DIAGONAL: A numerical solution of the Stokes transfer equation

In a previous paper (Semel & Lopez Ariste 1999, hereafter referred to as Pa per I) the authors found a new solution for the integration of the radiativ e transfer equation for polarized light (RTE). The present paper is a conti nuation of that work. and shows how this solution has been incorporated int o a new numerical code for the integration of the RTE. This code, called DI AGONAL, is characterized by an analytical handling of most model atmosphere s, differing in that from other codes. It gives exact solutions in new, non -trivial cases. In the general case when no complete analytical solution is available, the code resorts to a multi-layer technique, thus providing a s olution that combines an approximated analytical model and a minimized resi dual treated linearly. The algorithm is shown both to be very stable and to reach a high precision even for a small number of integration layers. This results in a very fast code. A comparison with DELO has been carried out. DIAGONAL is available, upon request to the authors.