In a recent paper1in this Journalwe described a simple photographic method of comparing the intensities of two beams of X rays of approximately the same wavelength. This method was based on one used in Ornstein's laboratory at Utrecht for the microphotometric investigation of spectral lines in the visible and ultra-violet region. It involves the use of ray filters which diminish the intensity of the initial X-ray beams by known amounts There is, however, a still simpler method of obtaining a quantitative estimate of the relative intensities of the two beams which is somewhat less accurate since it depends on the ratios of fairly small intervals of time, but which has the advantage of not requiring calibrated ray filters.It is well known that the Roscoe-Bunsen law,i.t= constant (where i = intensity of light andt= time of exposure), holds in many photo-chemical reactions, but not for the effect produced by the action of light on a photographic emulsion. If a plate or film is exposed to a beam of light for a certain time, then doubling the intensity of the beam and halving the time of exposure does not produce the same effect on the same plate (after developing and fixing), or the same “density” (darkening of the film or plate). In the case of X rays, however, this lawdoesapply very closely; the time which depicts the relationship between the photographic density and time is straight and passes through the origin (Fig. 1).*