A system of surgery : theoretical and practical . Fig. 3.—Hypermetropic -eye; too short in its antero-posterior diameter. Parallel rays,tending towards a focus at b, form on the retina circles of dispersion at b, V. Divergentrays would tend towards a focus stUl more distant than We have hitherto supposed the refraction of the eye to be equal in every meri-dian. This is not absolutely true ; usually the refraction in the vertical mei-idian isstronger than that in the horizontal, owing to the radius of curvature of the cornea,in the former meridian being somewhat shorter. In Emmetropia this
A system of surgery : theoretical and practical . Fig. 3.—Hypermetropic -eye; too short in its antero-posterior diameter. Parallel rays,tending towards a focus at b, form on the retina circles of dispersion at b, V. Divergentrays would tend towards a focus stUl more distant than We have hitherto supposed the refraction of the eye to be equal in every meri-dian. This is not absolutely true ; usually the refraction in the vertical mei-idian isstronger than that in the horizontal, owing to the radius of curvature of the cornea,in the former meridian being somewhat shorter. In Emmetropia this difiei-ence ofrefraction is so small that pencils of rays incident in these opposite meridians meet,if not with absolute exactitude, yet so very nearly at the same distance behind thecommon nodal point of the eye that the sharpness of the retinal image does not sen-sibly suffer. When, however, the refraction in any meridian differs by more than aminute quantity from that in the other meridians, the rays incident on that meridiando not meet in the same focal plane as those incident in other meridians, and the con-dition known as Astigmatism results. The difference of refraction may be one ofexcess or deficiency, so causing myopic and hypermetropic astigmatism. These mayoccur
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Keywords: ., bookcentury1800, booksub, booksubjectsurgicalproceduresoperative
