DIOPTRIC MECHANISMS OF THE EYEBALL 



589 



B 



principal focus the rays of light passing through it to the lens will take 

 the reverse course and leave the lens as a bundle of parallel rays. Any 

 point of light situated between infinite distance and the principal 

 focus will have a corresponding point on the other side of the lens to 

 which its rays will converge. Such corresponding points are known as a 

 conjugate foci (Fig. 257). In a thin lens, with the same media on 

 each side, the anterior and posterior focal distances are the same, so 

 that from whichever direction a parallel beam falls on the lens the point 

 to which its rays are converged on the other side of the lens is constant. 

 If instead of a point of light we have a series of points such as 

 that coming from a bright line in the plane of the paper (as in Fig. 258), 



FIG. 258. 



each of these points will have a corresponding point on the opposite 

 side of the lens. Thus from the point p three rays may be taken, viz. : 



(1 ) The ray PO, which passes through the centre of the lens and does 

 not deviate. 



(2) The ray PE, which is parallel to the axis and therefore is 

 converged to the principal focus 2 - 



(3) The ray PG, which passes through the focus on the front 

 surface of the lens A and therefore takes a course on the other side of 

 the lens parallel to the axis. The intersection of any two of these three 

 lines will be the situation of the image p to the point P. In the same 

 way all the other points in the object will have a corresponding image 

 on the opposite side, so that an inverted real image of the luminous 

 object is formed on this side. 



The size of the image as compared with the object will depend on 

 the distance of the object from the lens. It is greater than the object 

 if the latter is less than 2/ (twice the focal distance) from the lens, 

 equal if the distance is 2/, and diminished if the distance is greater 



