Page:The wonders of optics (1869).djvu/78

 with a headless man beneath. The head belonging to the body was lying before it, and the body itself was depicted with every limb—legs, thighs, and arms—perfect. He frequently told his crew of these illusions, adding that it was evidently a prediction of the fate in store for them. He was often in such a state of terror, that on calm days he would drop down into the hold and wrap himself up in a spare sail in order not to catch sight of the horrible image that he constantly saw in the shining surface of the tar.

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,