The clinical use of prisms : and the decentering of lenses / by Ernest E. Maddox.

  • Maddox, Ernest E. (Ernest Edmund), 1860-1933.
Date:
1907
Catalogue details

Licence: In copyright

Credit: The clinical use of prisms : and the decentering of lenses / by Ernest E. Maddox. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    3.—It has a certain relation to the metre-angle, in that half the number of centimetres between the pupils indicates approximately the number of centrads in one metre-angle. The centrad is taken from the well-known circular measure. It is the hundredth part of a radian, a radian being the angle subtended at the centre of a circle by an arc which is equal in length to the radius. Its ensign is an inverted delta. Circular measure has a certain theoretical interest. While the author is inclined, it will be seen, to deprecate the invention of special units for tiny departments of science, wishing rather to preserve the breadth and unity of the whole, it must be confessed that a departure or gradient of one in a hundred has much to recommend it. Next to our old and well-tried friend, the degree, probably no better unit could be chosen. The unit proposed by Mr. C. F. Prentice is the Prism-Dioptrey It has much to recommend it, and its publication by him preceded and no doubt suggested the centrad. Its ensign is an erect delta. The size of a single prism-dioptre differs inappreciably from that of a centrad, both being the departure of I in 100, that is, a deviation of one centimetre at one metre distance, but the centrad is measured on the arc, and the prism-dioptre on the tangent. The difference between the two units is shown diagrammatically in Fig. 20. In popular language, one may be said to be measured on the circle and the other on the square. The relation to the metre-angle of the P.D. is practically the same