The optical manual, or Handbook of instructions for the guidance of surgeons in testing the range and quality of vision of recruits and others seeking employment in the military services of Great Britain, and in distinguishing and dealing with optical defects among the officers and men already engaged in them / by T. Longmore.

  • Longmore, T. (Thomas), Sir, 1816-1895.
Date:
1885
Catalogue details

Licence: Public Domain Mark

Credit: The optical manual, or Handbook of instructions for the guidance of surgeons in testing the range and quality of vision of recruits and others seeking employment in the military services of Great Britain, and in distinguishing and dealing with optical defects among the officers and men already engaged in them / by T. Longmore. Source: Wellcome Collection.

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    the directions in which the movements affecting the field of view occur. Normal Range of Motion of the Eye consistent with Vision. I find that my own eye can turn through an angle of about 140° horizontally, and of 138° vertically, and perception of objects be retained. On my eye being directed to a point straight before it, it can turn from it 50° towards the nose and nearly 9Cf outwards in the horizontal plane, or through an angle of 48° from it upwards and 90° downwards. These measurements are not, however, universal; they are subject to variations according to individual circumstances, viz. to personal peculiarities in the shape and amount of projection of the parts near to the eye, as the nose, the margins of the orbit, the eyebrows, and other structures. Objects on the Visual Field.—The place and space occupied by objects in the field of vision are measured by the visual angles under which they are seen. The apparent size, or lineal measure, of an object is estimated by the size of the visual angle alone. To estimate the true size, the distance of the object, as well as the size of visual angle, must be known. Conversely, the true size of an object being known, the visual angle enables us to form a judgment of the distance at which it is placed from us. The apparent size or lineal measure, is to be distinguished from the apparent super- ficial size, or measure of surface of an object. The lineal measure as before mentioned, varies inversely as the distance of the object ; the measure of surface varies in proportion to the squares of the lineal measure at different distances. Illumination of Objects.—The quantity of light received on a given object, or unit of surface, is reduced inversely as the square of the distance from the source of light to which the object is re- moved. Let a printed paragraph be placed at a certain distance from a lighted candle, and the paragraph be then removed to double the same distance from the light, the intensity of illumi- nation of the paragraph in the more distant of the two positions will be only one-fourth of what it was in the nearer one; and the rule that applies to the whole paragraph equally applies to each letter of the print composing it. Apparent Brightness of Objects at Different Distances in open Daylight.—The vividness of the light under which objects appear in open daylight at different distances does not, however, vary with the distances at which they are respectively placed, excepting so far as those which are more remote may be affected by the thicker stratum of air through which they are seen, provided that all the objects face in the same direction. Supposing the atmosphere to be per- fectly clear and transparent, the apparent illumination of a series of targets, one more remote than the other but facing in the same direction, or of a line of men one in front of the other, will be exactly similar. A transverse section of the cone of rays entering