Of Light and Colours.

the second Sort, Orange ; the third Sort,Yellow ; the fourth Sort, Green ; the fifth

Sort,

found, the Proportion of the Sines FS and AR will be dis-covered, which Ratio is always the fame while the Mat-ter of the Prism remains the fame, as was before fheWJJfrom the Theory, and may by this Instrument be provedby Experiment. For Example, let the Prism beWatery it will be necessary to elevate each Arm 12 D e 'grees upon the Limb, before the Image of the Object a *F can be seen by the Eye at E ; then 12 + 3° =

= F K A + A K I = F K I, the Angle of Incidence-But the Sine F S of 42 0 is to the Sine A R of 30° as 4to 3 very nearly.

19. Now it is plain, if the Ratio of the Sines A stand F S were not fixed, since F S might be in any Rati^greater or lesser than A R, the incident Ray F K m 2 /make an Angle F K I greater or less than 42 0 , and y 6 *"the Object at F be seen by the Eye at E ; but this ^find by Experiment to be impossible, becaiife there isother Elevation of the Arms of the Index that will e3£ ”hibit the Appearance of the Object, but the one aboV c>mentioned.

20. If G PI D were a Prism of Glass, as that < s ^

denser Body than Water, so its refractive Power wist *greater, and consequently it will act more strongly 11 Pon the Ray K M at its Exit into the Air, and cause ^so be refracted farther from the Perpendicular I K/fM L. Therefore the Angle of Incidence out of AInto Glass, viz. the Angle F K I, ought to be greatsand so to require a greater Elevation of the Legs of t£1Index than before in the Prism of Water : And this ^find by Experiment is the Cafe ; for then the Elevati 0 ’ 1 ’instead of 12 0 , must be about 22° or 23 0 . $

21. Hence ’tis plain, the Sine of Incidence F S rnube in a constant Ratio to the Sine of Refraction A j* 'because, since the Angle AKL is invariable, ( f’palways equal toGDO) and in the fame Medium "H, the Angle F K I must always be the fame, bec^j 1

^1__T>_7 . > r. .1_ Cr.re- tu-

les'

the refractive Power is every where so; therefore

Ang