236 LAB OR A TOR Y G UJDE IN PH YSIOL OGY. 



from the chart. Hold before his eye a -f-0.75 D. 

 lens, it will probably make indistinct and blurred 

 distant objects which were, without the lens, clear. 

 If such be the case it is likely that refraction of the 

 eye is normal and for our purpose it may be re- 

 corded as an emmetropic eye. 



(<) If, however, the vision remains perfectly clear 

 for distant objects, with the +0.75 D. or the +1 D. 

 lens before the eye it is evident that the refraction 

 of the eye is not normal. 



(<:) Suppose, on the other hand, that distant objects 

 cannot be clearly seen with the unaided eye; but, 

 with the help of concave lenses, clearly seen, it is 

 evident again that the refraction of the eye is ab- 

 normal. 



(8) In case (7 c), where were the parallel rays 

 focused when the concave lens was used ? Where 

 were the parallel rays focused in the unaided eye ? 

 Would it be possible for the condition to be cor- 

 rected by an exercise of the accommodation? If 

 the punctum remotum is 2m., and if the refractive 

 indices and curvatures of the refracting surfaces are 

 all normal, in what way must the eye differ from the 

 normal eye ? This condition is called nearsighted- 

 ness or myopia. 



(0) In case (7 ), if a subject can read all of the letters 

 expected of the normal eye one credits him with 

 V=-|; but, the eye may have accomplished the re- 

 sult at the expense of more or less effort. 



If the eye have a punctum remotum beyond infin- 

 ity; /. e. , if the rays of light from a distant object 

 are not yet converged to a focus by the time they 

 reach the retina in the resting eye it will require a 

 certain effort of accommodation to produce a clear 



