ht and dawn while his wife begged him
to take some rest. In the first thirty years of the fifteenth century,
Masaccio contributed to the knowledge of anatomy by his painting
of the nude form; and the study of the nude was continued by
Pollaiuolo and Luca Signorelli, in the second half of the century.
Masaccio, also, was the first to place his figures in _air,_ enveloping
them in atmosphere. Verrocchio, a generation later than Masaccio,
was one of the first of the Florentines to understand landscape and
the part played in it by air and light. The realistic spirit, which
suffices itself with subjects drawn from every-day actual experience,
finds expression in the first half of the fifteenth century in the work
of Andrea del Castagno. And so down through that century of spring
and summer. Each painter in his own way carries some detail of his
craft to a further point of development and prepares the path for the
supreme triumphs of Michelangelo, Leonardo, and Raphael.

The growing mastery of the principles and technique of painting
accompanied a change in the painter's attitude toward his art.
Originally, painting, applied in subjection to architecture and
employed in the service of the Church, was decorative in scope; its
purpose was illustration, its function was to teach. As painters, from
generation to generation, went deeper into the secrets of their craft,
they became less interested in the didactic import of their work, and
they concerned themselves more and more with its purely artistic
significance. Religious subjects were no longer used merely as
symbols for the expression of piety and as incitements to devotion;
they became inherently artistic motives, valued as they furnished the
artist an opportunity for the exercise of his knowledge and skill and
for the exhibition of lovely color and significant form. A change in
the mechanical methods of painting, also, had its influence on a
change in the conception of the function of art. With a very few
exceptions, the works of Giotto were executed in fresco as wall
decorations. The principles of mural painting require that the
composition shall be subordinated to the architectural conditions of
the space it is to fill and that the color shall be kept flat. The fresco
method meets these requirements admirably, but because of its
flatness it has its limitations. The introduction of an oil vehicle for
the pigment material, in the fifteenth century, made possible a much
gre