2. _Contrast._ The contrast capable of development without fog should be from 1.5 to 2. This contrast should be produced by light of daylight quality, and, in orthochromatic and panchromatic plates, with the yellow or orange filters intended to be used with them. This contrast means a gamma infinity approaching 2.5. 3. _Speed of development._ A gamma of nearly 2 should be developed in 2½ minutes at 20 degrees C. in the developers recommended below. 4. _Fog._ Not over .25 for this degree of development, and not over .40 for six minutes development. 5. _Color sensitiveness._ This should in general be as high as possible. In terms of certain representative filters (described in a subsequent chapter) color sensitiveness should be such that with the white light speed above specified the relative exposures through the filters shall not be greater than as follows: No filter Aero 1 Aero 2 #21 #23a #25 Panchromatic plate 1 3 4.5 7 9 12 Ortho plate 1 2.5 3.5 6 =Relative Behavior of Plates and Films.=—The advantages of film from the standpoint of weight and bulk have been discussed in connection with aerial cameras. Were there no other considerations film would unquestionably be the most appropriate medium for aerial photography. There is, however, the question of ease of handling, to be treated in a subsequent chapter, and the question whether the purely photographic characteristics of film are satisfactory. Can the same speed, contrast, and color sensitiveness be obtained on film as on glass? Is the picture so obtained as permanent or reliable as the plate image? It must be confessed that up to the present emulsions on film have not proved the equal of those on glass. It has been found by emulsion manufacturers that the same emulsion flowed on film and on glass gives better quality on the glass. Emulsions specially prepared for film fall somewhat short of the best plate emulsions. It has also been found harder to color-sensitize film, and to insure good keeping quality in the color sensitized product. In addition to the question of photographic quality there arises the matter of shrinkage and distortion. These are negligible with plates, but are a more or less unknown quantity in film. Irregular shrinkages of as much as two per cent. are found on experiment. This defect, of course, would be an obstacle only in exact mapping work.