SIGHT. 167 



near vision, and the image is blurred as in myopia for distant 

 vision. 



What are the causes of hypermetropia ? 



Shortening of the antero-posterior axis of the eye or abnormal 

 flattening of the lens, which does not allow accommodation for near 

 vision. 



How is hypermetropia corrected ? 



By the use of convex lenses, which add to the refractive power 

 of the eye. 



What is astigmatism? 



A defect in the vision due to the irregularity in the globe of the 

 eye, whereby the diameter in one plane is greater than in another. 

 Thus, the retina may be an uneven surface, and the image focus 

 accurately in one part and falsely in another. In this condition 

 vertical and horizontal lines are not seen with equal distinctness. 



How is astigmatism corrected? 



By the use of cylindrical or prismatic glasses, which have to be 

 accurately adapted to the needs of each case. This error, if seri- 

 ous, is usually combined with other defects of vision, frequently 

 myopia. 



What is presbyopia? 



Defective vision due to the loss of power in advanced years. The 

 elasticity of the lens becomes less, and the convexity cannot be in- 

 creased for near vision. The ciliary muscle may also be weaker 

 and aid in the production of the error. A weak convex glass com- 

 monly corrects the lack of refraction-power. 



Is the eye achromatic ? 



Yes. It may, however, be said that there may be a visible band 

 of color seen by some defective eyes where there is considerable 

 fault in the focus of the image on the retina. 



What is the " blind spot " ? 



The point of entrance of the optic nerve is not sensitive to light, 

 and at this point an image thrown upon the retina is not seen ; 

 therefore it is called the blind spot. 



How is the blind spot demonstrated? 



If the left eye is covered and the right directed steadily upon 



