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May 1951

DISTANCE DISCRIMINATIONVIII. Influence of Rod Brightness and Background Illumination in Howard-Dolman Test

AMA Arch Ophthalmol. 1951;45(5):523-527. doi:10.1001/archopht.1951.01700010535003

Hirsch, Horowitz and Weymouth1c have demonstrated that the apparatus rod size (width) does not affect the threshold but does significantly influence the position of subjective equality in tests of distance discrimination. It is known, in addition, that the apparent size of an object varies with the brightness of the object, this effect being due to irradiation (Duke-Elder2). In the Howard-Dolman apparatus the rods are painted dull black and have a different and weaker source of illumination than the background. Because of this arrangement the rods are seen essentially as silhouettes. Should the contrast between the rods and the background be lessened by decreasing the illumination of the background or by increasing the reflectivity of the rods, the factor of apparent rod size due to irradiation might alter the results obtained in testing with the apparatus, particularly if the reflectivity or brightness of the two rods differed. The purpose