The Zwicky catalog was made from photographic plates taken primarily
at the 18-inch telescope at Mt. Palomar.  The catalog contains
high surface brightness galaxies exclusively and the magnitude estimates
were made by eye.  The Zwicky magnitude system appears to estimate
total galaxy luminosity to an accuracy of 30--35%; however, there can
be large errors for individual galaxies.  There is a semi-coherent
discussion of the Zwicky magnitude scale in The Astronomical Journal,
1990, volume 99, page 1004.