ON EDUCATIONAL AND DOCUMENTARY FILMS. 28!) 



Practically speaking, all safety base at present available consists of cellulose 

 acetate — ordinary film is made from nitro-cellulose — the difference between one 

 safety base and another consisting mainly in the methods adopted to overcome the 

 defects of the plain acetate. It is non-inflammable in the sense that it does not 

 flare up when a light is applied, but slowly chars away so long as the high temperature 

 is maintained. 



It has the further advantage that while the heat of a normal beam of light in a 

 projector is sufficient to cause ordinary film to burst into flames, if kept there without 

 movement or special means of protection, this is not the case v,-iih safety base, 

 excessive heat produced in this way merely producing charring. There is also the 

 difference between the two bases in that at a comparatively low temperature nitrate 

 base decomposes with the production of dangerous decomposition products. This is 

 not the case with acetate base. 



At the Seventh International Congress of Photography, held in London in Julv 

 1928, there was a considerable discussion on the definition of safety film, but it was 

 found impossible to arrive at any agreed conclusions. The matter is now under 

 consideration by the various national committees, which are expected to report at 

 the next Congress at Dresden in 1931. 



A film, after exposure, has to go through the processes of development, fixing, 

 washing and drying, after which it must return to its original dimensions, within very 

 narrow limits of tolerance : otherwise difficulties would occur in its passage through 

 the gate of the projector. During these processes it is subject to various strains wliicli 

 may affect the dimensions. The chief strain is due to the fact that the base itself is 

 not entirely impervious to moisture, and alters, therefore, to some extent in size 

 according to its moisture content. This creates some difficulties in the use even of 

 the nitrate base, but they are very marked in the case of the acetate base. Minor 

 strains may also be caused by the swelling of the sensitive layer during development, 

 variations in this swelling during fixing and washing, and then de-swelling (contraction) 

 on drying. 



When passing through the projector the film is subject to considerable strain, of 

 an alternating character, and yet, after being used many times, its dimensions must 

 still remain within the limits of tolerance allowed. It must also withstand, as much 

 as possible, marking due to scratching, abrasure, etc. 



It is found that the mechanical properties of the nitrate base are such that it 

 stands up to the above treatments far better than the acetate base. The latter has 

 the further disadvantages that it is fiabby in handling (in the developing, etc., pro- 

 cesses) in comparison with the former, and also that when drj- it tends to become 

 brittle and break easilj'. 



At the present time the nitrate base film is the only one which satisfies the call 

 on it when made of the standard size, 35 mm., though cellulose acetate base film of 

 standard size is usable and will be improved as time goes on. When, however, the 

 size of the film is reduced to 16 mm. (sub-standard), the differences between the two 

 bases are not so marked. The strain of wear and tear is not so great in the sub- 

 standard film, and consequently the non-inflammable base is found to satisfj' all 

 requirements within reasonable limits. 



B. Cinematograph Appaeatusj 



Acetate film of 16 mm. or less in width is suitable for use by the amateur and for 

 educational (classroom) purposes. There is a large selection of reliable apparatus for 

 producing such films made bj' firms of repute, and there are also libraries of well 

 selected, though not specifically educational, films. This 16 mm. standard equipment 

 will give excellent results in a classroom not exceeding a seating capacity of 80, provided 

 the room can be well darkened (down to one desk light for the master and a red lamp 

 over exit doors). The projector is easj' to manipulate by the master or an assistant. 

 It is quite safe if reasonably used, and can be coupled up to any electric light suppl}-. 

 It is easily portable, and several makers fit their projectors with a stopping device 

 so that any one frame (picture) can be held stationary on the sheet without heat 

 from the light source giving trouble. Care must, however, be taken to make sure 

 that the rigidity of the projector under operative conditions has not been sacrificed 

 to portability. It is known that demand is causing concentration on improvements 

 that will increase the size, definition and brilliancj' of the projected picture and 



1930 U 



