A question which arises in connection with all photography of detail is the size of the grain of the photographic emulsion. Dependent on the size of the grain is the resolving power, or ability to separate images of closely adjacent objects. This varies with the speed, fast plates being of coarser grain than slow ones; with the exposure; and with the method and time of development. In general, it may be said that the resolving power of the plate does not enter practically into aerial work, because the resolving power of all plates so far found usuable corresponds to a smaller distance than the size of a point image as limited by the performance of the camera lens and the speed of the plane. Remembering that mm- is a fair value for the size of a point image as rendered by the lens, the role of plate-resolving power is shown by consideration of the following table. Resolving powers are given in terms of lines to the millimeter just separable.