298 DETAILED SYNTHESIS. 



781 Inf. THE MUSCLES ix THE EYE are three in number: the cir- 

 cular and radiate of the iris, and the ciliary. 



Effect of the Eye and Light upon each other. 



782. THE USE OF THE MEDIA OF THE EYE is to cause 

 the light coming from different visual points through the 

 pupil to be brought to corresponding points or foci in 

 the retina. The cornea has the greatest effect on the 

 light, the lens considerable, the aqueous and vitreous hu- 

 mors but little. 



783. THE EFFECT OF THE LIGHT BROUGHT TO A FO- 

 CUS ix THE RETINA is to produce changes in the cells act- 

 ed upon, different in case of different kinds of light ; 

 resulting in corresponding sensations, and also in the 

 perception that the light has come from a certain direc- 

 tion. 



784. Ittus.-^Red. light causes a sensation of redness ; blue and yel- 

 low acting together cause a sensation of green, etc. If a point in the 

 lower back part of the eye is acted upon, it causes a perception that the 

 light comes from above ; a point in the right side of the retina causes a 

 perception of light coming from the left, etc. 



785. Ulus. IF ONE EYE IS PRESSED WITH THE FINGER, the point On 



which the light will be focused will be changed, and the direction of the 

 object from which the light comes will seem to change ; the object will 

 also appear to move, and, if the other eye is open, will appear double. 

 If both eyes are pressed at the same time, the object will appear double, 

 and seem to move in two directions. 



786. THE USES OF THE EYE, then, are to gather the 

 light that from minute or visual points comes into its 

 pupil, to corresponding points in the retina ; to excite 

 corresponding sensations of color; and to cause percep- 

 tions of the directions of the minute or visual points. 

 These are the complete work of the eye, the only data 

 furnished by it, and all that are necessary ; at the same 

 time very pleasurable sensations are excited in connec- 

 tion with the fulfilment of these offices. 



781. How many ? 782. What is -? 788. What is ? 781 Illus. 785 Effect 

 78S. What are ? 



