640 



THE SENSES 



refracted in a line parallel with the axis, df; such a point, F 2 , is called the 

 anterior principal focus. 



In any given system the principal foci can be found by erecting verticals 

 at the nodal and principal points of the optical axis and laying off lengths 

 on each, a and Z>, proportional to the refractive indices of the media. A line 

 drawn through a on the principal vertical and b on the nodal vertical will cut 

 the optical axis at the posterior principal focus, and vice versa. 



FIG. 453. Diagram of a Simple Optical System. (Foster.) The curved surface, bd, is sup- 

 posed to separate a less refractive medium toward the left from a more refractive medium toward 

 the right. 



If a luminous point outside the anterior principal focus is considered, 

 it is obvious that rays from it will be so refracted when they enter the convex 

 surface that they will become converging and will ultimately meet again in 

 a point or focus. Two such points form conjugate foci, figure 454. If the 

 anterior focus of a conjugate is moved away from the anterior principal focus, 

 then the posterior conjugate will move toward the posterior principal focus, 

 and the converse. If one conjugate is known, the other can be found as 



FIG. 454. Diagram to Show the Relations of Conjugate Foci, cd. Refracting surface; AB 

 and ba, conjugate foci; o, nodal point; F", posterior principal focus. 



follows: From a point in the plane of the known conjugate, but outside the 

 principal axis, draw two rays, one perpendicular to the refracting surface 

 which will pass through the nodal point, the other parallel to the principal 

 axis. The latter will be refracted through the posterior principal focus and 

 when prolonged will meet the first ray in the plane of the second conjugate, 

 figure 454, a. This relationship between conjugate foci is played upon in the 

 focussing of a camera. 



