On the MOTION of LIGHT. 109 



the fquare of the relative velocity of the refracted light. This 

 is therefore a conftant quantity, or the relative velocity of the 

 refracted light is the fame in all the relative angles of re- 

 fra<ftion. 



Cor. 3. If the relative velocities of the incident light be the 

 fame in all angles of incidence, the fines of relative incidence 

 and refraction are in a conftant ratio, namely, the inverfe ratio 

 of the relative velocities of the incident and refracted light. 

 This appears by combining the laft corollary with the flrft. 



Cor. 4. When light moving with the fame velocity in all di^ 

 rections, is refracted by a medium at reft, the fines of incidence 

 and refraction are in the conftant ratio of the velocities of the 

 refracted and incident light. This appears from the laft co- 

 rollary. 



Cor. 5. If the light moving in a medium A be refracted by 

 a medium B, which is in motion, and emerge from it into the 

 medium A, it will regain the relative velocity which it formerly 

 had when in this medium. For the fquare of its relative velo- 

 city while in the medium B, differs from the fquare of each of 

 its relative velocities in the medium A by the fame quantity, 

 viz. by the fquare of the fpecific velocity of the medium B. 

 It is evident that the fame thing will happen when the light 

 paffes through feveral contiguous mediums in motion before it 

 emerges again into the medium A. 



Cor. 6. If the relative paths of the light before its entering 

 into the medium B, and after its emergence from it, be equally 

 inclined to the direction of the medium, the abfolute velocities 

 of the incident and emergent light will be equal, but in no 

 other cafe. This is eafily feen by refolving the relative motions 

 of the incident and emergent light. Hence we learn, that if the 

 plane furface of a plano-convex lenfe be turned towards a fixed 

 liar to which the earth is approaching, or from which it is re- 

 ceding, the abfolute velocity of the lateral emergent rays will 

 be increafed or diminifhed. 



Cor., 



