160 On Light in some of its Relations to Disease, 



angles with the arm, and in a short time, varying from one 

 to several minutes according to the muscular power of the 

 arm, fatigue will be experienced ;^ and, if the experimenter 

 is not accustomed to much exercise of the muscles of the 

 arm, a rather painful sense of exhaustion may continue for 

 a considerable time. 



Now, it not unfrequently happens that such a constant 

 and unnataral tension is forced upon this ciliary muscle 

 which regulates the accommodation. For all eyes are not 

 constructed upon the most perfect plan : some are too long 

 some too short, and some have not uniform refractive 

 surfaces in different meridians. 



The diagram here shown (Fig. 3) will assist in under- 

 standing this. 



Fig. 3. 



A 



Normal. 



B 



Myopic. 



Hypermetropic. 



Donders on Asthenopia. 



