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based on a thorough understanding of the objects observed. In no
case is this more surely effected than in the processes of
mechanical drawing, where the intended structure has to be portrayed
so exactly in plan, elevation, side view, and sections, that the
workman has simply to copy the drawing in metal, wood, or stone, as
the case may be. It is undoubtedly the fact that mechanicians,
engineers, and architects usually possess the faculty of seeing
mental images with remarkable clearness and precision.

A few dots like those used by the Bushmen give great assistance in
creating an imaginary picture, as proved by our general habit of
working out ideas by the help of marks and rude lines. The use of
dolls by children also testifies to the value of an objective
support in the construction of mental images. The doll serves as a
kind of skeleton for the child to clothe with fantastic attributes,
and the less individuality the doll has, the more it is appreciated
by the child, who can the better utilise it as a lay figure in many
different characters. The chief art of strengthening visual, as well
as every other form of memory, lies in multiplying associations; the
healthiest memory being that in which all the associations are
logical, and toward which all the senses concur in their due
proportions. It is wonderful how much the vividness of a
recollection is increased when two or more lines of association are
simultaneously excited. Thus the inside of a known house is much
better visualised when we are looking at its outside than when we
are away from it, and some chess-players have told me that it is
easier for them to play a game from memory when they have a blank
board before them than when they have not.

There is an absence of flexibility in the mental imagery of most
persons. They find that the first image they have acquired of any
scene is apt to hold its place tenaciously in spite of subsequent
need of correction. They find a difficulty in shifting their mental
view of an object, and examining it at pleasure in different
positions. If they see an object equally often in many positions the
memories combine and confuse one another, forming a "composite" blur,
which they cannot dissect into its components. They are less able to
visualise the features of intimate friends than those of persons of
whom they have caught only a single glance. Many such persons have
expressed to me their grief at finding themselves power