First draw the circular base _CD_. Divide the circumference into equal parts, according to the number of steps in a complete round, say twelve. Form scale _ASF_ and the larger scale _ASB_, on which is shown the perspective measurements of the steps according to their positions; raise verticals such as _ef_, _Gh_, &c. From divisions on circumference measure out the central line _OP_, as in the other figure, and find the heights of the steps 1, 2, 3, 4, &c., by the corresponding numbers in the large scale to the left; then proceed in much the same way as in the previous figure. Note the central column _OP_ cuts off a small portion of the steps at that end.
In ordinary cases only a small portion of a winding staircase is actually seen, as in this sketch.
[Illustration: Fig. 259. Sketch of Courtyard in Toledo.]
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.