88 On the MOTION of LIGHT. 



dolite. Therefore an eye placed behind the hole B will receive 

 the light which paffes through it when in the fituation /3, and 

 the obferver will fee the object through the fights of the inftru- 

 ment. He will fee it by means of light moving in the di- 

 rection O/S ', but he will not imagine that the object lies from 

 him in the direction (BO ; for he eftimates the direction of the 

 object by the poiition of the fiducial line of his inftrument, 

 which is always parallel to the line joining the centres of the 

 holes which are its fights. When he fees the object, thefe 

 holes are at /3 and a, and therefore the obferver afligns to the 

 object the direction /3a. Let BA and /3a cut the line O^ in the 

 points D and £ ; it is evident, that, when the object is at O, D 

 and £, the anterior fight of the theodolite, is at A', A, and a, and 

 the other fight is at B", B and /3 ; therefore, when the object is 

 feen through the inftrument, it is always feen in its real place, 

 and in its true direction, although not in the direction of the 

 light by which it is feen % confequently, if the index be di- 

 rected fo that the object may be feen through it, that is, if the 

 index be pointed to the object in the common way, and the whole 

 inftrument be kept firm in its pofition, it will always point to 

 the object, although both the inftrument and object are conti- 

 nually changing their places by the motion of the earth round 

 the fun and round its own axis. 



But now let us fuppofe, with Mr Boscovich, that the in- 

 terval between the two fights is filled with water contained in a 

 tube, and bounded by two plates of glafs at A and B, perpen- 

 dicular to the line AB, and covered externally with paper, 

 pierced with two fmall holes at A and B. Now, fays Mr Bos- 

 covich, the light which enters at A will not be at B when the 

 hole B arrives there ; for, if the velocity of light in water be 

 to the velocity of light in air, as the fine of incidence in air to 

 the fine of refraction in water, then, while the hole B comes 

 to /3, the light will move from A to E, fo that AE is to A/3 as 

 four to three nearly ; confequently, when the light has come 



to 



