II. 



MECHANICS AND OPTICS. 

 I. 



ABERRATION. 



We know the nature of the phenomenon of aberration 

 discovered by Bradley. The light emanating from a 

 star takes a certain time to traverse the telescope. 

 During this time the telescope is displaced by the 

 Earth's motion. If, therefore, the telescope were 

 pointed in the true direction of the star, the image 

 would be formed at the point occupied by the crossed 

 threads of the reticule when the light reached the 

 object-glass. When the light reached the plane of the 

 reticule the crossed threads would no longer be in the 

 same spot, owing to the Earth's motion. We are there- 

 fore obliged to alter the direction of the telescope to 

 bring the image back to the crossed threads. It 

 follows that the astronomer will not point his telescope 

 exactly in the direction of the absolute velocity of the 

 light from the star — that is to say, upon the true position 

 of the star — but in the direction of the relative velocity 

 of the light in relation to the Earth — that is to say, upon 

 what is called the apparent position of the star. 



The velocity of light is known, and accordingl}- we 



