3S2 HITCHCOCK'S ANATOMY 



eyes, and almost instantly, by turning them to a mountain 

 top several miles distant, we can see an object there with per- 

 fect cleaarness. No optical instrument can be made which 

 will so quickly do this, for it is necessary to make a new ad- 

 justment of the lenses to adapt it to different distances, but 

 the eye is at once ready. It is supposed that this adjustment 

 is effected by a shortening or lengthening of the diameter of 

 the crystalline lens, or by drawing the lens towards the pos- 

 terior part of the eye, by means of a few muscular fibers 

 called the ciliary muscle, running from the bones of the nose 

 to the cornea, which by their contraction would force the 

 aqueous humor upon the crystalline lens in such a manner as 

 to flatten it, and by a relaxation of the same fibers a dilata- 

 tion in an antero-posterior direction is effected, and that in- 

 stantly. 



701. Experiment for Seeing the Arteries of one's own 

 Eye. — The image of the arteries of one's own eye can be 

 readily seen in the following manner. In a dark room place 

 the left hand over the left eye, and in the right hand hold a 

 lighted candle by the right side of the head, and very near 

 to it. Then with the right eye open, looking towards the 

 darkened wall, move the candle up and down rather quickly, 

 and in a few seconds dark branches will appear at a short dis- 

 tance from the eye, looking like the limbs of a tree, which are 

 images of the arteries distributed on the retina. 



702. Limits of Vision. — What is the size of minute ob- 

 jects that can be seen by the naked eye ? Ehrenberg, an 

 eminent microscopist, says that nearly all eyes have equal 

 power to discern minute objects, whether long or short-sighted. 

 The smallest square magnitude visible to the naked eye,; 

 either of white particles on a dark ground, or the reverse, is 

 about the T ^th of an inch. Brilliant particles which pow- 



How is the telescopic and the. microscopic power of the eye explained ? 701. How can 

 we see the image of the arteries of our own eyes? 702. What is the smallest square sur- 

 face that can be seen with the naked eye ? 



