  The perturbation approach is used to derive the exact correlation length
$\xi$ of the dilute A_L lattice models in regimes 1 and 2 for L odd. In regime
2 the A_3 model is the E_8 lattice realisation of the two-dimensional Ising
model in a magnetic field h at T=T_c. When combined with the singular part f_s
of the free energy the result for the A_3 model gives the universal amplitude
$f_s \xi^2 = 0.061~728...$ as $h\to 0$ in precise agreement with the result
obtained by Delfino and Mussardo via the form-factor bootstrap approach.
