THE SENSE OF SIGHT. 



243 



The Eyeball a Living Camera Obscura. The eyeball may be com- 

 pared in a general way to a camera obscura. The anatomic arrange- 

 ment of its structures reveals many points of similarity. The 

 sclerotic and choroid may be compared with the walls of the chamber ; 

 the combined refractive media, cornea, aqueous humor, lens, and 

 vitreous humor, to the lens for focusing the rays of light ; the retina, 

 to the sensitive plate receiving the image formed at the focal point ; 

 the iris, to the diaphragm, which, by cutting off the marginal rays, pre- 

 vents spheric aberration and at the same time regulates the amount 



d 



FIG. 31. DIAGRAM OF A VERTICAL SECTION OF THE EYE. (From Holden's 

 "Anatomy.") 



i. Anterior chamber filled with aqueous humor. 2. Posterior chamber. 3. 

 Canal of Petit, a. Hyaloid membrane. b. Retina (dotted line). c. 

 Choroid coat (black line), d. Sclerotic coat. e. Cornea, f. Iris. g. 

 Ciliary processes, h. Canal of Schlemm or Fontana. i. Ciliary muscle. 



of light entering the eye ; the ciliary muscle, to the adjusting screw, by 

 which distinct images are thrown upon the retina in spite of varying 

 distances of the object from which the light rays emanate. The 

 structures just enumerated are those essential for normal vision. 



The relationship of the various structures composing the eyeball 

 is shown in Figure 31. 



The dioptric or refracting apparatus, by which the rays of light 

 entering the eye are so manipulated as to produce an image on the 



