94 On the MOTION of LIGHT. 



be three-fourths of the aberration obferved in the fame manner 

 with the common telefcope. This account of the experiment 

 is precifely the fame with that given by Mr de la Lande, and 

 alfo with that given by Mr Boscovich in the DhTertation above 

 mentioned. 



It is eafy to fee, that Mr Boscovich has made the fame 

 overfight here as in the cafe of terreftrial aberration. For, in 

 the frjl place, if the tube has the pofition AB, the light which 

 enters at A, in the direction OA, will not proceed in the direc- 

 tion AE, but be refracted at A, and proceed in the direction As, 

 and will meet with the hole B in the point i. The itar will, 

 therefore, be feen through the water tube, having the fame po- 

 fition with the plane fights ; and if the aberration be obferved 

 with a common telefcope, and a telefcope filled with water, by 

 bringing the image of the flar to the interfection of the crofs 

 wires, it will be the fame in both. In the next place, if the 

 water tube have the pofition A<p, which Mr Boscovich propofes, 

 the light which enters at A will not pafs through the pofterior 

 fight. For the angle BA/3 being 20", and the angle BA<p being 

 5", the angle of incidence will be 15", and the light will ftill 

 be refracled towards the perpendicular, making with it an angle 

 fomewhat greater than 1 1". 



Thus it appears, that the refult of this experiment, made in 

 the manner propofed by Mr Boscovich, will be different from 

 what he announces upon the phyfical principles affumed by 

 him. What I have here determined to be the refult of this 

 experiment, perfectly agrees with Mr Wilson's determination 

 in the Philofophical Tranfactions, where it is propofed by him 

 as a proof that light is accelerated by refraction out of air into 

 water, in the inverfe proportion of the fines of incidence and 

 refraction. Thus, thefe gentlemen differ fo widely in their 

 opinions, both with refpect to the refult of the experiment, and 

 the conclufions drawn from it, that the one cannot be fufpected 

 of having borrowed from the other. 



I 



