On the MOTION of LIGHT. 99 



I now begin with an examination of the Newtonian hypothe- 

 cs. But I fhall content myfelf with one confequence of it on- 

 ly, viz. That in refractions and reflections, the light is actuated 

 by forces whofe direction is perpendicular to the furface of the 

 refracting or reflecting body. I fhall here confider the effect 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 effects of forces tending from the medium. 



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

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

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

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

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

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

 While the light is between the planes AQ_and BS, it is affected 

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

 AQ^and XY, it is acted 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 acted on in all direc- 

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

 refracting forces. 



Let us call the fpace contained between the planes AQ and 

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

 at firft, that the refracting forces act uniformly through the 

 whole extent of the ftratum. 



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

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

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

 uniformly defcribe during the time that the refracting 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 refracting ftratum, be called the 



n 1 fpeciflc 



