THE EYE AS AN OPTICAL INSTRUMENT 265 



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 at 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. Astigmatism is another fruitful source of eye 

 strain. Although sharp focussing is impossible the eye constantly 

 strives for it. This involves great activity of the muscles of accom- 

 modation, which suffer from the effort. The occurrence of head- 

 ache at frequent intervals, particularly in connection with the use 

 of the eyes, as in reading or sewing, is more often than not an indi- 

 cation of visual defects which proper glasses would overcome. 

 Sufferers from such headaches should therefore have their eyes 

 examined and if glasses are necessary should wear them. 



Short-sighted eyes appear to be much more common now than 

 formerly, especially in those given to literary pursuits. Myopia 

 is rare among those who cannot read or who live mainly out of 

 doors. It is not so apt to lead to permanent injury of the eye as is 

 the opposite condition, but the effort to see distinctly objects a 

 little distant is apt to produce headaches and other symptoms of 

 nervous exhaustion. If the myopia become gradually worse the 



