The time interval between absorption and emission of light in fluorescence

The more recent theories of the absorption and emission of light, as in cases of resonance radiation and fluorescence, make the two processes quite distinct. According to Bohr the absorption of radiation of frequency corresponding to D2 by non-luminous sodium vapour, is associated with the passage of an electron from an inner to an outer stable orbit; resonance radiation resulting when the electron returns to the original orbit. The time interval between these two processes is probably too short to be detected experimentally, for the faintness of the light makes it appear doubtful whether methods, similar to the one devised by Abraham and Lemoine, can be used in these cases. In the case of the fluorescence, or rather phosphorescence, of mercury vapour, I have succeeded in measuring the time interval, the vapour remaining non-luminous during the process of the absorption of light, and bursting into luminosity about 1/15000th of a second later. This, I believe, is the first case ever observed of a photo-luminescent body remaining dark during the period of excitation. In the study of processes of this nature two methods are available: we may give the substance a very high velocity of translation or rotation and cause it to pass through a very narrow and intense beam of light, or we may keep the substance stationary, illuminating it with flashes of very brief duration and examining it during the moment of excitation, or at definite moments later, through a perforated disc rotating at high speed (phosphoroscope). Both methods have been employed in the present work, the former being preferable in some respects.