4.2. The Photographic Gradation of the Prints
The requirements for photographic reproductions most suitable for classification differ from those commonly considered for pictorial purposes; the straight-line portion of the characteristic curves should be used in the successive copying stages, so that gradation differences may have as accurate relative spacing as possible. Since the latitude of the photographic print is far less than that of the original negative, this implies that the tonal range will have to be compressed (lower contrast); in addition, if the approximately straight-line part of the characteristic curve of the final print is to be utilized, the background of the print itself cannot be white, but must be of some intermediate shade of gray. These considerations detract greatly from the pictorial quality of the prints and result in the loss of all the fainter parts of the galaxies illustrated; if the tonal separation in the inner, brightest parts is to be preserved, however, some such procedure seems to be necessary.