On the MOTION of LIGHT. 99 



I NOW begin with an examination of the Newtonian hypothe- 

 fis. But I fliall content myfelf with one confequence of it on- 

 ly, viz. Tliat in refradions and refledlions, the light is aduated 

 by forces whofe diredion is perpendicular to the furface of the 

 refracting or refledting body. I fhall here confider the effedt of 

 fuch forces only as tend toward the body; becaufe it will be 

 evident that the fame reafoning will apply, without any varia- 

 tion, to the effedls of forces tending from the medium. 



Let XZY (fig. 2.) be a plane furface which feparates a re- 

 fradling medium, fuch as glafs, from a void, the medium be- 

 ing fuppofed to be below the plane XY. Let it be fuppofed that 

 ZA, perpendicular to ZY , is the greateft diftance at which a particle 

 of the medium ads upon light. Make ZB, on the other fide of 

 XY, equal ZA, and draw the planes AC^ BS, parallel to XY. 

 While the light is between the planes AQ^and BS, it is affedted 

 by the refrading forces. For while it is between the planes 

 AQ^and XY, it is adled on by all the particles of the medium, 

 whofe diftance from it does not exceed ZA ; and while it is be- 

 tween the planes XY and DS, there are more particles below it 

 whofe diftance does not exceed ZA than there are above it. But 

 when the light is below BS, it is equally adled on in all direc- 

 tions, and its motion through the medium is not aflfedted by the 

 refrading forces. 



Let us call the fpace contained between the planes AQ^ and 

 BS by the name of the refradling ftratum ; and let us fuppofe, 

 at firft, that the refradting forces adl uniformly through the 

 whole extent of the ftratum. 



Let the light, moving in the diredtion HAF, enter the re- 

 fradling ftratum at A, while the medium is moving in the di- 

 redtion AI ; and let AF, AI, be the fpaces which they would 

 uniformly defcribe dui'ing the time that the refradting forces 

 would impel a particle of light, from a ftate of reft in A, 

 through AB. Let the velocity which the light would acquire 

 by this acceleration acrofs the refradting ftratum, be called the 



n 1 fpecific 



