The Boy's Book of New Inventions

by Harry E. Maule

The scientist then went on to explain that in attacking the problem the investigators first had to learn all they could about colours, and how they are reflected by light rays. As we have seen in the colour process for motion pictures there are really only three fundamental or primary colours, and all other shades and tints are made up from combinations of these. The three are blue-violet, green, and orange-red, and a screen of these forms the foundation of all the colour plates now used.

In the autochrome process the lowly potato, which we generally think of merely as a common article of our food, forms the first factor. The starch of the potato is ground down and sifted so that the grains are the same size—not more than 0.0004 to 0.0005 of an inch in diameter. These grains then are divided into three equal portions, and each portion is dyed, respectively, blue-violet, green, and orange-red. The three little piles of starch grains are then mixed together in suitable amounts and dusted on to a plate, which has previously been coated with a substance to make them stick. The difficulty in dusting on the starch grains is great, for they must cover the whole plate equally and yet not make any piles of starch at any one point, for to have several grains on top of one another would spoil the effect. The extreme delicacy of this operation will be appreciated when it is realized that there are over five million grains to the square inch. When the starch is all properly placed it makes the colour screen, though in appearance the plate is a dark gray.

The plate is next put through a rolling process so that all the grains are flattened out to form a mosaic covering over the whole surface. In spite of all the manufacturers can do there will still be some microscopic spaces between the particles, and these are filled up with a fine powder of carbon to prevent the passage of light.

The screen is then coated with a very thin layer of varnish and upon this is laid a thin and extremely sensitive photographic emulsion.

"And so that is the way these autochrome plates we have here were made," concluded the scientist. "Now our troubles begin, for we must be careful to give them a fair trial with the proper kind of an exposure and the proper kind of development."

As the plates are extremely sensitive to all kinds of light the scientist cautioned the boy against loading the camera carelessly. It is better, he said, to load in a dark room.

In putting the plates in the camera the plates are reversed and instead of placing the sensitized side toward the lens, the uncoated glass is put in front and the photograph is taken through the glass. Thus, the image first passes through the glass, next, through the grains of coloured starch, and, lastly, is recorded on the sensitive photographic emulsion.

Before loading the camera, however, the scientist fitted a yellow colour screen over the lens, explaining that this was necessary to absorb some of the overactive blue-violet light rays, to which the emulsion is extremely sensitive.

In exposing the plate what happens is this: Suppose a green field is to be photographed. The green rays of light, reflected from the field, pass through the lens, and through the glass support of the plate. But when they reach the coloured starch, the green rays pass through the green particles of starch, but not through the violet-blue particles, or the orange-red particles, for the grains of other colours absorb the green rays and hold them. Thus, development would show that the green light rays passing through the green starch particles caused the emulsion to darken under the green particles in just the proportion in which the green light reached them, and to record the image they carried. As the light would not pass through the other coloured particles they would not record any image. Thus a negative is produced, as we have seen, not the colour we see in life but the complement. By treating the plate with a solvent of silver the tiny black specks that were brought out behind each green particle are removed and each starch grain is allowed to transmit exactly the colour we see in life. In other words, we have a positive.

This is just as true of all the shades and hues as it is of the three fundamental colours, for the various rays of light will penetrate the starch in just the proportion of the hues they represent in the scene before our eyes. While the silver solvent will remove the dark images built up by the penetration of green light, it will leave behind the particles of red-orange, and blue-violet, backed up by the creamy silver bromide of the emulsion. If above the green field we had a blue sky, the blue-violet particles would let the blue-violet rays penetrate them, and record the image of the sky.