us, thus suffering from similar distance biases.

The Sample
==========

Imaging in the light of the $\rm H\alpha$+\[NII\] line of galaxies selected primarily in the region of the Coma supercluster is reported in this work. Out of the 262 spiral galaxies brighter than $\rm m_p=15.7$ belonging to the “Great Wall”, listed in the catalogue of Gavazzi & Boselli (1996), we observed 90 objects (8 additional early-type galaxies were observed serendipitously in the target fields. These will be not used in the following analysis). By themselves these objects do not constitute a complete sample. However, from the observations presented in this paper in conjunction with the $\rm H\alpha$ aperture photometry measurements taken from the literature (KK83, KBS84, Moss, Whittle & Irwin 1988 (MWI88), Romanishin 1990, GBK91), two complete subsamples can be extracted:

the “cluster” sample contains 40 (28 imaged in this work +12 from the literature) spiral galaxies with $\rm m_p\leq15.4$, belonging to either A1367 or to the Coma cluster (A1656);

the “isolated” sample contains 66 (44 imaged in this work + 22 from the literature) galaxies with $\rm m_p\leq15.4$ in the “Great wall” defined as isolated, i.e. those whose nearest companion lies at a projected distance of more than 300 