The
Pins also TT were drill'd with small holes through the _Axis_, and
through those holes was stretcht and fastned a small Wire. There was
likewise a small Pipe of Tin loosly put on upon the end of V, and
reaching down to the sight G; the use of which was only to keep any
false Rayes of light from passing through the bottom of V, and only
admitting such to pass as pierced through the sight G: All things being
placed together in the manner describ'd in the Figure; that is, the
Ruler AB being fixt perpendicular, I fill'd the Box CC with Water, or
any other Liquor, whose refraction I intended to try, till the Wire
passing through the middle of it were just covered: then I moved and
fixt the Ruler FE at any assignable Angle, and placed the flame of a
Candle just against the sight G; and looking through the sight I, I
moved the Ruler RS to and fro, till I perceived the light passing
through G to be covered, as 'twere, or divided by the dark Wire passing
through PP: then turning the Screw in K, I fixt it in that posture: And
through the hole S, I observed what degree and part of it was cut by
the cross threads in S. And this gave me the Angle of Inclination, APS
answering to the Angle of Refraction BPE: for the surface of the Liquor
in the Box will be alwayes horizontal, and consequently AB will be a
perpendicular to it; the Angle therefore APS will measure, or be the
Angle of Inclination in the Liquor; next EPB must be the Angle of
Refraction, for the Ray that passes through the sight G, passes also
perpendicularly through the Glass _Diaphragme_ at F, and consequently
also perpendicularly through the lower surface of the Liquor contiguous
to the Glass, and therefore suffers no refraction till it meet with the
horizontal surface of the Liquor in CC, which is determined by the two
Angles.
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