The determination of the necessity for wearing glasses / by D.B. St. John Roosa.

  • Roosa D. B. St. John (Daniel Bennett St. John), 1838-1908.
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: The determination of the necessity for wearing glasses / by D.B. St. John Roosa. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    require to avoid skepticism on the one hand and ex- cessive confidence on the other. Medio tutissimus ibis. A part of the foregoing chapter was read before the New York Academy of Medicine in March, 1886, and published in the N. Y. Medical fournal, vol. xvi., No. 11. THE DIOPTRIC SYSTEM OF MEASURING THE FOCAL POWER OF GLASSES, AS COMPARED WITH THE OLD SYSTEM OF MEASURING THE FOCUS BY INCHES. In this old system of measuring the focal power of lenses, the inches of the various countries were made the standard. A strong lens was taken as the unit. A convex glass of one inch focuses parallel rays at one inch, one of ten at ten inches, and so forth. For the sake of convenience fractions are employed, and a glass of ten inches focal distance is called For many reasons it has been thought ad- visable, if possible, to introduce a uniform method for all countries, which would be impossible so long as the inch is the unit, differing as it does in different countries. In the new or dioptric system, the unit is a weak lens of one metre focal length. This is called a diopter. The numbers of glasses by the old system really indicate the radius of curvature of the surface of the lens, and not its exact refractive power. The great objection to these lenses, however, is the fact