THE EYE. 



It has been stated that the diameter of the pupil varies from 

 to an inch in magnitude, the variation depending upon a 

 power of dilatation and contraction with which the iris is endued. 

 Taking that diameter at its greatest magnitude of a quarter of an 

 inch, pencils proceeding from an object placed at the distance of 

 three feet would have an extreme divergence amounting to less 

 than half a degree ; and if the pupil be in its most contracted 

 state when its diameter is only the one-eighth of an inch, then 

 the divergence of the pencils proceeding from such an object would 

 amount to about fifteen minutes of a degree. It may therefore 

 be concluded, that pencils proceeding from all objects more distant 

 from the eye than two or three feet, may be regarded as consisting 

 of parallel rays. 



20. If the refracting power of the humours of the eye be so 

 feeble that rays proceeding from such objects, and which enter the 

 eye in parallel directions, are not rendered sufficiently convergent 

 to come to a focus on the retina, or on the contrary, are so strong 

 as to bring them to a focus before they arrive at the retina, the 

 image produced upon the retina will be confused from the cause 

 just explained. 



21. The remedy for such a defect in vision is supplied by tho 

 properties of convex and concave lenses. 



If the eye possess too little convergent power, a convergent or 

 convex lens is placed before it, which, receiving the parallel 

 pencils, renders them convergent when they enter the pupil, and 

 this enables the eye to bring them to a focus on the retina, pro- 

 vided the power of the lens be equal to the deficient convergence 

 of the eye. 



If, on the other hand, the convergent power of the eye be too 

 great, so that the parallel rays are brought to a focus before 

 arriving at the retina, a divergent or concave lens is placed before 

 the eye, by means of which parallel pencils are rendered divergent 

 before they enter the pupil; and the power of the lens is so adapted 

 to the convergent power of the eye, that the rays shall be brought 

 to a focus on the retina. 



The two opposite defects of vision here indicated are generally 

 called, the one weak-sightedness or far-siyhtedness, and the other 

 near- sigh tedn ess. 



If the objects of vision be placed so near the eye that the rays 

 composing the pencils which proceed from them have sensible 

 divergence, then the foci of these rays within the eye will be at a 

 distance from the optical centre greater than the principal focus, 

 which is the name given to the focus of parallel rays. If, there- 

 fore, in this case, the principal focus fall upon the retina, tho 

 focus of rays proceeding from sucli near objects would fall 

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