Chap. XXIX.] SlZE OF RETINAL IMAGES, 381 



The distance between the anterior principal focus 

 and the anterior principal point ( = the anterior focal 

 length) is 15-0072 mm. ; and the distance between the 

 posterior principal focus and the posterior principal 

 point (= the posterior focal length) is 20 '0746 mm. 



From these data may be shown the course of rays 

 through the eye and the position and size of images. 



The size of the retinal image. In consider- 

 ing simple lenses, we saw that the size of the image 

 was obtained by the formula, 



size of image distance of image from lens 



size of object distance of object from lens 



The same rule applies to the eye when we calculate 

 the distances of image and object as from the nodal 

 points. The two nodal points may be taken as coin- 

 ciding ; therefore 



size of image distance of image from nodal point 



size of object distance of object from nodal point 



The distance of the image from the nodal point is 

 the posterior focal distance ; this distance in distinct 

 vision may be counted as the distance between retina 

 and cornea, less the distance between cornea and 

 nodal point ; while the distance of the object from the 

 nodal point is the distance of the object from the 

 cornea + the distance between cornea and nodal 

 point. 



Let I = size of image, = size of object, P = dis- 

 tance between object and cornea, P' = distance between 

 retina and cornea, and B, distance between cornea 

 and nodal point ; then 



I ZjzJ? 



o P + R' 



NY>w the distance P' 22 '6470 mm., and the 



