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# Long description

This figure is identical to Figure 9, except for the addition of a vertical plane, parallel to the x-axis and intersecting the curved surface. It consists of a perspective drawing of a three-dimensional set of Cartesian axes with a smooth surface sketched on it. The complex plane is drawn as horizontal, with axes labelled x and y. The x-axis points to the left and the y-axis to the right from the origin. The vertical axis is labelled s. A smooth surface shaded darker on top and lighter underneath starts like a sheet with the top edge attached along the y-axis. At this point it is flat. As the x-coordinate increases, the surface rises up in a saddle shape above the x-axis, and dips down on either side. The vertical cross-section of the surface parallel to the s-y plane is shaped like an inverted parabola, symmetrical about a vertical line through the x-axis. On the surface a point is marked with a solid dot and labelled with its coordinates: capital P equals, open bracket, 2 comma 1 comma 2, close bracket. The vertical plane is shaded red, and labelled with its equation, y equals 1. A curve is drawn on the surface to show where the surface intersects the plane. It passes through the point marked P.