Experimental psychology has long been dominated by the proposition that a knowledge of sensory thresholds would provide a framework for the study of human behaviour. Some serious shortcomings of the traditional psychophysical methods for ascertaining these thresholds have been pointed out by Bartlett (1947) , who stressed the fact that the discrete stimulus-response situation of psychophysical experiments can provide little information about the sensory processes involved in life, where the

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... ect is dealing always with a series of stimuli. At the same time, psychology has been equally guilty of failing to recognize that under natural conditions perception of a single series of stimuli is not much more than a theoretical possibility. In fact, behaviour must be determined by the perception not only of concurrent series of stimuli in different sensory modes, but also of concurrent, but not necessarily related, series of stimuli in the same mode. Particularly one cannot assume that any one sensory threshold determines the framework of behaviour without taking into account the operation of the other thresholds which are almost certain to be present. It is almost a century ago that the philosopher William Hamilton (1859) attempted to analyse the numerical limit beyond which attention could not be stretched and proposed an experimental technique which led to an immense number of researches into the span of apprehension. This field of research has yielded a great deal of information relating to the spatial arrangements, with particular reference to groupings, of static visual displays. But the data which it can provide must assume that the parts of the display being apprehended remain in a constant attentional relationship. However true this may be in some situations, it is obvious that even in the most mundane forms of behaviour, such as cycling in traffic, the attentional demands of the different parts of the perceptual field are continually changing relative to one another. It is not therefore surprising that side by side with experiments on the span of apprehension there proceeded others which attempted to study the nature of the " division " of attention which appears to occur when an individual adapts to several concurrent but independent series of events. Unfortunately, for every hundred experiments of the former kind there was perhaps one of the latter. Research in this field began with the experiments of Paulhan (1887) who set himself the task of reciting one poem orally whilst at the same time writing another. From then onwards, the emphasis lay either in the analysis of the effects on performance of a concurrent second task, or on studies of individual differences of ability to perform two tasks at once. Had the same question which inspired the experiments on the span of apprehension been asked-i.e. how many tasks can be simultaneously performed-the early workers might have quickly discovered the presence in their experimental situations of variables which, unrecognized, may have contributed to the relative sterility of. research in this important field.