THE EYE. 



extremity of the line of motion connected with, the object would 

 be carried forward faster than the extremity connected vith the 

 observer, and the object would appear to move in the direction 

 of the observer's motion, with a velocity equal to the difference ; 

 but if, on the contrary, the velocity of the observer were greater 

 than that of the object, the extremity of the line connected with 

 the observer would be carried forward faster than that connected 

 with the object, and the change of direction would be the same as 

 if the object were moved in a contrary direction with the difference 

 of the velocities. 



It is easy to perceive that a vast variety of complicated rela- 

 tions which may exist between, the directions and motions of the 

 observer and of the object observed, will give rise to very com- 

 plicated phenomena of apparent motion. Thus, relations may 

 be imagined between the motion of the observer and that of th3 

 object perceived by which, though both are in motion, the object 

 will appear stationary ; the motion of the one affecting the lino 

 of direction in an equal and contrary manner to that with which 

 it is affected by the other ; and, in the same manner, either motion 

 may prevail over the other more or less, so as to give the lino 

 of direction a motion in accordance with or contrary to the real 

 motion of the object. 



90. All these complicated phenomena of vision are presented in 

 the problems which arise on the deduction of the real motion of 

 the bodies composing the solar system from their apparent motions. 

 The observer placed in the middle of this system is transported 

 upon the earth in virtue of its annual motion round the sun with 

 a prodigious velocity, the direction of his motion changing from 

 day to day according to the curvature of the orbit. The bodies 

 which he observes are also affected with various motions at various 

 distances around the sun, the combination of which with the motion 

 of the earth gives rise to complicated phenomena, the analysis of 

 which is made upon the principles here explained. 



91. It is usual to express the relative position in which objects 

 are seen by the relative direction of lines drawn to them from the 

 eye ; and the angle contained by any two such lines is called tho 

 angular or visual distance between the objects. Thus, the angular 

 distance between the objects A and B, fig. 14, is expressed by the 

 magnitude of the angle A E B. If this angle be 30, the objects 

 are said to be 30 asunder. It is evident from this that all objects 

 which lie in the direction of the same lines will be at the same 

 angular distance asunder, however different their real distance 

 from each other may be. Thus, the angular distance between A 

 and B, fig. 14, is the same as the angular distance between 

 A' and B'. 



92 



