Latent Image

The earliest exposures were very long indeed, and photographers would peer into the camera to inspect the image as it appeared. At the time it had been assumed that if the image did not appear, no change had occurred. Then Daguerre and Fox Talbot both independently and, it seems, accidentally, discovered that whilst short exposures produced no visible image, if one were to treat exposed plates with the fumes of heated mercury, an image would start to appear. This was a major breakthrough, because it enabled much shorter exposures to be made. In his record dated 23 September 1840 Talbot wrote about his calotype: "Some very remarkable results were obtained. Half a minute suffices for the Camera, the paper when removed is often perfectly blank but when kept in the dark the picture begins to appear spontaneously, and keeps improving for several minutes, after which it should be washed and fixed with (iodine of potassium)." By Dr. Robert Leggat

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