  Critical point scaling in a field $H$ applies for the limits $t\to 0$, (where
$t=T/T_c-1$) and $H\to 0$ but with the ratio $R=t/H^{2/\Delta}$ finite.
$\Delta$ is a critical exponent of the zero-field transition. We study the
replicon correlation length $\xi$ and from it the crossover scaling function
$f(R)$ defined via $1/(\xi H^{4/(d+2-\eta)}) \sim f(R)$. We have calculated
analytically $f(R)$ for the mean-field limit of the Sherrington-Kirkpatrick
model. In dimension d=3 we have determined the exponents and the critical
scaling function $f(R)$ within two versions of the Migdal-Kadanoff (MK)
renormalization group procedure. One of the MK versions gives results for
$f(R)$ in d=3 in reasonable agreement with those of the Monte Carlo simulations
at the values of R for which they can be compared. If there were a de
Almeida-Thouless (AT) line for $d \le 6$ it would appear as a zero of the
function $f(R)$ at some negative value of R, but there is no evidence for such
behavior. This is consistent with the arguments that there should be no AT line
for $d \le 6$, which we review.
