A rougher idea of color sensitiveness than is given by spectrograms is furnished by the _tri-color ratio_, which is the ratio of exposure times necessary with white light to give equal photographic action through a certain set of red, green and blue filters, expressed in terms of the blue exposure as unity. In an excellent panchromatic plate the three exposures would be equal. In an orthochromatic plate the red exposure will be too large to be figured. In interpreting either spectrograms or tri-color ratios care must be taken that the _absolute_ exposures necessary are known. Thus a relatively high red sensitiveness may mean merely low absolute blue sensitiveness. Two methods are used in imparting color sensitiveness. Either the sensitizing dye is incorporated in the plate emulsion before it is flowed; or the plate is bathed in a dye solution not long before using. The latter method gives higher color sensitiveness but poorer keeping quality, and is not a practical method for field operations. Greatly enhanced sensibility may be given by treatment with ammonia, but this again is a method for laboratory rather than field use. =Resolving Power.=—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 usable 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 ⅒ mm. is a fair value for the size of a point image as rendered by the lens, the rôle 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. Emulsion. Resolving Power. Seed Graflex 25 Eastman Aerial Film 37 Hammer Ortho 44 Cramer Isonon 48 Cramer Spectrum Process 57 Eastman Portrait Film 61 =Tabulation of Requirements for Aerial Emulsions.=—In terms of the sensitometric quantities just discussed the general requirements for aerial plates may be listed as follows: 1. _Speed._ The speed usually connected with the contrast and density required for the exposure times available is about 150 H & D. Faster plates in general have too low contrast, but the highest speed that will give the necessary contrast is desired.