loo On the MOTION of LIGHT. 



fpecific velocity of the medium. It is known that, if it moved 

 uniformly with this velocity, it would defcribe AO, double of 

 AB, during the time of defcribing AB with the uniformly ac- 

 celerated motion. Therefore, (taking this for the unit of time) 

 AF, AI and AO, will exprefs the velocity of the incident light, 

 the progreflive velocity of the n^edium, and the fpecific velocity. 

 It is alfo known, that the light, in paffmg through the refradl- 

 ing ftratum, will defcribe a parabola ACL, which AF touches 

 in A, and of which AB is a diameter, having BL, equal and 

 parallel to AF, for an ordinate. 



Produce IA till Ai is equal to AT; complete the parallelo- 

 gram AFfi, and draw the diagonal Af. The motion Af, com- 

 pounded of AF, the real motion of the incident light, and Ai^ 

 the oppofite to the motion of the medium, is the relative mo- 

 tion of the incident light. This motion will be uniform, be- 

 caufe it is compounded of motions which are uniform. There- 

 fore Af will exprefs the relative velocity of the incident light. 

 Draw FL. It is evidently equal and parallel to AB. Let CP 

 be drawn from any point C of the parabola, parallel to AB, 

 cutting AF in P. Complete the parallelograms FLlf, flBA. 

 Draw C c parallel to Ff, and make Ff : Cc = AF : AP. I fay 

 that the point c is in a parabola Acl, of which AB is a diame- 

 ter, and Af a tangent, and which is the relative path of the 

 light, and that L and 1, C and c, are contemporaneous places of 

 the light in its real and relative paths. Draw Pp parallel to Ff, 

 cutting Af in p. Draw the ordinate CK, and draw pc, Kc. 

 Then Ff : Pp = AF : AP, — LI : Cc. But Ff is equal to LL 

 Therefore Pp is equal to Cc, and pc is equal and parallel to PC 

 and AK, and Kc is equal and parallel to Ap. Therefore 

 Kc' : Bl'=r Ap' : Af%nAP= : AF%= KC= : BL% = AK : AB. 

 Therefore the points A, c, 1, are in a parabola, of which AB 

 ' is a diameter, Af a tangent, and Kc, Bl, are ordinates. Alfo* 

 becaufe LI : Cc, =r Ff : Cc, =r AF : AP, r= T, AF : T, AP, (by 

 the fymbol T, AF, i^c. is exprefled the time of moving along 



AF, 



