Certain factors which enter into comparisons of this sort in other lines of work, such as astronomical photography, play little part here. These are, first, the optical resolving power of the lens, which is conditioned by the phenomena of diffraction, and is directly as the diameter; and, second, the size of the grain of the plate emulsion. The first of these does not enter directly, because the size of a point image on the axis of the lens, due merely to diffraction, is very much less than that given by any photographic lens which has been calculated to give definition over a large field, instead of the minute field of the telescope. Yet it may contribute toward somewhat better definition with a long focus lens because of the actually larger diameter of such lenses. The second factor is not important, because, as will be seen later, the resolving power of the plates suitable for aerial photography is considerably greater than that of the lens. The emulsion grain is in fact only a quarter or a fifth the size of the image as given by a 25 centimeter lens, and enlargements of more than two or three times are rarely wanted. A series of experiments was made for the U. S. Air Service to test out these questions, using a number of representative lenses of all focal lengths, both at their working apertures and at identical apertures for all. With regard to lens defining power, as shown by the size of a point image, the answer has already been reported in a previous section. Lenses of long focus give a relatively smaller image than lenses of the same design of short focus. In regard to the whole process of making a small negative and enlarging it, the loss of definition is quite marked, as compared to the pictures of the same scale made by contact printing from negatives taken with longer focus lenses. This answer is clear-cut only for lenses calculated to give the same angular field. Thus a 10 inch lens covering a 4 × 5 inch plate has about the same angle as a 50 centimeter lens for an 18 × 24 centimeter plate. When, however, it comes to the longer foci, such as 120 centimeters, the practical limitation to plate size (18 × 24 cm.) has been passed, and the angular field is less than half that of the 50 centimeter lens. The 120 centimeter lens need only be designed for this small angle, with consequent greater opportunities for reduction of spherical aberration. It is therefore an open question whether a 50 centimeter lens designed to cover a plate of linear dimensions 50/120 times that used with the regular 50 centimeter lens could not be produced of such quality that it would yield enlargements equal to contacts from a 120 centimeter lens. If so, lenses of larger aperture could be used, and a considerable saving in space requirements effected.