and the recent discovery of distinct brain-areas for the various orders of sensation would seem to provide a physical basis for such variations and discrepancies.  The facts, as I said, are nowadays so popularly known that I need only remind you of their existence.  They might seem at first sight of practical importance to the teacher; and, indeed, teachers have been recommended to sort their pupils in this way, and treat them as the result falls out.  You should interrogate them as to their imagery, it is said, or exhibit lists of written words to their eyes, and then sound similar lists in their ears, and see by which channel a child retains most words.  Then, in dealing with that child, make your appeals predominantly through that channel.  If the class were very small, results of some distinctness might doubtless thus be obtained by a painstaking teacher.  But it is obvious that in the usual schoolroom no such differentiation of appeal is possible; and the only really useful practical lesson that emerges from this analytic psychology in the conduct of large schools is the lesson already reached in a purely empirical way, that the teacher ought always to impress the class through as many sensible channels as he can.  Talk and write and draw on blackboard, permit the pupils to talk, and make them write and draw, exhibit pictures, plans, and curves, have your diagrams colored differently in their different parts, etc.; and out of the whole variety of impressions the individual child will find the most lasting ones for himself.  In all primary school work this principle of multiple impressions is well recognized, so I need say no more about it here.

This principle of multiplying channels and varying associations and appeals is important, not only for teaching pupils to remember, but for teaching them to understand.  It runs, in fact, through the whole teaching art.

One word about the unconscious and unreproducible part of our acquisitions, and I shall have done with the topic of memory.

Professor Ebbinghaus, in a heroic little investigation into the laws of memory which he performed a dozen or more years ago by the method of learning lists of nonsense syllables, devised a method of measuring the rate of our forgetfulness, which lays bare an important law of the mind.

His method was to read over his list until he could repeat it once by heart unhesitatingly.  The number of repetitions required for this was a measure of the difficulty of the learning in each particular case.  Now, after having once learned a piece in this way, if we wait five minutes, we find it impossible to repeat it again in the same unhesitating manner.  We must read it over again to revive some of the syllables, which have already dropped out or got transposed.  Ebbinghaus now systematically studied the number of readings-over which were necessary to revive the unhesitating recollection of the piece after five minutes, half