1.04 On the MOTION of LIGHT:. 



"Thus we fee that when the light within the refracting medi- 

 um either meets or overtakes the refracting ftratum, or is over- 

 taken by it, and the relative perpendicular velocity is not 

 greater than the fpecific velocity, the light does not emerge 

 from the medium, but is reflected back into it. 



It is farther to be obferved, that in thefe cafes, the angle of 

 relative reflection is equal to the angle of relative incidence, 

 and the relative velocity of the light after reflection is the fame 

 as before reflection. For the tangents At, tc, (fig. 3.4. 5.) are 

 equally inclined to the axis of the parabola Ale, and equal 

 portions of them will be intercepted by the diameters AB, FL, 

 and thefe portions exprefs the relative velocities of the light in 

 A and c. Alfo, the relative perpendicular velocity of the re- 

 flected light is equal to the relative perpendicular velocity of the 

 incident light, but in the oppofite direction. 



Let us fuppofe, that the refracting ftratum is divided into 

 feveral partial ftrata, by planes parallel to AQ^and BS, and that 

 the forces are different in each ftratum, but uniform through 

 its whole extent. 



The relative motion of the light, emerging from the pofterior 

 furface of the firft partial ftratum, is the relative motion of the 

 light immerging into the fecond ftratum. Therefore, by the 

 preceding reafoning, the relative motion of the light emerging 

 from the pofterior furface of the fecond ftratum, is the fame as 

 if the medium had been at reft, and the light had approached 

 it with the fame relative initial motion. The fame muft be 

 affirmed of all the partial ftrata in fucceflion, and is therefore 

 true with refpect to the final motion of the refracted light. 



Further, the whole change which is made on the fquare 

 of the relative velocity of the incident light, in thofe cafes 

 where it paffes through and emerges from the refracting ftra- 

 tum, is equal to the fquare of the velocity which a particle of 

 light would acquire if impelled by the variable refracting 

 forces from a ftate of reft through the whole refracting ftratum. 



For 



