its splendor obscured the Rings made by the Light of the Clouds, unless
when the Speculum was removed to a great distance from the Window, so
that his Light upon it might be broad and faint. By varying the position
of my Eye, and moving it nearer to or farther from the direct beam of
the Sun's Light, the Colour of the Sun's reflected Light constantly
varied upon the Speculum, as it did upon my Eye, the same Colour always
appearing to a Bystander upon my Eye which to me appear'd upon the
Speculum. And thence I knew that the Rings of Colours upon the Chart
were made by these reflected Colours, propagated thither from the
Speculum in several Angles, and that their production depended not upon
the termination of Light and Shadow.
_Obs._ 7. By the Analogy of all these Ph nomena 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 Ph nomenon
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
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