84 On the MOTION of LIGHT. 



There is one deduction which he makes from his premifes, ex- 

 tremely curious in itfelf, and having the jnoft furprifing confe- 

 quences. It is this : If a telefcope be conftructed, having its 

 tube filled with water, and be directed to a terreftrial object 

 properly fituated, it will be found to deviate from that object by 

 a certain determined quantity every day. It will follow from 

 this, that a perfon fhut up in a mine or dungeon, may, without 

 feeing the fun or heavens, difcover the motion of the earth 

 round the centre of the folar fyftem, and alfo whether this cen- 

 tre be in motion, and the velocity and direction of this mo- 

 tion. 



The contrivance of a telefcope filled with water, has been 

 long familiar to my thoughts, (as a means of difcovering whe- 

 ther light be accelerated when refracted towards the perpendi- 

 cular) in confequence of the fpeculations of my ingenious 

 friend ProfefTor Wilson of Glafgow. But all my attempts to 

 conflruct fuch a telefcope have hitherto proved abortive, for 

 want of a fubftance fufficiently tranfparent to admit of the 

 neceffary magnifying power. I faw that this rendered ufelefs 

 the beautiful theory of their conftruction which is contained in 

 this paper of Mr Boscovich. But, at the fame time, I faw 

 that this aberration of terreftrial objects would enable us to de- 

 cide the fame queflion by means of a compound microfcope of 

 a very eafy conftruction. If a cylindrical piece of glafs be 

 ground fpherical at one end, and plane at the other, and if the 

 plane furface be fituated a fmall diftance beyond the principal 

 focus of the fpherical furface, and a fcratch be made on the 

 plane furface, and confidered as a vifible object, an image of 

 this fcratch will be formed in the conjugate focus of the fphe- 

 rical furface, which image may be viewed by means of a deep 

 eye-glafs, as in the ordinary compound microfcope. If this 

 image be formed on a frame of wires, like the wires of an aftro- 

 nomical telefcope, there muft be obferved the fame diurnal de- 

 viation that Mr Boscovich announces with refpect to his tele- 

 fcope, 



