  The universal scaling function of the square lattice Ising model in a
magnetic field is obtained numerically via Baxter's variational corner transfer
matrix approach. The high precision numerical data is in perfect agreement with
the remarkable field theory results obtained by Fonseca and Zamolodchikov, as
well as with many previously known exact and numerical results for the 2D Ising
model. This includes excellent agreement with analytic results for the magnetic
susceptibility obtained by Orrick, Nickel, Guttmann and Perk. In general the
high precision of the numerical results underlines the potential and full power
of the variational corner transfer matrix approach.
