no On the MOTION of LIGHT. 



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

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

 the incident and reflected light. 



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

 and alfo the abfolute velocities of the incident and reflected 

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

 reflecting fubftance is in the direction of the reflecting 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 reflected light 

 will exceed that of the incident light by twice the perpendicular 

 velocity of the reflecting plane. The contrary will happen if 

 the plane be moving towards the oppofite fide. 



The application of the foregoing proposition to Mr Bosco- 

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

 fo directed that the image of the object is formed upon the 

 interfection 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 direction of the relative 

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

 object. Now, when the water telefcope has the pofition AB, 

 (fig. 1.) 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 

 direction and with the velocity DA, it would have proceeded 

 in the direction AB, and therefore the telefcope mud always be 

 directed to the real contemporaneous place of the terreftrial 

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

 Mr Boscovich afferts. 



Thus 



