210 HAND-BOOK OF PHYSIOLOGY. 



the same as if the retina were shifted forward to mm. Two images, h, g, 

 are formed, one from each hole. It is interesting to note that when two 

 Images are produced, the lower one g really appears in the position Q, 

 while the upper one appears in the position p. This may be readily 

 Terified by covering the holes in succession. 



The contents of the ball of ths eye are surrounded and kept in posi- 

 tion by the cornea, and the dense, fibrous membrane before referred to as 

 the sclerotic, which, besides thus encasing the contents of the eye, serves 

 to give attachment to the various muscles by which the movements of 

 the eyeball are effected. These muscles, and the nerves supplying them, 

 have been already considered (p. 138 et seq., Vol. II.). 



Course of a Ray of Light. With the help of the diagram (Fig. 

 379) representing a vertical section of the eye from before backward, the 

 mode in which, by means of the refracting media of the eye, an image 



FIG. 379. Course of a ray of light. 



of an object of sight is thrown on the retina, may be rendered intelligible. 

 The rays of the cones of light emitted by the points A B, and every other 

 point of an object placed before the eye, are first refracted, that is, are 

 bent toward the axis of the cone, by the cornea c c, and the aqueous 

 humor contained between it and the lens. The rays of each cone are 

 again refracted and bent still more toward its central ray or axis by the 

 anterior surface of the lens E E; and again as they pass out through its 

 posterior surface into the less dense medium of the vitreous humor. For 

 a lens has the power of refracting and causing the convergence of the 

 rays of a cone of light, not only on their entrance from a rarer medium 

 into its anterior convex surface, but also at their exit from its posterior 

 convex surface into the rarer medium. 



In this manner the rays of the cones of light issuing from the points 

 A and B are again collected to points a and b; and, if the retina F be 

 situated at a and J, perfect, though reversed, images of the points A and 

 B will be formed upon it: but if the retina be not at a and I, but either 

 before or behind that situation, for instance, at H or G, circular lumi- 

 nous spots c and /, or e and o, instead of points, will be seen; for at H 



