850 



THE SENSE OF SIGHT 



ina. Inside this circle of light upon the retina lies the shadow of the 

 pin in its natural position. It appears inverted for the obvious reason 

 that the retinal elements stimulated by this shadow, are associated in 

 an inverse manner. Consequently, any shadow falling upon the lower 

 expanse of the retina, is interpreted as having been produced by an 

 object situated in the upper visual field. 



FIG. 450. DIAGRAM TO ILLUSTRATE THE FORMATION AND PROJECTION OF THE SHADOW OF A 



PIN. 



A, Pin; J, shadow of it upon lower retina; P, projected as if moving into the visual 

 line from above. 



The Size of the Retinal Image. The dimensions of the image of 

 an object upon the retina may readily be ascertained if the size of the 

 object and its distance from the cornea are known (Fig. 451). Sup- 

 posing that the object AB is focused upon the retina in A'B', then AB 

 and A'B' really form the bases of two similar triangles, the apices of 

 which are situated at the nodal point of the lens, while its sides are 

 formed by the secondary axes AB' and BA'. If C stands for the dis- 

 tance of the nodal point from A, and D for the distance of this point 

 from B', then: 



AB _ A'B' 

 C '' D 



B 

 FIG. 451. DIAGRAM TO SHOW HOW THE SIZE OF THE RETINAL IMAGE MAY BE DETERMINED. 



As has been stated above, the distance of the image from the nodal 

 point may be reckoned at about 15 mm. Consequently, an object 

 120 feet in height, placed at a distance of 25 miles, forms an image upon 

 the retina, the dimension of which is 



120 ft. 120 ft. . , _ 1 w t s 



;r- :T X 15 mm., i.e. - OQn - ... X 15 mm., or ^r^ X 15 mm. = 

 25 miles 5280 X 25 ft. 



0.013 mm. 



