On the MOTION of LIGHT. 85 



fcope, but in the oppofite direction ; as in the microfcope, there 

 would be no want of light, we fhould have the mod fatisfacto- 

 ry decifion of this important queflion in optics, and alfo the 

 opportunity of detecting any hitherto unknown motions of the 

 globe which we inhabit. It may alfo be fhown, that, if any 

 of thefe motions be very confiderable, we (hall determine ano- 

 ther very important queflion in optics, viz. Whether the mo- 

 tion of light be affected by the motion of the luminous body. 



On thefe, and many other accounts, I was eager to conftruct 

 this microfcope, and fet about it accordingly. But I happened 

 at that time to be engaged in that part of my courfe of lectures 

 where I had occafion to confider the apparent motions of bo- 

 dies. I confider it as the fundamental propofition on this fub- 

 ject, that " the apparent motion of a body is compounded of its real 

 " motion^ and the oppofite to the real motion of the obferver." The 

 confequence is, that, fince the motions of the terreftrial object 

 and of the obferver are always nearly equal, there fhould be no 

 apparent motion in the object, and therefore no apparent diurnal 

 change of place. This ftartled me, and caufed me to confider the 

 matter more minutely. ProfefTor Wilson, to whom I com- 

 municated my doubts, raifed other objections, founded on the 

 application of mechanical principles to that hypothefis, with re- 

 fpect to light, which the Abbe Boscovich profeffes to main- 

 tain. In my fubfequent fpeculations on this fubject, I found, 

 that the application of the above mentioned propofition was not 

 flrictly jufl with refpect to the apparent place of the terreftrial 

 object ; but I was led by it to difcover the real flate of the 

 matter, by applying it to the determination of the apparent 

 motion of the light by which the object is feen. I thus detect- 

 ed the circumflance which Mr Boscovich had overlooked, and 

 which unfortunately puts an end to the hopes which I had en- 

 tertained of many curious and important difcoveries. I flatter 

 myfelf that this Society will not think this fubject unworthy of 

 their notice ; but am extremely forry that my infirm flate of 



health 



