“Much difference of opinion has long existed in respect to the explanation of certain {{w|Science of photography#Chemical processes|phenomena of photographic action}}. In the vast majority of cases the action of light is a reducing one. {{w|Salt (chemistry)|Salt}}s of iron, of uranium, and of other metals are reduced from a higher to a lower stage of oxydation, and the same is the case with the combinations of certain metallic {{w|acid}}s, such as bichromates and {{w|tungstate}}s. These phenomena present no difficulty. It is only when we come to the silver haloids that obscurity commences.”