constitute its Thickness when the Rays are perpendicular. And the Rule
found by the seventh Observation agrees well with these Measures, if
duly apply'd; namely, that the Thickness of a Plate of Water requisite
to exhibit one and the same Colour at several Obliquities of the Eye, is
proportional to the Secant of an Angle, whose Sine is the first of an
hundred and six arithmetical mean Proportionals between the Sines of
Incidence and Refraction counted from the lesser Sine, that is, from the
Sine of Refraction when the Refraction is made out of Air into Water,
otherwise from the Sine of Incidence.
I have sometimes observ'd, that the Colours which arise on polish'd
Steel by heating it, or on Bell-metal, and some other metalline
Substances, when melted and pour'd on the Ground, where they may cool in
the open Air, have, like the Colours of Water-bubbles, been a little
changed by viewing them at divers Obliquities, and particularly that a
deep blue, or violet, when view'd very obliquely, hath been changed to a
deep red. But the Changes of these Colours are not so great and
sensible as of those made by Water. For the Scoria, or vitrified Part of
the Metal, which most Metals when heated or melted do continually
protrude, and send out to their Surface, and which by covering the
Metals in form of a thin glassy Skin, causes these Colours, is much
denser than Water; and I find that the Change made by the Obliquation of
the Eye is least in Colours of the densest thin Substances.
_Obs._ 20. As in the ninth Observation, so here, the Bubble, by
transmitted Light, appear'd of a contrary Colour to that, which it
exhibited by Reflexion. Thus when the Bubble being look'd on by the
Light of the Clouds reflected from it, seemed red at its apparent
Circumference, if the Clouds at the same time, or immediately after,
were view'd through it, the Colour at its Circumference would be blue.
And, on the contrary, when by reflected Light it appeared blue, it would
appear red by transmitted Light.
_Obs._ 21. By wetting very thin Plates of _Muscovy_ Glass, whose
thinness made the like Colours appear, the Colours became more faint and
languid, especially by wetting the Plates on that side opposite to the
Eye: But I could not perceive any variation of their Species. So then
the thickness of a Plate requisite to produce any Colour, depends only
on the density of the Plate, and not on that of the ambient Medium. And
hence, by the 10th and 16th Observations, may be known the thickness
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