( i3 9 )

It will not (I hope) be thought more than neceflary, that in this Accountof thc Apparatus, I have mention'd so many Minute Circumstanccs; for,we found it' difficult cnough to Exclude tne Air, and almost impoflibleto dücover the very little Holes through which so Subtile a Fluid wouldfreely Enter, and Possefi the Spaces deserted by the Subsiding Mercury. Butwith all tbis Precaution, the Experiment liicceeded at last, as I wilht: asterthis manner.

We placed the Object a, ( which was a blackThread fasten’d in a little Frameover a piece of White Paper) in the Axis of the Cylinder xca ; We filled the.Pipes and Cylinder with Mercury ; and having stopt the upper end of the Pipe atF, with the little Iron-Stopple K, and closed-it, as the upper part of the Tubeand other Joints, we let the Mercury ruri out gently at O, (into the Bladderu,) till itremained sospended at theusoal Height, (as in the Barometer) leavingthe upper. part of the Tube and the Cavity of the Cylinder between thc ,Glal-fes o o , and / f, void of Air. We then law the Object, . which before ap-peared in the Axis at x, raised coniiderably above it; and we Reduced k toappear again at x, by removing it from a to a. The Axis therefore ofthe Visual Ray, (which was also the Axis of the Cylinder) x c a, falling Per-pendicularly on the void Space, past through it without any Refraction: ButEmerging Obiiquely into the Air, it was Refractcd towards the Perpendicu-lar f c, and received a new Direction to «. And therefore the Distance a «,Subtended the Angle of Refraction ac a j all which we measor’d, and foundas follows j vt\.

■ Inches. dec. faxt:.

The Heighth. of the Object above the Axis, or the un-?refractcd Visoal Ray, a a. -:-———-5 >°’

The Distance of the Object from the Refracting Pia in ? ,

Cy about yi Feet,,or —■ —»■ ■■ ■ -— 5 6l2 ’ cca

Deg*

•oo

A -litL Scc.

OZ .

Therefore the Angle of Refraction «c«, was —— __

The Angle of Emersion p c a, (by the Construction o(l r

the Cylinder) was ----- l 6z ' ?° ’ °°

the Cylinder) was

Therefore the Angle of Incidence j> c a. —pca-bwas - - - - -»-—

*7 • 37

And therefore

universali?,- (according to the known Laws o£ Refraction)

The Sines- of the Angles of Incidence bemg-The Sines of the Angles of Emersion are—And the Refractive Power, of the Dense Air-

IOOCOO

tOOOZ6

Z6

By the Bcfraftivc Power of a PellucicL Bocly, L mean that Property iri.it whereby thc . Oblique Rays-of Light are Diverted from their DirectCourse, and which is meafured.by the Proportional Differences,- (alwaysobserved) between the Sines- of the Angles of Incidence and Emer-sion

Tly.