5.4. The D Systems (Plates XX-XXIV)
This group is defined in a less precise manner than the preceding ones, and the problem of correlation of form types and spectroscopic appearance must be considered a problem for the future. In addition to spectroscopic changes along the D sequence, the problem of the strength of the metallic lines must be investigated; there appears to be a distinct possibility that the "weak line" characteristic observed in certain old stars and globular clusters (3) in our galaxy may be present in some D systems such as NGC 4762.