ASTIGMATISM. 95 



consequence of this, when the irregularities are very great, 

 there is impairment of the sharpness of vision. The circles 

 of diffusion, which are regular in normal vision, become ir- 

 regularly radiated, and single points appear multiple, an ir- 

 regularity described by Donders, under the name of polyopia 

 monocularis. 1 Accurate observations have shown that this 

 condition exists to a very slight degree in normal eyes ; but 

 it is so slight as not to interfere with ordinary vision. In 

 what is called normal, irregular astigmatism, the irregularity 

 depends entirely upon the crystalline lens. If we place 

 before the eye a card with a very small opening, and move 

 this before the lens, so that the pencil of light falls succes- 

 sively upon different sectors, it can be shown that the focal 

 distance is different for different portions. The radiating 

 lines of light observed in looking at remote luminous points, 

 as the fixed stars, are produced by this irregularity in the 

 curvatures of the different sectors of the lens. 



While regular astigmatism, both normal and abnormal, 

 may be perfectly corrected by placing cylindrical glasses 

 before the eyes, it is impossible, in the great majority of cases, 

 to construct glasses which will remedy the irregular form. 



For a complete account of the different forms of astig- 

 matism, the reader is referred to the more elaborate works on 

 ophthalmology. "We have considered the subject briefly, as 

 illustrating some of the aberrations in the normal eye, which, 

 though they do not interfere materially with distinct vision, 

 indicate clearly enough that the eye is by no means a perfect 

 optical instrument. 



1 BONDERS, On the Anomalies of Accommodation and Refraction of the Eye, 

 The New Sydenham Society, London, 1864, p. 643. 



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