Emulsions coated on to films should be parti cularly resistant to abrasion (§ 199), as all rubbing of the film tends to cause phenomena similar to those caused on plates by sliding pressure and friction.
Films, even of the best quality, expand and contract according to circumstances. In the course of development and the subsequent treatment, films may increase in length by from o•5 per cent (celluloid) to 1-5 per cent (cellulose acetate of good quality), while after drying they may exhibit a contraction of from 0.2 to o-5 per cent. This shrinkage continues for a long time, and, in the case of an old film, may exceed I•25 per cent (limit of tolerance for cinematograph apparatus) ; it is not uniform, and in some rare cases the local variations may amount to zoo per cent. 2 As a rule, film cannot be used when it is wished to make exact measure ments from the images except by registering, at the same time as the image, a scale (e.g. a squared ruling of known size) against which to check the measurements. To reduce the local variations for this work, the film should be kept for some time in a moist atmosphere before commencing measuring.
243. The constituents of films are not, as is glass, inert materials, and the preservation for a long period of the qualities of the emulsion with which the film is coated is not as certain as it is in the case of plates. Chemical fog usually appears after some time, but in tem perate climates it takes two years or even longer for this to acquire appreciable density. To safeguard themselves, manufacturers mark on the packets a date beyond which the film should not be used. The period thus indicated is much shorter than that which can probably be allowed, so as to provide for the risk of retailers keeping stocks under unsuitable conditions. Frequently films (not panchromatic or infra-red sensitized ones) have given excellent results after a period considerably in excess of the time limit allowed.
244. Negative Paper. Negative paper, which is deserving of notice owing to its comparatively low price compared with that of plates and films, is generally kept for work in large sizes (direct negatives, or negatives made by enlargement). For small sizes, the grain of the paper, which always appears to some extent when prints are made by contact, masks some of the finer details of the image. Printing and enlarging by epi
scopic projection (§ 799) can give good results, but the range of tones of the subject that can be differentiated in the final image is then much more limited than in contact printing.
In addition to negative paper proper, and the various stripping negative papers (forming sup ports for thin films which are stripped from the paper after processing), other papers are pre pared for special purposes. These include papers for certain scientific and industrial recording apparatus, and papers for use with the simplified cameras for the copying of documents (in banks, for copying current accounts ; in drawing-offices for the duplication of sketches, etc.). These negative copies are obtained " the right way round " by the use of a prism (§123) in front of the lens, and, as a rule, are kept as they are. The emulsion of these papers is considerably hardened, so that it can rapidly be dried by heat without risk of melting the gelatine.
245. Relative Merits of Plates and Films. We will consider here only the point of view of the amateur, with particular reference to the novice, leaving to the professional and other users the responsibility for deciding for themselves in each particular case to which type of sensitive material they will give preference.
To amateurs who restrict themselves to in stantaneous photography in good weather, the roll film offers the advantages of daylight loading, lightness, and almost complete freedom from halation. On the other hand, for those who wish to improve their work it is a drawback that it is impossible, without almost acrobatic opera tions, to develop each negative as soon as it is taken. The novice who, during the day, has found two or three subjects worth photographing is always tempted to " finish the spool " by photographing anything in order not to be obliged to postpone the development of his negatives. Roll-film cameras are usually not fitted with any means of focussing the picture on a screen, and so it is at a disadvantage in doing all that is best for an exposure. Also, a long strip of film is much less easily handled than a plate.
It is possible to take advantage of the light ness and convenience of the daylight changing associated with films, by using a film pack with a plate camera, when need arises.