146 THE DISCOVERY OF THE CHILD
the child makes of it. These two extremes, when brought together,
demonstrate the most striking difference which exists in the series,
and thus establish the most outstanding contrast which the material
renders possible. The contrast, being a striking one, makes the
difference very evident and even before he has used the things
the child is interested in them.
ISOLATION OF A SINGLE QUALITY IN THE MATERIAL
Whatever object we wish to use for the education of the senses,
it, of necessity, presents many diverse qualities like weight, rough-
ness, colour, form, size, etc. How then ought we to proceed so
that the series will bring one quality only into prominence? We
must isokte, from among the many, one single quality. This
difficulty is overcome by the series itself and its gradations; we
must prepare objects identical among themselves in all respects
except the variable quality.
If we want objects suitable for teaching colour differences, we
must have them made of the same substance, form and size and
differing in colour only. Or, if we want tp prepare objects with
a view to teaching the various tones of the musical scale, it is
necessary that they be perfectly alike in appearance, as are the
bells which we use in our system; these are of the same shape and
size and are mounted on identical supports, but when struck with
a small mallet, they give out different sounds and these sounds
constitute the only difference, perceptible to the senses.
For this reason the little instruments which are put into the
hands of children as musical toys, which have longer or shorter
rods or tubes of different heights arranged like organ pipes, do
not lead themselves to a real exercise in musical sense, tending to
differentiate sounds; for the eye is able to help in distinguishing
them, being guided by the different dimensions, whereas the ear
ought to be the sole receiver and the sole judge.
This method is successful in differentiating between things very
clearly; it is evident that clearness constitutes the principal factor
for raising interest in making distinctions.