On the MOTION of LIGHT. 93 



tested againfl every imputation of plagiarifm, as I mall pre- 

 fently make appear, by returning again to the considera- 

 tion of the apparent place of objects viewed through a theo- 

 dolite. 



Suppose that the object O is a fixed ftar, and that, while a 

 ray of light proceeds from it in the direction OA/3, with a uni- 

 form motion, the theodolite moves uniformly from the fituation 

 A'B' to the fituation a/2. It is evident, that the light will enter 

 the anterior fight at A, and pafs through the pofterior fight at 

 /3. An obferver, therefore, will fee the liar by looking through 

 the theolodite. But he will judge falfely of its place; for he 

 will imagine that it lies in the direction /•&, while it is really in 

 the direction /30. The angle 0/3£ is called the aberration of 

 the ftar, and the proportion of the velocity of light to the ve- 

 locity of the earth is fuch that this angle is about 20". Let us 

 now fuppofe that the water tube is ufed. Then, fays Mr Bos- 

 covich, if the water tube have the fame pofition with the 

 plane fights, the ftar will not be obferved through it ; becaufe, 

 when the hole B has come to /3, the light which entered at A 

 has got to E, and when this light has got to (3, the hole has got 

 only to y : Therefore, in order to fee the ftar, the water tube 

 muft have a different pofition, which he thus determines : — 

 Make B<p equal to /3y, and draw A<p. This will be the proper 

 pofition of the tube. For, while the light which enters at A 

 moves from A to (3, the pofterior fight of the theodolite will 

 have moved from <p to (B, where it will meet the light, and al- 

 low it to pafs through to the eye of the obferver. If the velocities 

 of light in air and water are as three to four, the angle BA<p 

 will be about 5", in the opinion of Mr Boscovich, and the 

 aberration of the ftar will be about 15", namely, three-fourths 

 of the aberration difcovered by the plane fights. In like man- 

 ner, when the aberration of the ftar is obferved with a tele- 

 fcope filled with water, by bringing its image to the interfection 

 of the crofs wires in the eye-piece of the telefcope, it will 



be 



