ese Phaenomena with those of the like
Rings of Colours described in the first part of this Book, it seemed to
me that these Colours were produced by this thick Plate of Glass, much
after the manner that those were produced by very thin Plates. For, upon
trial, I found that if the Quick-silver were rubb'd off from the
backside of the Speculum, the Glass alone would cause the same Rings of
Colours, but much more faint than before; and therefore the Phaenomenon
depends not upon the Quick-silver, unless so far as the Quick-silver by
increasing the Reflexion of the backside of the Glass increases the
Light of the Rings of Colours. I found also that a Speculum of Metal
without Glass made some Years since for optical uses, and very well
wrought, produced none of those Rings; and thence I understood that
these Rings arise not from one specular Surface alone, but depend upon
the two Surfaces of the Plate of Glass whereof the Speculum was made,
and upon the thickness of the Glass between them. For as in the 7th and
19th Observations of the first part of this Book a thin Plate of Air,
Water, or Glass of an even thickness appeared of one Colour when the
Rays were perpendicular to it, of another when they were a little
oblique, of another when more oblique, of another when still more
oblique, and so on; so here, in the sixth Observation, the Light which
emerged out of the Glass in several Obliquities, made the Glass appear
of several Colours, and being propagated in those Obliquities to the
Chart, there painted Rings of those Colours. And as the reason why a
thin Plate appeared of several Colours in several Obliquities of the
Rays, was, that the Rays of one and the same sort are reflected by the
thin Plate at one obliquity and transmitted at another, and those of
other sorts transmitted where these are reflected, and reflected where
these are transmitted: So the reason why the thick Plate of Glass
whereof the Speculum was made did appear of various Colours in various
Obliquities, and in those Obliquities propagated those Colours to the
Chart, was, that the Rays of one and the same sort did at one Obliquity
emerge out of the Glass, at another did not emerge, but were reflected
back towards the Quick-silver by the hither Surface of the Glass, and
accordingly as the Obliquity became greater and greater, emerged and
were reflected alternately for many Successions; and that in one and the
same Obliquity the Rays of one sort were reflected,