After this discussion do not jump to the conclusion that just because you find some difficulty in using one sense avenue for impression, it is therefore impossible to develop it.  Facility in using particular senses can be gained by practice.  To improve ability to form visual images of things, practise calling up visions of things.  Try to picture a page of your history textbook.  Can you see the headlines of the sections and the paragraphs?  To develop auditory imagery, practise calling up sounds.  Try to image your French instructor’s voice in saying eleve.  The development of these sense fields is a slow and laborious process and one questions whether it is worth while for a student to undertake the labor involved when another sense is already very efficient.  Probably it is most economical to Arrange impressions so as to favor the sense that is already well developed and reliable.

Another important condition of impression is repetition.  It is well known that material which is repeated several times is remembered more easily than that impressed but once.  If two repetitions induce a given liability to recall, four or eight will secure still greater liability of recall.  Your knowledge of brain action makes this rule intelligible, because you know the pathway is deepened every time the nervous current passes over it.

Experiments in the psychological laboratory have shown that it is best in making impressions to make more than enough impressions to insure recall.  “If material is to be retained for any length of time, a simple mastery of it for immediate recall is not sufficient.  It should be learned far beyond the point of immediate reproduction if time and energy are to be saved.”  This principle of learning points out the fact that there are two kinds of memory—­immediate and deferred.  The first kind involves recall immediately after impression is made; the second involves recall at some later time.  It is a well-known fact that things learned a long time before they are to be recalled fade away.  If you are not going to recall material until a long time after the impression, store up enough impressions so that you can afford to lose a few and still retain enough until time for recall.  Another reason for “overlearning” is that when the time comes for recall you are likely to be disturbed.  If it is a time of public performance, you may be embarrassed; or you may be hurried or under distractions.  Accordingly you should have the material exceedingly well memorized so that these distractions will not prove detrimental.

The mere statement made above, that repetition is necessary in impression, is not sufficient.  It is important to know how to distribute the repetitions.  Suppose you are memorizing “Psalm of Life” to be recited a month from to-day, and that you require thirty repetitions of the poem to learn it.  Shall you make these thirty repetitions at one sitting?  Or shall you distribute them among several sittings?