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.
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.