  Form factor representation of the correlation function of the 2D Ising model
on a cylinder is generalized to the case of arbitrary disposition of
correlating spins. The magnetic susceptibility on a lattice, one of whose
dimensions ($N$) is finite, is calculated in both para- and ferromagnetic
regions of parameters of the model. The singularity structure of the
susceptibility in the complex temperature plane at finite values of $N$ and the
thermodynamic limit $N\to\infty$ are discussed.
