THE EYE AS AN OPTICAL INSTRUMENT. 525 



Short Sight and Long Sight. In the eye the range of 

 accommodation is very great, allowing the rays from points 

 infinitely distant up to those from 

 points about eight inches in front 

 of the eye to be brought to a 

 focus on the retina. In the nor- 

 mal eye parallel rays meet on the 

 retina when the ciliary muscle is 

 completely relaxed (A, Fig. 152). 

 Such eyes are emmetropic. In 

 other eyes the eyeball is too long 

 from before back; in the resting 

 state parallel rays meet in front 

 of the retina (B}. Persons with 



Such eyes, therefore, Cannot See and a hypermetropic (C) eye. 



distant objects distinctly without the aid of diverging (con- 

 cave) spectacles; they are short-sighted or myopic. Or the 

 eyeball may be too short from before back; then, in the rest- 

 ing state, parallel rays are brought to a focus behind the 

 retina (C). To see even infinitely distant objects, such per- 

 sons must therefore use their accommodating apparatus to 

 increase the converging power of the lens; and when objects 

 are near they cannot, with the greatest effort,, bring the di- 

 vergent rays proceeding from them to a focus soon enough. 

 To get distinct retinal images of near objects they therefore 

 need converging (convex) spectacles. Such eyes are called 

 hypermetropic, or in common language long-sighted. 



Hygienic Remarks. Since muscular effort is needed by 

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

 longed contemplation 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 dis- 

 tance such that enough light is reflected from it into the eye 

 in order to enable the letters to be seen at all, requires an ex- 

 traordinary 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 



