On the MOTION of LIGHT. 105 



For if the medium were at reft, and the light approached it 

 with the fame relative motion, we have feen that the abfolutt 

 velocity with which the light enters any one of the partial 

 itrata, is the fame with the relative velocity with which it en- 

 ters it when the medium is in motion. Now, when the medi- 

 um is at reft, the change made in the fquare of the abfolute 

 velocity (whatever this velocity be) is equal to the fquare of 

 the fpecific velocity of that ftratum. Therefore, if the velocity 

 with which the light enters this ftratum be that which it would 

 iiave acquired if impelled by the refracting forces from a ftate 

 of reft at A, in the anterior furface of the firft ftratum, the 

 change made on the fquare of this velocity would have been 

 (till the fame, and the whole velocity would be that acquired 

 by the varied impulfe from a ftate of reft in A. This is true 

 with refpect to the laft ftratum ; and therefore if the light enter 

 the refracting ftratum of the quiefcent medium with any velo- 

 city, and in any direction whatever, the change made in the 

 fquare of its velocity, when it has pafled through all the partial 

 ftrata, and emerged from the laft of them, is equal to the fum 

 of the fquares of their refpective fpecific velocities, and this 

 fum is equal to the fquare of what may be called the fpecific 

 velocity of the whole refracting ftratum. Now, the abfolute 

 velocity with which the light emerges from the refracting ftra- 

 tum of the quiefcent medium is the fame with the relative ve- 

 locity with which it emerges from the refracting ftratum of the 

 medium in motion. Therefore, the change made on the fquare 

 of the relative velocity of the incident light is equal to the 

 fquare of the fpecific velocity of the medium. 



We need not employ any time to (how that this is alfo true 

 with refpect to the relative perpendicular velocity. Nor will it 

 be neceflary to {how that when the light, moving within the 

 medium, meets with or overtakes the refracting ftratum, or is 

 overtaken by it, and the relative perpendicular velocity of the 



Vol. H. incident 



