SENSE OF SIGHT. 



427 



cornea, converging again after passing through this dioptrical 

 organ, and uniting in a point which, in the normal state, and 

 under circumstances which we will mention, is situated 

 directly on the retina: it is here that external objects are 

 represented in smaller dimensions. If the convergence, how- 

 ever, does not take place exactly on the retina, but rather in 

 front or behind it, we can easily understand that every point 

 of the object presented to the eye will be represented on the 

 membrane, not as a point, but as a small circle, which corre- 

 sponds to the plan of section by the retina of the converging 

 cone, which these rays form before their union, or of the 

 diverging cone which they form after their union (Fig. 114). 

 In order to make this perfectly plain, let us give the name 

 of objective cone to the cone of luminous rays which, pro- 

 ceeding from the luminous point, diverge as they fall upon 



Fig. 114. Ocular and objective cones.* 



the cornea, and that of ocular cone to the cone which repre- 

 sents these rays after undergoing the converging influence of 

 the ocular lens (Fig. 114) : the slightest familiarity with 

 optics will show us that if the rays of light come to us from 

 a great distance, as from a star, the objective cone will attain 



* A, B, The luminous points under consideration, c, c, Cornea. D D, Iris. 

 E E. Lens. 



At first the luminous rays which set out from the points A or B are bent by 

 the cornea C C and by the watery humor contained between this membrane and 

 the crystalline, that is, they are brought near to the median ray which travels on 

 a line parallel with the axis. A second refraction takes place in the lens, and 

 the final result appears in the form of ocular cones, the summit of which is at 

 and 6, or exactly on the retina: but we also see that if the retina, instead of 

 exactly corresponding with the summit of the ocular cones, intersected them 

 either farther forward (at H) or backward (at G), the image formed upon 

 this membrane would no longer be a point, but a small circle (circle of 

 diffusion). 



