The subject of thought, once started, develops all sorts of fascinating consequences. The attention is led along one of these to another in the most interesting manner, and the attention never once tends to stray away. In him, therefore, the faculty of voluntary attention receives abundant opportunity for cultivation in daily life. A genius, on the contrary, is the man in whom you are least likely to find the power of attending to anything insipid or distasteful in itself. He breaks his engagements, leaves his letters unanswered, neglects his family duties incorrigibly, because be is powerless to turn his attention down and back from those more interesting trains of imagery with which his genius constantly occupies his mind.
Voluntary attention is thus an essentially instantaneous affair.
To keep them where you have called them, you must make the subject too interesting for them to wander again. From an unchanging subject the attention inevitably wanders away. You can test this by the simplest possible case of sensorial attention. Try to attend steadfastly to a dot on the paper or on the wall. This is what the genius does, in whose hands a given topic coruscates and grows.