| Sumario: | 16 men counted the number of bright symbols of a particular kind in a simulated electronic display presenting a total of between 19 and 59 symbols of 8 different kinds. In some displays all the symbols were bright, in others bright and dim symbols were mixed. 4 sets of 8 symbols were compared. Targets were missed less often from sets of capital letters than from sets of closed shapes (p < .01) thus the letters were the more easily discriminated from each other. The probability of missing at least 1 target increased approximately linearly with the number of targets present (p < .01). The rate of increase was approximately proportional to the total number of bright symbols displayed, and was somewhat greater when dim symbols were present also (p < .05). For a combined total of 57-59 bright and dim symbols, the probability of missing at least 1 target increased from about .03 per target when 10 of the symbols were bright, to .09 when 26 were bright. 35% of the underestimations were of size 2 or greater. The time taken to count the targets increased with the total number of bright symbols present (p < .01). Dim symbols tended to be counted as bright (p < .01).
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