OPTICAL DEFECTS OF THE EYE. 505 



minute bodies floating in space outside the eye, but chang- 

 ing their position when the position of the eye changes, 

 by which fact their origin in internal causes may be recog- 

 nized. Many persons never see them until their attention. 

 is called to their sight by some weakness of it, and then 

 they think they are new phenomena. Visual phenomena 

 due to causes in the eye itself are called entoptic; the most 

 interesting are those due to the retinal blood-vessels (Chap. 

 XXXII.). Tears, or bits of the secretion of the Meibo- 

 mian glands, on the front of the eyeball often cause distant 

 luminous objects to look like ill-defined luminous bands or 

 patches of various shape. The cause of such appearances is 

 readily recognized, since they disappear or are changed 

 after wiuking, 



