_The Intervals of the Fits of easy Reflexion and easy Transmission,
propagated from points of Reflexion into any Medium, are equal to the
Intervals of the like Fits, which the same Rays would have, if refracted
into the same Medium in Angles of Refraction equal to their Angles of
Reflexion._
For when Light is reflected by the second Surface of thin Plates, it
goes out afterwards freely at the first Surface to make the Rings of
Colours which appear by Reflexion; and, by the freedom of its egress,
makes the Colours of these Rings more vivid and strong than those which
appear on the other side of the Plates by the transmitted Light. The
reflected Rays are therefore in Fits of easy Transmission at their
egress; which would not always happen, if the Intervals of the Fits
within the Plate after Reflexion were not equal, both in length and
number, to their Intervals before it. And this confirms also the
proportions set down in the former Proposition. For if the Rays both in
going in and out at the first Surface be in Fits of easy Transmission,
and the Intervals and Numbers of those Fits between the first and second
Surface, before and after Reflexion, be equal, the distances of the Fits
of easy Transmission from either Surface, must be in the same
progression after Reflexion as before; that is, from the first Surface
which transmitted them in the progression of the even Numbers 0, 2, 4,
6, 8, &c. and from the second which reflected them, in that of the odd
Numbers 1, 3, 5, 7, &c. But these two Propositions will become much more
evident by the Observations in the following part of this Book.