no 0>i the M or ION of LIGHT. 



Cor. 7. When the light is refleded, the relative angles of in- 

 cidence and reflecflion are equal, as alfo the relative velocities of 

 the incident and retletSled light. 



Cor. 8. But the abfolute angles of incidence and reflection, 

 and alfo the abfolute velocities of the incident and refleded 

 light, are xineqiial, except in the cafe where the motion of the 

 refleding fubllaace is in the diredlion of the refleding plane. 



If the plane be moving towards that fide from which the 

 light comes, the angle of reflection will be lefs than that of in- 

 cidence, and the perpendicular velocity of the refle(51ed light 

 will exceed that of the incident light by twice the perpendicular 

 velocity of the refleding plane. The contrary will happen if 

 the plane be moving towards the oppofite fide. 



The application of the foregoing propofition to Mr Bosco- 

 vich's experiment is extremely fimple. When the telefcope is 

 fo direded that the image of the objed is formed upon the 

 interfedion of the crofs wires in the eye-piece, the relative mo- 

 tion of the light is performed along the axis of the telefcope ; 

 or the axis of the telefcope is in the diredion of the relative 

 motion of the light, and indicates the apparent pofition of the 

 objed:. Now, when the water telefcope has the pofition AB, 

 (fig. I.) the relative motion of the light in the telefcope is the 

 fame as if the telefcope had been at reft, and the light had ap- 

 proached it with the fame relative motion. Now, the motion 

 DA is evidently the relative motion of the incident light. For 

 it is compofed of OA, the real motion of the light, and DO, 

 the oppofite to the real motion of the telefcope. Now, if the 

 telefcope had been at reft, and the light had entered it in the 

 diredion and with the velocity DA, it would have proceeded 

 in the diredion AB, and therefore the telefcope muft always be 

 direded to the real contemporaneous place of the terreftrial 

 objed, and there will be none of that diurnal deviation which 

 Mr BoscovicH aflerts. 



Thus 



