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