of incidence and emergence GH, Ik in G and K. In GH take TH equal to IK, and to the plane Aa let fall a perpendicular Tv. And (by cor. 2. of the laws of motion) let the motion of the body be reſolved into two, one perpendicular to the planes Aa, Bb, Cc, &c. and another parallel to them. The force of attraction or impulſe, acting in directions perpendicular to thoſe planes, does not at all alter the motion in parallel directions; and therefore the body proceeding with this motion will in equal times go through thoſe equal parallel intervals that lie between the line AG and the point H, and between the point I and the line JK; that is. they will deſcribe the lines GH, IK in equal times. Therefore the velocity before incidence is to the velocity after emergence as GK to IK or TH, that is as AH or Id to vH, that is (ſuppoling TH or IK radius) as the force of emergence to the ſine of incidence. Q. E. D.
Proposition XCVI. Theorem L.
The ſame thing being ſuppoſed, and that the motion before incidence is ſwifter than afterwards; I ſay, that if the {{ls]]ine of incidence be inclined continually, the body will be at laſt reflected, and the angle of reflexion will be equal to the angle of incidence.
For conceive the body paſſing between the parallel planes Aa, Bb, Cc, &c. (Pl. 25. Fig. 4.) to deſcribe parabolic arcs as above; and let thoſe arcs be HP, PQ, QR, &c. And the obliquity
