第86章

"On the skin I easily succeed in bringing out suggested sensations wherever I will.But because it is necessary to protract the mental effort I can only awaken such sensations as are in their nature prolonged, as warmth, cold, pressure.Fleeting sensations, as those of a prick, a cut, a blow, etc., I am unable to call up, because I cannot imagine them ex abrupto with the requisite intensity.The sensations of the former order I can excite upon any part of the skin;

and they may become so lively that, whether I will or not, I have to pass my hand over the place just as if it were a real impression on the skin."

Meyer's account of his own visual images is very interesting; and with it we may close our survey of differences between the normal powers of imagining in different individuals.

"With much practice," he says, "I have succeeded in making it possible for me to call up subjective visual sensations at will.I tried all my experiments by day or at night with closed eyes.

At first it was very difficult.In the first experiments which succeeded the whole picture was luminous, the shadows being given in a somewhat less strong bluish light.In later experiments I saw the objects dark, with bright outlines, or rather I saw outline drawings of them, bright on a dark ground.I can compare these drawings less to chalk drawings on a blackboard than to drawings made with phosphorus on a dark wall at night, though the phosphorus would show luminous vapors which were absent from my lines.

If I wished, for example, to see a face, without intending that of a particular person, I saw the outline of a profile against the dark background.When I tried to repeat an ex- periment of the elder Darwin I saw only the edges of the die as bright lines on a dark ground.Sometimes, however, I saw the die really white and its edges black; it was then on a paler ground.I could soon at will change between a white die with black borders on a light field, and a black die with white borders on a dark field; and I can do this at any moment now.After long practice...these experiments succeeded better still.I can now call before my eyes almost any object which I please, as a subjective appearance, and this in its own natural color and illumination.I see them almost always on a more or less light or dark, mostly dimly changeable ground.Even known faces I can see quite sharp, with the true color of hair and cheeks.It is odd that I see these faces mostly in profile, whereas those described

were all full-face.Here are some of the final results of these experiments:

"1) Some time after the pictures have arisen they vanish or change into others, without my being able to prevent it.

"2) When the color does not integrally belong to the object, I cannot always control it.A face, e.g., never seems to me blue, but always in its natural color; a red cloth, on the other hand, I can sometimes change to a blue one.

"3) I have sometimes succeeded in seeing pure colors without objects; they then fill the entire field of view.

"4) I often fail to see objects which are not known to me, mere fictions of my fancy, and instead of them there will appear familiar objects of a similar sort; for instance, I once tried to see a brass sword-hilt with a brass guard, instead of which the more familiar picture of a rapier-guard appeared.

"5) Most of these subjective appearances, especially when they were bright, left after-images behind them when the eyes were quickly opened during their presence, For example, I thought of a silver stirrup, and after I had looked at it a while I opened my eyes and for a long while afterwards saw its afterimage.

"These experiments succeeded best when I lay quietly on my back and closed my eyes.I could bear no noise about me, as this kept the vision from attaining the requisite intensity.The experiments succeed with me now so easily that I am surprised they did not do so at first, I feel as though they ought to succeed with everyone.The important point in them is to get the image sufficiently intense by the exclusive direction of the attention upon it, and by the removal of all disturbing impressions."

The negative after-images which succeeded upon Meyer's imagination when he opened his eyes are a highly interesting, though rare, phenomenon.So far as I know there is

only one other published report of a similar experience. It would seem that in such a case the neural process corresponding to the imagination must be the entire tract concerned in the actual sensation, even down as far as the retina.This leads to a new question to which we may now turn -- of what is THE NEURAL PROCESS WHICH UNDERLIES

IMAGINATION

The commonly-received idea is that it is only a milder degree of the same process which took place when the thing now imagined was sensibly perceived.Professor Bain writes:

"Since a sensation in the first instance diffuses nerve-currents through the interior of the brain outwards to the organs of expression and movement, -- the persistence of that sensation, after the outward exciting cause is withdrawn, can be but a continuance of the same diffusive currents, perhaps less intense, but not otherwise different.The shock remaining in the ear and brain, after the sound of thunder, must pass through the same circles, and operate in the same way as during the actual sound.We can have no reason for believing that, in this self-sustaining condition, the impression changes its seat, or passes into some new circles that have the special property of retaining it.Every part actuated after the shock must have been actuated by the shock, only more powerfully.With this single difference of intensity, the mode of existence of a sensation existing after the fact is essentially the same as its mode of existence during the fact....Now if this be the else with impressions persisting when the cause has ceased, what view are we to adopt concerning impressions reproduced by mental causes alone, or without the aid of the original, as in ordinary recollection?