ATTENTION. 409 



If, then, by the original question, how many ideas or 

 things can we attend to at once, be meant how many entirely 

 disconnected systems or processes of conception can go on 

 simultaneously, the answer is, not easily more than one, 

 unless the processes are very habitual ; but then two, or 

 even three, without very much oscillation of the attention. 

 Where, however, the processes are less automatic, as in the 

 story of Julius Caesar dictating four letters whilst he writes 

 a fifth,* there must be a rajjid oscillation of the mind from 

 one to the next, and no consequent gain of time. Within 

 any one of the systems the parts may be numberless, but 

 we attend to them collectively when we conceive the whole 

 which they form. 



When the things to be attended to are small sensations, 

 and when the efibrt is to be exact in noting them, it is 

 found that attention to one interferes a good deal with the 

 perception of the other. A good deal of fine work has been 

 done in this field, of which I must give some account. 



It has long been noticed, when expectant attention is 

 concentrated upon one of two sensations, that the other 

 one is apt to be displaced from consciousness for a moment 

 and to appear subsequent ; although in reality the two may 

 have been contemporaneous events. Thus, to use the stock 

 example of the books, the surgeon would sometimes see 

 the blood flow from the arm of the patient whom he was 

 bleeding, before he saw the instrument penetrate the skin. 

 Similarly the smith may see the S23arks fly before he sees 

 the hammer smite the iron, etc. There is thus a certain 

 difliculty in perceiving the exact date of two impressions 

 when they do not interest our attention equally, and when 

 they are of a disparate sort. 



Professor Exner, whose experiments on the rninimal per- 

 ceptible succession in time of two sensations we shall have to 

 quote in another chapter, makes some noteworthy remarks 

 about the way in which the attention must be set to catch 

 the interval and the right order of the sensations, when the 

 time is exceeding small. The point was to tell whether 



* Cf. Chr. Wolff: Psycbologia Empirica, § 245. Wolff's account of ihe 

 phenomena of atteuliou is in general excellent. 



