92 On the MOTION of LIGHT. 



tlon As, cutting the line «/3 in the point s, £o 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 

 imiformly 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 objecfl through 

 the water tv;be or water telefcope, having the fame pofition 

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

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

 however fitviated, and be firmly fixed in its pofition, the objedl 

 will always be feen on the interfeclion 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 terreflrial objedts 

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

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

 water telefcope, for deciding the quefiiion concerning the acce- 

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

 the fixed ftars. 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, publillied at 

 Baflano in 1785, mentions his having heard that fome perfon 

 had publifhed a diflertation in the Philofophical Tranfadions of 

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

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

 formation, which had been publiflied fome time before. This 

 alludes to a Diflertation by our worthy member Mr Wilson, 

 now profeflTor 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 moft 

 pviblic manner. In the prefent inflance, he is fortunately pro- 



teded . 



