HEFHACTION IN THE EYE 457 



these rays be parallel to the axis they will unite 

 in the posterior principal focus. Conversely, all 

 rays parallel in the second medium unite in one 

 point on the first or anterior focal surface, and if 

 parallel to the axis they unite, at the anterior 

 principal focus (Gauss). 



1. Find the course in the vitreous humor of 

 any ray, a b, which enters the eye. 



Draw in the first medium a ray, a 1 b', parallel 

 to a I, directed to the first nodal point. In the 

 second medium draw this ray, parallel to its origi- 

 nal direction, from the second nodal point to the 

 posterior focal surface. Then a b must also meet 

 the posterior focal surface at this same place ; for 

 all rays parallel in the first medium converge in 

 the second medium to one point in the posterior 

 focal surface. Produce a b to the first principal 

 surface, thence, parallel with the optical axis, to 

 the second principal surface, thence, through the 

 vitreous humor, to the point already found in the 

 posterior focal surface. 



2. Let i be any point in the first medium (the 

 air). Find its image (for convenience i should 

 be placed at least 10 cm. in front of the cornea). 



Draw from i a ray, i\j\, through the first and 

 second nodal points, as directed above. Draw 

 from i a second ray, 4*2/2, parallel with the optical 

 axis. This ray will cut the second focal surface 



