92 On the MOTION of LIGHT:. 



tion As, cutting the line a/3 in the point s, fo fituated that A/3 

 is to As as the velocity of light in air is to its velocity in water. 

 This line As will cut the line B/3 in fome point i. Now, fince 

 the light moves uniformly along As, while the hole B moves 

 uniformly along B/3, it follows that they will meet in i, where 

 the light will pafs through the hole, and enter an eye placed 

 behind it. The obferver, therefore, will fee the object through 

 the water tube or water telefcope, having the fame pofition 

 with the plane fights or common telefcope ; and if the water 

 telefcope be directed to the real place of a terreftrial object, 

 however fituated, and be firmly fixed in its pofition, the object 

 will always be feen on the interfection of the crofs wires, and 

 no aberration will be obferved in confequence of the earth's 

 motion. 



I may here obferve, that this aberration of terreftrial objects 

 occurred to Mr Boscovich but very lately, namely, while he 

 was writing for the prefs his DifTertation on the ufe of the 

 water telefcope, for deciding the queflion concerning the acce- 

 leration or retardation of light, by means of the aberration of 

 the fixed flars. It appears that Mr Boscovich has long had 

 this ingenious thought; for M. de la Lande, in the fourth 

 volume of his Aftronomy, mentions his having received a let- 

 ter to that purpofe from Mr Boscovich in the year 1769. Mr 

 Boscovich, in the firfl volume of his Opufcula, publiihed at 

 Bafiano in 1785, mentions his having heard that fome perfon 

 had publiihed a difTertation in the Philofophical Tranfactions of 

 London upon the fame fubject, and feems difpofed to think, 

 that the author had been indebted to Mr de la Lande's in- 

 formation, which had been publifhed fome time before. This 

 alludes to a DifTertation by our worthy member Mr Wilson, 

 now profefTor of Aftronomy at Glafgow. But this gentleman's 

 ingenuity is too eminent to make him need any information 

 which his candour would not difpofe him to avow in the mofl 

 public manner. In the prefent inftance, he is fortunately pro- 

 tected 



