We study a C(1) parabolic and a C(2) quartic spline which are determined by
solution of a tridiagonal matrix and which interpolate subinterval midpoints. In
contrast to the cubic C(2) spline, both of these algorithms converge to any continuous
function as the length of the largest subinterval goes to zero, regardless of mesh ratios.
For parabolic splines, this convergence property was discovered by Marsden [1974]. The
quartic spline introduced here achieves this convergence by choosing the second
derivative zero at the breakpoints. Many of Marsden's bounds are substantially
tightened here. We show that for functions of two or fewer coninuous derivatives the
quartic spline is shown to give yet better bounds. Several of the bounds given here are
optimal.