On the M On ON of L IG H T. 101 



AF, &c.) it is evident that Cc is equal and oppofite to the mo- 

 tion of the point A, while the light defcribes the parabolic arch 

 AC, and that LI is equal and oppofite to the motion of A, while 

 the light defcribes the arch ACL. Therefore L and 1, C and c, 

 are contemporaneous places of the real and relative paths of 

 the light, and the parabola Acl is its relative path. 



We have feen that Af is the relative motion of the incident 

 light during the time of defcribing AB by the impulfe of the 

 refracting forces acting on a particle of light at reft in A. Let 

 us now fuppofe that the medium is at reft, and that the light 

 enters the refracting ftratum at A , with the velocity and in the 

 direction Af. It muft defcribe a parabola, which Af touches in 

 A, and of which AB is a diameter and Bl an ordinate ; that is, 

 it muft defcribe the very parabola Acl, and it muft defcribe it 

 in the fame time that the light incident with the velocity, and 

 in the direction AF, defcribes the parabola ACL. Its motion, 

 therefore, both before and after refraction, is the fame with the 

 relative motion of the light having the velocity and direction 

 AF, incident on the medium moving with the velocity and in 

 the direction AI. 



Let c be the point of interfection of the parabola Acl and 

 the plane BS. Draw cC parallel to Ai, cutting the parabola 

 ACL in C. C muft be the point of that parabola, where the 

 refraction by the moving medium is completed. For LI : Cc 

 s= Af : Ap, = AF : AP, = T, AL : T, AC, - T, Al : T, cC. 

 Therefore, while the light moves from A to c, the point c moves 

 from c to C, where the light will pafs through it, and the re- 

 fraction be completed, the plane BS having now gotten into the 

 fituation bs, and the plane AQ^into the fituation aq. 



Draw the ordinates ADE, Ade, to the diameters PC, pc, 

 and draw mr, the directrix of the parabola Acl, and join Dd» 

 It is known that AF is to AE as the velocity in A to the velo- 

 city in C. Now, AE : AD = AF : AP, - Af : Ap, = Ae : Ad. 

 Therefore, Dd is parallel to Cc. Therefore the velocity Ae, 



compounded 



