It is not enough to notice, we must notice and remember. This implies attention, and attention leads directly to the second great principle in mental photography—which is Concentration. Beyond question, this is one of the most important, probably the most important factor in the whole subject of memory-training.
Let us refer again to our illustration of the camera, as no simile can be clearer. In his excellent book, A Perfect Memory, Dana draws a very interesting parallel.
" The general principle of the camera is understood by most folk. We need not trouble ourselves over the chemical action involved in the effect of light on the plate, or the processes of development, any more than we need concern ourselves with the precise constitution and metabolism of the brain-cells. It is enough for our purpose to consider merely the mechanics of the camera in its simplest phase.
" In the taking of a photograph, a sensitive plate is introduced into the camera. Hitherto, the prepared glass or film has been carefully protected in such wise that no slightest ray of light could fall upon any portion of its surface. This sensitized recipient is of a sort corresponding in some measure to the brain-tissues that receive these impressions which we term memory.
" It must be borne in mind that the camera is strictly protected from the intrusion of the tiniest ray of light. Such protection, after the setting of the plate, is absolutely essential to the successful photographing of any scene. Ultimately, when the camera has been duly adjusted in such a position as to command the required objects to be photographed, the protecting cover is removed from the lens. Through that one point only is light allowed to fall upon the sensitive plate. By reason of this fact, the plate is subjected to the action of the actinic rays emanating from the objects to be photographed, while all other radiance of every kind is rigorously barred out. The plate, in consequence, receives only those vibrations of radiant energy, and no others. So simple is the operation of the camera with which we', are concerned.
" In the act of memory, one must shut out from his mind for an interval everything except the single thing to be remembered. By this means, the rays of thought, like the beams of light that pass through the lens of the camera, shine from the thing to be remembered directly upon the brain-cells that are to serve as the sensitive plate in recording the mental picture. If thought of other things intrude while the effort is being made to;concentrate on the given subject, the result in the brain will be a blurred image—just as in the camera a leak of light into the interior from any point other than through the lens must ruin the negative.
" We need to consider in this connection only one other requirement as vital to our purpose of illustration. For the making of a photograph, the camera must be trained on something concrete. Otherwise, there could be no successful picture. There must be some definite object in the perspective, or there will be no definite photograph. 1
" If, now, we bear in mind the operation of the camera in its successful work, we shall have a simple guide for the understanding of the manner in which the brain must operate for the successful storing of memories. When one would remember anything, he must give his whole attention to that particular thing for an appreciable length of time. This means concentration. The period of concentration may be long or short; often, it need be no more than the tiniest fraction of a second. But it must be absolute while it endures. The error of most persons is in the failure to concentrate. Some few individuals, indeed, seem born with the power of concentration as a natural possession in its perfection. But, in the vast majority of instances, the faculty of attention must be cultivated as an art. At whatever pains, the student must acquire this ability of concentration."
The inventor who triumphs over problems of supreme difficulty does it by a persistent, indomitable concentration on the obstacles to be overcome. By that concentration, he discovers the way to victory. Every great thinker in any line of thought has this mastery of concentration, by which alone he is able to employ all the powers within his brain for conquest over the subject to which he has set himself.
Personally, I consider concentration to be so great a factor in any rational system of memory training or mental discipline that a separate chapter of this book will be devoted to "How to concentrate."