render this Discourse short and
distinct, I have first described the principal of my Observations, and
then consider'd and made use of them. The Observations are these.

_Obs._ 1. Compressing two Prisms hard together that their sides (which
by chance were a very little convex) might somewhere touch one another:
I found the place in which they touched to become absolutely
transparent, as if they had there been one continued piece of Glass. For
when the Light fell so obliquely on the Air, which in other places was
between them, as to be all reflected; it seemed in that place of contact
to be wholly transmitted, insomuch that when look'd upon, it appeared
like a black or dark spot, by reason that little or no sensible Light
was reflected from thence, as from other places; and when looked through
it seemed (as it were) a hole in that Air which was formed into a thin
Plate, by being compress'd between the Glasses. And through this hole
Objects that were beyond might be see