THE EYE AS AN OPTICAL INSTRUMENT 229 



When this defect, known as astigmatism, is marked it causes seri- 

 ous troubles of vision and requires peculiarly shaped glasses to 

 counteract it. 



4. Opaque Bodies in the Refracting Media. In diseased eyes the 

 lens may be opaque (cataract) and need 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 muscce 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 in- 

 ternal 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 interest- 

 ing are those due to the retinal blood-vessels (Chap. XVI). Tears, 

 or bits of the secretion 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. 



Hygienic Remarks. Since muscular effort is needed by the 

 normal eye to see near objects, it is clear why the prolonged con- 

 templation of such is more fatiguing than looking at more distant 

 things. If the eye be hypermetropic still more is this apt to be the 

 case, for then the ciliary muscle has no rest when the eye is used, 

 and to read a book at a distance such that enough light is reflected 

 from it into the eye in order to enable the letters to be seen by all, 

 requires an extraordinary effort of accommodation. Such persons 

 complain that they can read well enough for a time, but soon fail 

 to be able to see distinctly. This kind of weak sight should always 

 lead to examination of the eyes by an oculist, to see if glasses are 

 needed ; otherwise severe neuralgic pains about the eyes are apt to 

 come on, and the overstrained organ may be permanently injured. 

 Old persons are apt to have such eyes; but young children fre- 

 quently also possess them, and if so should at once be provided 

 with spectacles. The occurrence of headache at frequent inter- 

 vals, particularly in connection with use of the eyes, as in read- 

 ing or sewing, is more often than not an indication of visual 

 defects which proper glasses would overcome. Sufferers from 



