OF THE LATE PROFESSOR ROBISON. 519 



maintained, that in ascertaining the effect of a water telescope 

 on the motion of light, the observation of celestial objects might 

 be dispensed with, and that of terrestrial substituted in its place. 

 He argued, that while light moves with an uniform velocity, 

 the telescope must be directed, not to the point of space which 

 the object occupied when the particle was sent off which is 

 entering the telescope, but to a point advanced before it by a 

 space just equal to that which both the object and the obser- 

 ver have passed over in the time in which the particle has pas- 

 sed from the object to the eye. It is therefore directed exact- 

 ly to the place which the object is in when the light from it 

 enters the eye. If, therefore, the ray, on entering the tele- 

 scope, is made to move faster than it did before, the telescope 

 must not be inclined so much, and the apparent place of the 

 object will fall behind its true place. If the ray is retarded on 

 entering the water, the contrary must happen. Hence a num- 

 ber of very unexpected phenomena Would result, affording, 

 without having recourse to the heavenly bodies, a direct proof 

 of the motion of the earth in its orbit, as well as a resolution 

 of the question, whether light is accelerated or retarded on 

 passing from a rarer to a denser medium *. 



On this reasoning Professor Robison has very well re- 

 marked, that it would be just, if the light, on entering the 

 water telescope, had only its velocity changed, and not its di- 

 rection. But this is not the case ; for the ray that is to go 

 down the axis of the telescope, is not perpendicular to the 

 surface of the fluid ; it makes an angle with it, depending on 

 the aberration, and therefore in some cases less by 20 " than a 

 right angle. On this account, the effect is not produced which 

 Boscovich's reasonings lead us to expect. 



4 U 2 The 



* Boscovich, Opera Math. torn. n. opusc. 3. 



