Of Light and Colours. 295

cular and the reflected Ray CI: Or theAngle of the Incidence is equal to the Angleof Reflection in every Inclination of theRay of Light. This is evidently shewn byExperiment; and it is very well worth ourObservation, that in this Case only, the saidRay takes the Jhorteji Way pojjible fromany Point H, to any other Point I, if it«uist, in its Passage, touch any of those Sur-faces (CXVIj.

The

12. Hence we fee the Possibility of Bodies being soExceeding porous, as to be rare enough to transmit■Light with all that Freedom pellucid Bodies are foundto do. Though what their real Structure or inwardframe may be, is yet unknown to us.

(CXVT) 1. The Demonstration of this is as sol- Platelows : Let A C be the incident Ray, and C B the re- XXXVI.flected one ; from A and B let fall the Perpendiculars Fig. 3.A E, B D, and let A E = a, B D = b, E D — r, andf L —.V ; then-CD — c — X, and AC — aa- f-xx,a nd also CB = \/ b'b + cc — zcx st- xx. Thenf nee A C -{- C B is to be a Minimu m , we m ustsoake the Fluxion of its Expression y/ a a _i_ ^ x

V bb-^-cc — 2cx-\-xx equal to nothing, viz.

XX t X X—ex

\y~ 1 - = H - - - — 0; whence

y aa'\-xx b bc c — Zcx-\-xx

O'viding by x, and multiplying crofs-wife, we have x Xbb-\-cc — 2 ex xx -j- x — cX%/ aay-xx, conse-quently X X \/b b-\- cc —2 ex- j- xx~c—x X \/aa-\-xx,

.^at i s , ECXCB = CDXAC; and so we havef C ; AC :: C D : C B. Consequently (by Euclid, 6and 7.) the Triangles A EC and B D C are equiangu-ar , and therefore the Angle of Incidence ACE = BCD*ae Angle of Reflection.

U 4 2- ?ince