On the MOTION of LIGHT. ot 



would be a great deficiency of light ; for a telefcope of this 

 length, magnifying thirty times, would not render 10" fufE- 

 ciently diftinguifhable. 



But this aberration of terreftrial objects, announced by Mr 

 Boscovich, mufl be obferved in a compound microfcope filled 

 with water, or conftructed in the manner defcribed in the be- 

 ginning of this difcourfe. In fuch a microfcope, we can have 

 abundance of light by external illumination, and little will be 

 loft in its pafTage through the fhort column of lime-water or 

 glafs. 



The intelligent reader will eafily fee that this aberration of 

 an object placed before the microfcope mud be obferved even 

 at the bottom of a mine. He will alfo fee that, if the fun, 

 with his attending planets, be carried along in any direction, 

 with a velocity much greater than that of the earth in its orbit, 

 another aberration, will be obferved, greater than the former, 

 and diftinguifhable from it, although blended with it. Confe- 

 quently, we fhould be able to difcover, by means of this aber- 

 ration, fuch hitherto unknown motion of the folar fyftem. It 

 will readily be believed, therefore, that I engaged with eager- 

 nefs in preparations for this experiment, and in farther re- 

 fearches into its theory, and that I was greatly mortified when 

 I found my hopes of curious difcovery fruftrated by the detec- 

 tion of an overfight in Mr Boscovich's reafoning. This I 

 fhall now fubmit to the Society. 



Mr Boscovich fuppofes, that when the light, moving in the 

 direction OA, enters the water tube at A, it moves on in the di- 

 rection Aj8, defcribing AE uniformly, while the hole A moves 

 from A to a. But Mr Boscovich, in the fame difcourfe, pro- 

 fefTes to maintain the opinion advanced by Sir Isaac Newton, 

 viz. That light is accelerated in water by forces which act per- 

 pendicularly to its furface. If this be the cafe, the light en- 

 tering the water at A, in the oblique direction OA, will be re- 

 fracted towards the perpendicular, and will move in the direc- 



m i tion 



