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, and those of another
transmitted. This is manifest by the fifth Observation of this part of
this Book. For in that Observation, when the Speculum was illuminated by
any one of the prismatick Colours, that Light made many Rings of the
same Colour upon the Chart with dark Intervals, and therefore at its
emergence out of the Speculum was alternately transmitted and not
transmitted from the Speculum to the Chart for many Successions,
according to the various Obliquities of its Emergence. And when the
Colour cast on the Speculum by the Prism was varied, the Rings became of
the Colour cast on it, and varied their bigness with their Colour, and
therefore the Light was now alternately transmitted and not transmitted
from the Speculum to the Chart at other Obliquities than before. It
seemed to me therefore that these Rings were of one and the same
original with those of thin Plates, but yet with this difference, that
those of thin Plates are made by the alternate Reflexions and
Transmissions of the Rays at the second Surface of the Plate, after one
passage through it; but here the Rays go twice through the Plate before
they are alternately reflected and transmitted. First, they go through
it from the first Surface to the Quick-silver, and then return through
it from the Quick-silver to the first Surface, and there are either
transmitted to the Chart or reflected back to the Quick-silver,
accordingly as they are in their Fits of easy Reflexion or Transmission
when they arrive at that Surface. For the Intervals of the Fits of the
Rays which fall perpendicularly on the Speculum, and are reflected back
in the same perpendicular Lines, by reason of the equality of these
Angles and Lines, are of the same length and number within the Glass
after Reflexion as before, by the 19th Proposition of the third part of
this Book. And therefore since all the Rays that enter through the
first Surface are in their Fits of easy Transmission at their entrance,
and as many of these as are reflected by the second are in their Fits of
easy Reflexion there, all these must be again in their Fits of easy
Transmission at their return to the first, and by consequence there go
out of the Glass to the Chart, and form upon it the white Spot of Light
170