PHOTOGRAPHIC MATERIALS . 135 



Types of Negative Materials. — The various emulsions are supplied on plates, cut 

 films, roll films, film packs, and the special modifications of these such as motion- 

 picture films. 



Plates and cut films, which are loaded into holders or special magazines in the 

 darkroom, are most widely used commercially and are the only forms available in the 

 larger sizes. Since they are handled in holders, they are particularly suited for use 

 with the professional types of cameras using ground-glass focusing. Also, since each 

 negative is a separate entity, it can be exposed and developed individually, which is 

 often very desirable. Because of the wide variety of applications, most of the various 

 emulsion types are available in this form. Most of the special emulsions, such as the 

 spectroscopic materials which are coated on special order, are supplied only on plates. 



Film packs have many of the desirable characteristics of cut films, such as use with 

 ground-glass focusing and individual handling when necessary, but with the additional 

 advantage that the films in the pack are protected from light, both before it is used and 

 after the films have been exposed. Thus the film pack can be loaded into and removed 

 from the holder or adapter in the light, and a darkroom is necessary only for develop- 

 ment of the films. The films used in the packs must be fiexible and so have a thinner 

 base than cut films. The result is that the films are more difficult to hold flat; hence, 

 while film packs serve excellently in the smaller sizes, up to about 4 by 5 in., they do 

 not work particularly well in the large sizes. Onlj^ the more widely used emulsions are 

 furnished in film packs. 



Roll films consist of a long strip of film wound on a flanged wooden or metal spool 

 with an opaque backing paper. This backing paper is sufficiently longer than the 

 film to provide a leader for threading through the camera without exposing any film 

 to the light. Because of their great convenience, roll films are most widely used by 

 amateurs. Previously only one or two emulsions were available on roll films, but now 

 a wide selection of medium and high-speed emulsions, with orthochromatic or pan- 

 chromatic sensitizing, can be obtained. 



A comparatively new development is the daylight-loading magazine containing 

 a short length of 35-mm. film, with standard motion-picture negative perforation, for 

 use in miniature cameras. In this case the film is wound on a spool inside a lighttight 

 container, with the end of the film extending through a slit, light-locked with black 

 plush. The end of the film which is threaded into the camera in the light is, of course, 

 fogged, but after the camera is closed, fresh film is drawn from the magazine. When 

 the whole strip has been exposed, it is wound back into the magazine and so can be 

 removed from the camera in the light. 



Aerial films can be considered as a special type of roll film, supplied in greater 

 widths and in very much greater lengths than ordinary roll film. Instead of a full- 

 length backing paper, which would add unnecessary bulk and weight, they are supplied 

 with opaque leader strips cemented to the ends of the film and long enough to wrap 

 several times around the roll, thus protecting it from light. Because of the large 

 negative size, the film base is thicker than that for roll film and is often especially pre- 

 pared to have very low shrinkage. 



Motion-picture film consists of long narrow strips, having perforations along one 

 or both sides to allow accurate positioning of successive small portions of the film in 

 the exposure aperture of the camera. For commercial users the film is usually fur- 

 nished wound on simple cores and requires a darkroom for loading into the camera 

 magazine. The films supplied for amateur use, however, are wound on special reels 

 with solid flanges which allow daylight loading. The 35-mm. films are usually avail- 

 able on either nitrate or safety (acetate) base, while 16-mm. films can be furnished 

 only on safety base. 



