These are included whenever possible as subscripts respectively after the family and variety symbols. An advantage of the present notation system is that it is possible to retain significant information on objects poorly resolved by dropping one or more symbols which from right to left refer to stages,varieties, families and classes of increasing generality. Hence the nature and number of symbols used can be selected according to the amount of detail visible in any given object with any particular telescope. These are discussed in detail by Zwicky. It is nevertheless of some importance for the present discussion to determine the separation between neighboring objects within which their mutual interaction begins to cause significant structural distortion and thus sets a limit to the validity of the classification system established in the first instance for isolated galaxies.
Many parameters are obviously involved, such as separation, diameters, masses, velocities, etc.
The regular spirals SA , SB make up just over fifty percent of the sample, ellipticals and lenticulars close to a quarter each. The low apparent frequency of the Sd , Sm stages is certainly due to a large extent to the low absolute luminosity and surface brightness of galaxies at these stages, but it is not yet possible to correct the apparent frequencies for such selection effects. On the other hand the high frequency of ellipticals cannot be due in general to an abnormally high absolute luminosity, so that it probably reflects the actual great abundance of this class 6 . An example is NGC 3115 which, although selected as the type object for E7 in Hubble's original scheme, shows an absorption marking and a beginning of nuclear differentiation on 200 inch plates. The existence of dwarf lenticulars and spirals remains to be demonstrated.