530 PHYSIOLOGY 



with the same media on each side, the anterior and posterior focal distances 

 are the same, so that from which ever 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. 260), 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. : 



FIG. 260. 



(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 <f> 2 . 



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

 of the lens X 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 than 2/. 



A lens with a focal distance of 1 metre is taken as unit strength. 

 Such a lens is said to have a strength of one dioptre ; a lens of two 

 dioptres would have a focal distance of \ metre ; of four dioptres one 

 of \ metre, and so on. 



DIOPTRIC MECHANISMS OF THE EYEBALL 



The eye is not directly comparable to a camera in its optical arrange- 

 ments, since the image is formed, not in air, but in the fluids of the eye 

 itself. Moreover the conditions are complicated by the facts that it is 



