OPTICS 283 



If we present to each eye, from its own standpoint, a pic- * 

 ture of the same body, the eyes see the pictured object 

 as a solid body. The instrument by which this is done is 

 called the stereoscope. 



The judgment concerning the distance and the direction 

 of an object is based chiefly upon the degree of contraction 

 which the external eye muscles and the muscle of accom- 

 modation undergo in fixing the gaze upon the object. The 

 judgment of the size of the object is formed by comparison 

 with an object of known size, correction being made for the 

 distance of the object. Errors made in judging the distance 

 and direction of objects are called optical illusions. 



1 . The protective organs of the eye. By the closing of the 

 eyelid the eyeball is protected from injurious external influ- 

 ences. This is accomplished by the orbicularis palpebrarum, 

 which is innervated by the facial. The closing may be a 

 voluntary or a reflex act. The reflex closing is brought 

 about by too strong stimulation of the retina (blinking) or 

 by the stimulation of the cornea and conjunctiva. 



The surface of the eye is kept moist and clean by the 

 tears. The tears flow from the efferent duct of the lachry- 

 mal gland into the conjunctival sac and are distributed by 

 the closing of the lid and by the movements of the eye. In 

 this manner the closing of the lid keeps the cornea moist 

 and clean. From the conjunctival sac the tears flow through 

 the nasal duct into the nose. 



The Meibomian glands in the eyelids are sebaceous glands 

 whose secretion oils the borders of the lids. This prevents 

 the flowing of the tears over the lids. 



2. Blood and lymph circulation in tJie eye. The blood 

 enters the eye: 



(1) By the central artery of the retina, which supplies the 

 retina with blood. 



(2) By the ciliary arteries which pass to the choroid. 

 Communications exist between the branches of the vessels 



of the retina and of the choroid, especially near the entrance 

 of the optic nerve. 



