On the detection and measurement of astigmatism / by John Green.

  • Green, John, 1835-1913.
Date:
1867
Catalogue details

Licence: Public Domain Mark

Credit: On the detection and measurement of astigmatism / by John Green. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    [Extracted from the American Journal of the Medical Sciences for January 1867.] , -' ON THE DETECTION AND MEASUREMENT OF ASTIGMATISM. By JOHN GREEN, M. D., OF ST. LOUIS, MISSOURI. (with five wood-cuts.) V' Astigmatism (from a privative, and (stiypLa, a'point),^^4ia,;ai^ .proposed by Dr. Whewell to designate the visual defect dependent on a difoence in the refractive power of the eye in different meridians, has been long recog- nized as an occasional cause of imperfect visiom The conclusive demon, strations of Dr. Young {Philosophical Transactions, 1801) and of Mr. Airy {Transactions of the Cambridge Philosophical Society, 1821 and 1849) completed the history of astigmatism as a mathematical problem, and discovered the optical means necessary for its correction; but it is to Donders, pursuing the line of ophth.idmometrical research opened by Kra- mer and Helmholz, that we are indebted for the demonstration of its anatomical characters and the importance of its pathological relations. Far from being a rare or exceptional phenomenon, Donders has shown that astigmatism exists in an appreciable degree in almost every eye, and in its lower grades must be accepted as a normal condition, while in its higher or pathological degrees (^^^ or upwards) he has detected one of the most frequent and important of the refractive anomalies to which the eye is subject. Astigmatism may occur either alone or in connection with any of the refractive or accommodative anomalies ; it may exist in one eye only, or in both eyes; and it may be symmetrical or unsymmetrical, equal or unequal, in the two eyes. The different relations which astigmatism may bear to the refractive condition of the eye have been expressed by Donders in the form of several useful symbols, which are now' very generally adopted, for the sake of brevity and precision. Astigmatism is expressed by As. Positive or myopic astigmatism in one meridian only, the other meridian being emmetropic, is Am. Negative or hypermetropic astigmatism in one meridian is Ah. Positive astigmatism in one meridian, combined with negative astigmatism in the other meridian, is written Ahm or Amh, according as the hyper- metropic or myopic element predominates. Positive astigmatism in both meridians may be considered as myopia combined with myopic astigmatism, and is written M-f Am. So, also, negative astigmatism in both meridians is expressed by H-j-Ah