The imagination really seems to create for itself a sort of mental visual organ which is in intimate relation with that of the body, and which often takes its place so efficiently—as in the case of dreams—that the mind is utterly unable to perceive the substitution. It is on account of this that practical opticians are so unsparing in their endeavours to predispose their spectators to being deceived. When both the body and mind are healthy, the relative intensity of the two kinds of impressions is very unequally divided, mental images being more evanescent and comparatively weak, and with persons of ordinary temperament incapable of effacing or disturbing the reflections of visible objects. The affairs of life could not go on if the memory introduced amongst them brilliant representations of the past in the midst of ordinary domestic scenes or the objects familiar to us. We may account for this by supposing that the set of nerves which carries the efforts of the memory to the brain cannot execute their functions at the same time as those which take cognizance of the images reflected on the retina. In other words, the mind cannot accomplish two separate functions at one and the same time, and the mere act of directing the attention to one class of subjects causes all others to become instantly imperceptible. The exercise of the mind in these instances is, however, so rapid that the alternate appearance and disappearance of the two different impressions is completely unnoticed. Thus, for instance, while looking at the dome of St. Paul’s, if our memory suddenly evokes the image of some other object, Mont Blanc for instance, the picture of the cathedral, although still depicted on our retina, is momentarily effaced by the effort of the will, although we may not change the position of our eyes during the time. While the memory continues to dwell on the picture it has called up, it is seen with sufficient distinctness, although its details may be somewhat misty and its colours confused; but as soon as the wish to see it passes away the whole disappears, and the cathedral is seen in all its former distinctness.