The Cinema Industry

The cinema is no longer, as it was in its earliest days, merely a means of amusement; today it has acquired an educational character, the perfecting of which has required the efforts of many engineers and scientists, especially in what concerns talking pictures and color films. I shall therefore attempt to explain to our honored readers in this issue how ordinary and color films are prepared, and in the next issue I shall furnish them with an explanation of how sound is recorded on film and of the projection apparatus in the cinema houses.

Raw Film

Raw film is divided into two parts: there is the chemical compound that reacts with light, and there is the material on which this chemical compound is coated. This material on which the chemical compound rests is called cellulose nitrate; it is a transparent substance made from cotton (which contains about 95 percent cellulose) by mixing it with sulfuric acid and nitric acid (acide sulfurique et acide azotique): the cellulose reacts with the nitric acid to produce cellulose nitrate. This substance is then washed and dried thoroughly and placed in a liquid to dissolve it, this liquid being usually a mixture of ether and acetone (ether et acetone) — two liquids that evaporate easily. If we dissolve the cellulose nitrate in this liquid and spread the solution on smooth polished drums, the ether and acetone evaporate, leaving behind them a thin skin of transparent cellulose nitrate. (This material is also used in preparing nail polish, or what we call "ecladore.")

As for the sensitive chemical compound, it is called silver bromide (Bromure d'argent), a compound produced by a reaction between potassium or sodium bromide and silver nitrate — all of these being substances that dissolve easily in water. If we mix a solution of potassium bromide with a solution of silver nitrate, the bromine unites with the silver, and silver bromide is formed by this reaction; and since it is a compound that does not dissolve in water, it settles to the bottom of the vessel as a white substance inclining slightly to yellow.

This silver bromide is a chemical compound sensitive to the action of light and rays. If it is exposed to light, it is chemically affected, with the result that the silver and the bromine separate from each other: the bromine flies off (since it is a gas) or dissolves in the water when the film is developed, while the grains of silver remain clinging to the cellulose nitrate.

To prepare the light-sensitive emulsion (émulsion photo-sensitive), a solution of potassium bromide is prepared and gelatin is added to it in the proportion of three percent; this solution is then mixed with a solution containing silver nitrate, and the silver bromide precipitates in the form of fine grains. The quantity of gelatin is then increased to seven percent and the whole is mixed thoroughly, so that the silver bromide becomes evenly distributed in a viscous, semi-solid solution or emulsion of gelatin.

The gelatin is then washed to remove whatever foreign matter has clung to it, and a little alcohol is added to it so that it may more easily be used to coat the cellulose nitrate.

When this operation is finished, the sheets of cellulose nitrate are taken and coated with this light-sensitive emulsion, then dried in a current of clean hot air; the film then becomes fit for use, after being wrapped in paper that keeps light from seeping in to it.

It goes without saying that all the operations connected with preparing the sensitive compound must be carried out either in pitch darkness or in a room lit by a very faint light of orange or green color, according to the kind of film.

And now that the film is ready, it is wound on special reels whose shapes differ according to the purpose for which they are used: a cinema reel differs, for instance, from a camera spool.

Recording the Image

The film is now inside the camera. When light enters through the lens of this apparatus, it draws a miniature image of whatever persons or things may be before the lens. Naturally, the light that carries the image of the persons or things into the camera varies in strength according to what it is reflected from. The color white, for example, reflects a brighter light than brown or black, and shining metal reflects a stronger light than the same metal when its surface is not smooth. The light that passes through the lens, then, is unequal in intensity (Intensité) throughout the image it draws on the sensitive film. Its effect will therefore differ according to its strength, and consequently the degree of its reaction with the silver bromide will vary with that strength. I said at the beginning of this article that the reaction that takes place between light and silver bromide results in

a separation of the bromine and the silver, and the stronger the light, the more complete the separation. If the image the light draws on the film shows two men, one wearing a white garment and the other a black garment, it is self-evident that the light reflected from the white garment will bring about, in the spot of the film on which it falls, a separation of bromine and silver far greater than the separation brought about by the scant light reflected from the black garment.

Developing the Image

Now that the light has recorded the image on the film, the next operation is developing the film (Development). For this purpose the film is placed in a chemical solution containing organic compounds that increase the separation of the silver from the bromine in those spots of the film that were exposed to light; the bromine dissolves in the water, leaving behind black grains of silver.

As for the places that were not exposed to strong light, they still retain some silver bromide, which must be removed before the film is exposed to light. For this purpose the film is placed in a solution containing hyposulfate of soda (Hyposulfate de Soude), which reacts with the silver bromide to form a soluble substance.

The film, then, now contains black spots in the places where it reacted with strong light, white places where no light reached it, and spots ranging between black and white according to the strength of the light that fell on them. Or, in other words, if we return to the picture of the two men mentioned above, we see that the white garment will appear black and the black garment white. This is what we call the negative image (Negatif). To make it a positive image, the negative film is laid on a piece of glossy paper coated with the same sensitive substances the film contains; there take place on the surface of the glossy paper the same reactions that take place on the film, except that what was black becomes white and what was white becomes black, because the light does not penetrate to the sensitive paper through the black part of the film (the part which recorded the image of the white garment), while it passes easily through the white part (the part which recorded the image of the black garment).

Color Films

The light-sensitive part of these films is made in the same way as that of ordinary films. The addition here is that the film is coated with a transparent coating containing the colors blue, red, and green, for these are the primary colors — that is to say, we can obtain almost any color we wish if we mix these colors, or some of them, in the right proportions. The film is exposed to light in such a way that the rays reach the sensitive chemical compound after piercing the three colored layers. If we wish to photograph a red rose, the rays pierce the red color without affecting the blue or the green; and if we wish to photograph a green tree, the green passes through the film without affecting the red or the blue, but acts on the sensitive compound behind the green part of the film. If, however, the color is a compound one — yellow, for example — it passes through both the red and the green. This method thus separates the colors from one another. And when the color film is developed, the grains of silver increase behind the places on which the red light fell, for example, so that this color appears clearly, while the green or the blue does not appear, because the red light does not penetrate through them and so cannot act on the sensitive compound behind them.