In the case of spectroscopically composite systems such as galaxies it is to be expected that the spectral types deduced will depend upon the wavelength region observed; if a mixture of early- and late-type stars is present, the integrated spectral type will tend to become progressively earlier on passing toward regions of shorter wavelength. Moreover, the features observed in any one spectral region may indicate a considerable range in type for the contributing stars. We shall first review briefly some aspects of the spectral classification system of Humason. 2