A rapid method of measuring the density of negatives is a desideratum not only for the scientific investigator but also for the practical photographer. In a paper published in the Journal of the Society of Chemical Industry we have shown that the speed of photographic plates can be ascertained by a very simple method, provided an instrument be at hand which rapidly, and with a sufficient degree of accuracy, measures the relative amounts of silver deposited on a plate by a series of different exposures.