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4.1. The Amount of Information in a Photographic Image of a Galaxy

It is of the greatest importance to ensure that the criteria of classification do not have a high sensitivity to changes in resolving power. For example, the position of such nearby systems as M 31 with respect to the general classified category of galaxies is difficult to evaluate when a criterion such as the degree of resolution is utilized.

From considerations such as this, the criteria suggested here are of a coarse nature. This implies that the resulting classification will also be coarse; however, the revision does have two important characteristics: (a) it is sufficiently fine to permit segregation of groups of galaxies that appear to have similar physical characteristics as far as their stellar populations are concerned; and (b) it should make possible the segregation of closely similar groups of galaxies at greatly varying distances from the observer. This last point is most important for the investigation of fluctuations in the characteristics of galaxies in various regions of space.

As an additional step toward ensuring systematic accuracy, the range in linear size of the galaxies on the photographs used has been limited. If we define the number of information elements in a galactic image as being equal to the number of star images that could be placed in the area occupied by the galaxy, then the systems illustrated range in number of information elements from the order of several hundred to several thousand. This appears to be a convenient method of specifying the most general characteristic of the photographs used for the new form classification.

In the case of M 31 (Plate XVI), the plate reproduced was obtained with a telescope of 30 inches focal length; in spite of this small scale, the number of information elements is higher than in the other systems illustrated; however, the coarseness of the criteria used for classification makes it seem probable that M 31 has been located at its proper relative position with respect to the frame of reference defined by the other galaxies reproduced.

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