The teacher, in the same manner, should understand what is the full beneficial effect which it is possible, _in theory_, to derive from his instructions.

He should understand, too, that just so far as he falls short of this full effect there is waste.

It may be unavoidable; part of it unquestionably is, like the friction of machinery, unavoidable.

Still, it is waste; and it ought to be so understood, that, by the gradual perfection of the machinery, it may be more and more fully prevented.
Always bear in mind, then, when you are devoting your time to two or three individuals in a class, that your are losing a large part of your labor.

Your instructions are conducive to good effect only to the one tenth or one twentieth of the extent to which, under more favorable circumstances, they might be made available.