Now there are always some objects that for the time being _will not develop_. They simply _go out_; and to keep the mind upon anything related to them requires such incessently renewed effort that the most resolute Will ere long gives out and lets its thoughts follow the more stimulating solicitations after it has withstood them for what length of time it can. There are topics known to every man from which he shies like a frightened horse, and which to get a glimpse of is to shun. Such are his ebbing assets to the spendthrift in full career. But why single out the spendthrift, when to every man actuated by passion the thought of interests which negate the passion can hardly for more than a fleeting instant stay before the mind? It is like 'memento mori' in the heydey of the pride of life. Nature rises at such suggestions, and excludes them from the view:--How long, O healthy reader, can you now continue thinking of your tomb?--In milder instances the difficulty is as great, especially when the brain is fagged. One snatches at any and every passing pretext, no matter how trivial or external, to escape from the odiousness of the matter in hand. I know a person, for example, who will poke the fire, set chairs straight, pick dust-specks from the floor, arrange his table, snatch up the newspaper, take down any book which catches his eye, trim his nails, waste the morning _anyhow_, in short, and all without premeditation,--simply because the only thing he _ought_ to attend to is the preparation of a noonday lesson in formal logic which he detests. Anything but _that_!
Once more, the object must change. When it is one of sight, it will actually become invisible; when of hearing, inaudible,--if we attend to it too unmovingly. Helmholtz, who has put his sensorial attention to the severest tests, by using his eyes on objects which in common life are expressly overlooked, makes some interesting remarks on this point in his section on retinal rivalry. The phenomenon called by that name is this, that if we look with each eye upon a different picture (as in the annexed stereoscopic slide), sometimes one picture, sometimes the other, or parts of both, will come to consciousness, but hardly ever both combined. Helmholtz now says: