Let us suppose, for the purposes of explanation, that as far as seeing goes, any object is made up of countless infinitesimal points of light, and that the business of the eye is to gather them in and spread them out at the back of the eye in exactly the same relation they bore to each other on the object. The points of light, so duplicated, would thus form the image of the object.

The camera works very much the same way. The lens at the front of the camera is the eye, and the plate or film at the back of the camera corresponds to the back of the eye. The lens collects all the points of light of the object we wish to photograph, and directs them to the plate or film in such fashion that they occupy exactly the same relative position that they did before. An image of the object is formed.

Now if we could look inside the camera and the image were visible, we would see that it was upside down. The reason for this is very simple, as the accompanying diagram shows. The ray of light from "A" at the bottom of the object passes through the lens at an angle, and continues in a straight line until interrupted by the film or plate. It started at the bottom of the object and ended at the top of the image. The position of all the points of light is just reversed, although their relative position remains the same. "Then here," you say, "is where your analogy between the camera and the eye falls down."

Not at all. It is true that we do not see things upside down, but this is because of mental readjustment during the passage of the impressions from the eye to the brain.

Now let us suppose that we have our camera loaded with film, and that mother has succeeded in keeping the baby quiet long enough for us to uncover the lens for an instant and let the points of light through to the film. The next question is, how are we going to make the resulting image permanent. We know that it is there, but in its present state it is not going to do us a great deal of good. In fact, if we should peek in the back of the camera, and to do so would ruin the exposure, we could not even see it.

But let us go back a bit. We ought to know a little something about the composition of this film on which the image has been projected.

In brief, film is a cellulose base coated with silver bromide and gelatine. If we were using a plate the only difference would be that instead of cellulose as a base we would have a sheet of glass. The gelatine is there to afford lodgment to this sensitized silver. The silver, being sensitive to the action of light, is there to record the image. As soon as one of these silver particles has been touched by light, it becomes imbued with the power of holding whatever the lens has transmitted to it. The image was formed, we remember, by points of light grouped in the same relative positions as the points of light of the object we were photographing. Consequently it is only those silver particles within the image-forming area that are affected, because that is where the light struck.

Showing Inversion of the Image.

* Illustrations by courtesy of Eastman Kodak Company.

The lens, then, gathered in the points of light and dispersed them on the film so as to form an image. The silver particles held this image, but not visibly - it is a latent image, and it is the purpose of development to bring it out.

It is the particular business of a chemical called "pyro" to release this latent image. When attacked by pyro, those silver bromide particles which have been affected by light - and only those - change to black metallic silver. After all the silver bromide particles, the ones that held the image, have been transformed into metallic silver, another chemical called "hypo" effectively disposes of all the silver bromide that was not affected by light. Now only the image-forming silver bromide particles remain, and these have been transformed to metallic silver. The result is a permanent image - a negative.

But it is a negative, so called because everything in it is reversed - not only from left to right, but in the details of the image. Mother's dark blue gown looks light, for example, and baby's white dress, dark.

To get our picture as it should be, we must place the negative in contact with a sheet of paper coated with a gelatine containing silver. This emulsion, as the coating is called, is, as we might readily infer from the presence of the silver, sensitive to the action of light in much the same manner as was the original film. We place the negative and paper in contact, then, in what is called a printing frame, so that light may shine through the negative and impress the image on the sensitive paper. It is obvious that the light parts of the negative will let through the most light, and that consequently the silver emulsion on the paper underneath will be most blackened, while the dark parts will hold back the light and the emulsion on the paper underneath will be less affected. In other words, the very faults that we noted in the negative, from a picture point of view, automatically right themselves. Mother's dress looks dark and baby's dress white - just as the lens saw it.

We then have the picture in its finished form.

The story of the making of the camera is as interesting as that of the making of the pictures by the camera.

Back in 1732, J. H. Schulze discovered that chloride of silver was darkened by light and all unwittingly became the father of photography. In 1737, Hellot, of Paris, stumbled on the fact that characters written with a pen dipped in a solution of silver nitrate would be invisible, until exposure to light, when they would blacken and become perfectly legible. However, it was not until early in the nineteenth century that these two discoveries were put to any practical use, as far as photography was concerned.