edo.]

CXLVI

OF THE CYLINDRICAL PICTURE OR DIORAMA

[Illustration: Fig. 260.]

Although illusion is by no means the highest form of art, there is no
picture painted on a flat surface that gives such a wonderful appearance
of truth as that painted on a cylindrical canvas, such as those
panoramas of 'Paris during the Siege', exhibited some years ago; 'The
Battle of Trafalgar', only lately shown at Earl's Court; and many
others. In these pictures the spectator is in the centre of a cylinder,
and although he turns round to look at the scene the point of sight is
always in front of him, or nearly so. I believe on the canvas these
points are from 12 to 16 feet apart.

The reason of this look of truth may be explained thus. If we place
three globes of equal size in a straight line, and trace their apparent
widths on to a straight transparent plane, those at the sides, as _a_
and _b_, will appear much wider than the centre one at _c_. Whereas, if
we trace them on a semicircular glass they will appear very nearly equal
and, of the three, the central one _c_ will be rather the largest, as
may be seen by this figure.

We must remember that, in the first case, when we are looking at a globe
or a circle, the visual rays form a cone, with a globe at its base. If
these three cones are intersected by a straight glass _GG_, and looked
at from point _S_, the intersection of _C_ will be a circle, as the cone
is cut straight across. The other two being intersected at an angle,
will each be an ellipse. At the same time, if we look at them from the
station point, with one eye only, then the three globes (or tracings of
them) will appear equal and perfectly round.

Of course the cylindrical canvas is necessary for panoramas; but we
have, as a rule, to paint our pictures and wall-decorations on flat
surfaces, and therefore must adapt our work to these conditions.

In all cases the artist must exercise his own judgement both in the
arrangement of his design and the execution of the work, for there is
perspective even in the touch--a painting to be looked at from a
distance requires a bold and broad handling; in small cabinet pictures
that we live with in our own rooms we look for the exquisite workmanship
of the best masters.

BOOK FOURTH

CXLVII

THE PERSPECTIVE OF CAST SHADOWS

There is a pretty story of two lovers which is sometimes told as the
origin of art; at all events, I may tell it here as the origin