The different kinds of light afforded in cellars by various forms of
windows. The least useful and the coldest is the window at _a_. The
most useful, the lightest and warmest and most open to the sky is
the window at _b_. The window at _c_ is of medium utility.

[Footnote: From a reference to the notes on the right light for
painting it becomes evident that the observations made on
cellar-windows have a direct bearing on the construction of the
studio-window. In the diagram _b_ as well as in that under No. 510
the window-opening is reduced to a minimum, but only, it would seem,
in order to emphasize the advantage of walls constructed on the plan
there shown.]