Let two ordinary persons, A. and B., take a map of a district with which they are unacquainted, and let each be allowed half an hour to study the map. Desire A. to fix his attention undeviatingly to the map for this time; and at its expiration, the map being withdrawn, request him to put on paper the relative situations and names of the different places; and for the performance of his task, allow him another half hour. As the experiment has been repeatedly made, it may be confidently predicted, that A. would exhibit a very incorrect copy of the original map. Let B. take the same map to study for the same time; but instead of keeping his eyes undeviatingly fixed to the object, desire him to view it only for a few seconds; and then, shutting his eyes, let him endeavour to bring the picture of the map before his mind: his first efforts will convey a very confused notion of the actual and relative positions; but he will become sensible of his defects, and reinspect the map for their correction. If this successive ocular examination and review by the mind, be continued during the half hour, or even for a less time, B. will be competent to make a drawing of the map with superior accuracy to A., who endeavoured to fix his attention for the whole of the time allotted. In conducting this experiment some very curious phenomena may be observed. If A. had directed his eyes to the object intensely and undeviatingly, especially in a strong light, and had then covered or shut his eyes, in order to recollect the relative situations in the map, the straining of the organ to the object would defeat his endeavours; and instead of being able to bring the picture before his mind, he would be annoyed and interrupted by the intrusion of ocular spectra, undergoing the succession of changes described by Dr. Darwin.[14] Thus there are limits to the duration of our effective attention: if the organ of vision be too long directed to the object of perception, ocular spectra arise, fatigue and confusion ensue in the other senses; and if the subjects of recollection be too long and intensely contemplated, delirium will supervene.