s of form are probably derived through the different
colours of objects around us, by which they are thrown into relief upon
the background, or against other objects; and, as I mentioned in the
first chapter, we reach outline by observing the edges of different
masses relieved as dark or light upon light or dark grounds, so now, in
my last, we come again to the consideration of the definition of line
and form by colour, and their relief and expression upon different
planes or fields of colour.

There is first the colour of the object itself--the local colour--and
then the colour of the ground upon which it is relieved, both of which
in their action and reaction upon each other will greatly affect the
value of the local colour and the degree of relief of the form upon it.

One of the best and simplest ways to ascertain the real value of a
colour and its effect upon different grounds or fields is to take a
flower--say a red poppy, and place it against a white paper ground,
blocking in the local colour as relieved upon white, as near as may be
to its full strength, with a brush, and defining the form as we go
along. Then try the same flower upon grounds of different tints--green,
blue, yellow--and it will be at once perceived what a different value
and expression the same form in the same colour has upon different
tinted grounds. A scarlet poppy would appear clearest and darkest upon
white; it would show a tendency upon a blue ground to blend or blur at
its edges, and also on yellow and green to a less extent.

[Illustration (f132): Sketch to Show Effect of the Same Colour and Form
upon Different Coloured Grounds.]

It is this tendency to lose the edges of forms owing to the radiation of
colours, and to mingle with the colour of the background, which makes a
strong outline so constantly a necessity in decorative work. One may use
a black on a white, a brown, or a gold outline (as in cloisonne), the
nature of the outline being generally determined by the nature of the
work. In stained glass the outline must be black, and this black is of
the greatest value in enhancing by opposition the brilliance of the
colours of the glass it incloses, stopping out the light around it as it
does in solid lead when placed in the window.

[Illustration (f133): (1) Principle of the Effect of the Blending or
Blurring of Colours at Their Edges; (2) Use of Black and White Outline
to Clear the Edges of Coloured Forms upon Different Coloured