THE EYE AS AN OPTICAL INSTRUMENT. 529 



removal; or opacities from ulcers or wounds may exist 

 on the cornea. But even in the best eye there are apt to be 

 small opaque bodies in the vitreous humor causing musccz 

 volitantes; that is, the appearance of minute bodies floating 

 in space outside the eye, but changing their position when 

 the position of the eye changes, by which fact their origin in 

 internal causes may be recognized. 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 XXXIII. ). Tears, or bits of the secre- 

 tion of the Meibomian 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 winking. 



