Diseases of the eye : a hand-book of ophthalmic practice, for students and practioners / By G. E. De Schweinitz.

De Schweinitz, G. E. (George Edmund), 1858-1938.

Date:: 1899

Credit: Diseases of the eye : a hand-book of ophthalmic practice, for students and practioners / By G. E. De Schweinitz. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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$the vertical meridian is emmetropic it forms a distinct point in the image, of each point in the object, by bringing the rays which pass through it to a focus. A horizontal line thus appears as a succession of distinct points when the vertical meridian is em- metropic. Vertical lines, in the same way, appear most distinct when the horizontal meridian is nearest to emmetropia, or if oblique lines appear most distinct, the meridian at right angles to their direction is the one nearest to emmetropia. Luminous points are drawn out in the direction of the ametropic meri- dian, and luminous circles become elongated into ovals. Astig- matism may be responsible for the most aggravated types of asthenopia. Fully sixty per cent, of functional headaches are caused by this type of refractive error, either alone or in associa- tion with other forms of ametropia. The headache may vary from a moderate frontal distress to violent explosions of pain, and may be situated in any portion of the cranium. Further- more, all the symptoms detailed on page 556 are the frequent result of astigmatism, not only when the error is of high degree, but commonly, indeed, more commonly, when it exists in low grade, and often unassociated with any symptoms which prom- inently direct attention to the eyes as the cause of the distress. Regular Astigmatism.—Regular astigmatism is classified into five varieties, according to the relative position of the retina to the foci of the two principal meridians. The horizontal meri- dian focus is represented by H., the vertical meridian by V. Fig. 66. Fig. 67. Foci of the principal meridians in simple hypermetropic astigmatism. Foci of the principal meridians in simple myopic astigmatism. 1. Simple hypermetropic astigmatism.—In this variety one me- ridian, usually the vertical, is emmetropic, and the horizontal meridian is hypermetropic. The focus of the vertical meridian$