(This article appeared in Volume 25, Number 3, 2008, pp. 469–474).
The abscissa in Fig. 1 (p. 471) was labeled in units of cd/m2. The correct units are trolands (Td), and a new Fig. 1, so labeled, is shown below.
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On p. 473, column 1, the third sentence in the second paragraph under the heading Effects of stimulus duration should read as follows:
“The mean Tc-estimates for their two observers were 33.5 ms for SPP and 49 ms for PPP, compared with 26.3 (SPP) versus 38.4 (PPP) for our controls.”