When a preliminary classification is devised, which is based on an imperfectly understood physical characteristic, a choice must be made with regard to the most efficient procedure. In the present case, the new classification is expressed by means of groups formed according to certain observed characteristics, by a process of visual interpolation between two extreme categories; for the fundamental parameter, these are: systems having little or no central concentration of luminosity, and those in which the light is centrally condensed to the highest degree observed. Because of the diverse appearance of systems whose light has a similar degree of central concentration, it is necessary also to introduce parameters additional to the fundamental one.