On Light in some of its Relations to Disease, 159 



This lens is hung in its place by a delicate substance, which 

 attaches it to an annular ring of muscular fibres, called the 

 ciliary muscle (c, Fig. 2). When this ring of muscle con- 

 tracts, the lens, by its elasticity, becomes thicker and more 

 convex, and thus the rays passing through it are more 

 strongly refracted, and near objects are distinctly seen. 



Fig. 2. 



(Fig. 2 shows the condition of the lens under these diflFerent circumstances. At a the 

 ciliary muscle, c, is in repose, the lens is flattened, and the eye accommodated for 

 distant objects. At b the ciliary muscle c, is in a state of contraction, the lens is 

 thickened, and is accommodated for the near point.) 



This faculty of changing the focus is called the faculty 

 of accommodation, and is in constant use as we turn our 

 eyes from near to distant objects. In an eye constructed 

 on as strict optical principles as eyes should be, this 

 change is very easily produced, and vision is perfect at all 

 points, with only the slight exertion of nerve and muscle 

 force which can be maintained without fatigue or con- 

 sciousness of exertion. In such an eye — which, because 

 it is the best form of an eye, we will call a normal eye — 

 the slight tension of the ciliary muscle which is necessary 

 for near vision may be kept up for many hours with very 

 little if any inconvenience. But we know that a constant 

 tension upon a muscle or set of muscles, if it be even but 

 a little more than nature intended, soon produces fatigue. 

 A pound'Weight is but little for any one to hold ; but let 

 any person hold a pound weight in the palm of his hand 

 while he bends the elbow so that the forearm is at right 



