A survey for Ly alpha galaxies at z~3.1

We describe the results of a deep survey for Ly alpha emission line galaxies at z~3.1, carried out with the multislit imaging spectroscopy (MSIS) technique, with the FORS2 spectrograph on VLT-UT1. We discuss the criteria used to select the emission line galaxies and present the main physical characteristics of the sample: redshift, observed flux and equivalent width distributions.