The fluorescence has been recorded both photographically and photoelectrically, and the apparatus developed for each of these methods is described. The 3650 angstrom group of mercury lines has been found to excite fluorescence in all the type I and type IIa diamonds examined, but type IIb specimens have given no emission with this wavelength. The fluorescence spectra usually contain one or more 'systems' of lines, each system consisting of a main principal line accompanied by a banded ancillary structure. These systems are described and compared with features found in absorption spectra. Excitation spectra have been obtained for the two common fluorescence systems and are compared with absorption spectra. The effect of irradiation and of subsequent heat treatment on the fluorescence spectrum is described. Finally, the results are used to draw some conclusions about the nature of the centres responsible for the fluorescence.