This will depend upon the strength of the stimulus or force which
excites it. The sense of sight is the strongest of all the senses and
therefore can exert the strongest stimulus, and should be used in all
possible cases. In the exercises with the square the changes are all
visual and they continue the strongest stimulus.
Another strong stimulus can be induced by the feelings of either
pleasure or displeasure. Happy, joyful anticipation or fear, horror, or
disgust will arouse the attention.
Familiarity also aids the attention because of the feelings which it
incites. Visual pictures which contain familiar scenes are better and
all changes introduced should be of familiar ideas in order to take
advantage of this fact.
The more you embrace in the attention the less penetrating it will be.
Do not try to take in the whole picture or object all the time, but
change from one detail to another, centering the attention on one at a
time and thus building the perfect whole.