FOR every emulsion, at a given temperature, there is an optimum development time at which the minimum light-exposure is just rendered visible, that is at which the threshold (Schwellenwert) speed of the emulsion is a maximum. For a given slow emulsion the maximum threshold speed is primarily dependent on the actual reducing substance in the developing solution, and is scarcely affected by variation in the concentration of the sulphite, bromide, and other constituents. The various ordinary organic chemical developing agents form a series, of which elon (p-methylaminophenol) occupies the highest position, and pyrogallol the lowest. This series bears some resemblance to the Watkin's factor series, but with this marked distinction, that the former is independent of the bromide concentration. The resemblance to the reduction potential series is probably closer, and it is even possible that the two are identical. The threshold speed series is a continuous one, from alkaline chemical developing solutions, in which the actual reducing substance is an anion, to nearly neutral or acid silver-containing physical ones, in which the reducing substance is a cation. For emulsions faster than process, the same sequence probably holds, but the difference between individual chemical developers is usually less and sometimes only of the same order as the experimental uncertainty.
Beyond the optimum time of development, the developed image scale begins to recede, or in other words, the lowest exposure fields of a tablet exposure begin to disappear for visual observation such as before an illuminator, one by one, or the extreme shadow details of a camera exposure are lost, little by little. This effect is seen with every emulsion and every (chemical or physical) developer. It is scarcely appropriate to say that the image becomes lost or buried in fog, for the reasons that the * Communicated by the Director.