Citation

Johnson HM. J. Exp. Psychol. 1924; 7(1): 1-44.

Copyright

(Copyright © 1924, American Psychological Association)

DOI

10.1037/h0074831

PMID

unavailable

Abstract

On the horizontal diameter of a test-field having a practically constant brightness of 2.6 millilamberts two circles, with diameters subtending 24' at the eye of the observer, were projected by means of two miniature stereopticons. The removal of one or the other of these spots constituted the stimulus to reaction. The brightness of these circles was 0.09 millilambert, so that they constituted additions of 3.5 per cent to the brightness of the field on which they were projected. The remainder of the field of vision was filled by the interior surfaces of a polyhedral chamber painted "flat white" and illuminated from invisible sources. Their brightness was uniform with respect to each other but variable with respect to that of the test-field. The variable factor tested was the ratio of the surfaces ( Bs) to the test-field ( Bf). Experiments were made under six ratios of Bs/Bf, ranging from 0.015 to 9.45. The observers were instructed to press the appropriate bulb as quickly as possible when one of the spots disappeared. "The results yielded three criteria: the time required for selective reaction, the dispersion of the measurements and the relative number of errors; or expressed inversely, speed, constancy and accuracy. By all criteria taken both separately and integrally the results showed that performance is progressively impaired by increases in the ratio Bs/Bf above unity and that rather small excesses of this ratio produced an effect which is important as to magnitude and reliability. The effect of reducing the ratio below unity is small though the reduction be great." From Psych Bulletin 21:08:00783. (PsycINFO Database Record (c) 2012 APA, all rights reserved)