"They might be the name of a ship, or some sign. I do not think it was part of a ship. I tried to find something in the cave, on the day I went in while you went after the team, which would afford some clue, but so far nothing confirms me in any view which I may have."
"Isn't it curious that these letters should show through only after the slab was exposed to the light?"
"Why is that any more curious than photography is?"
"Because in photography something is put on the glass or the sheet that the negatives are made of, and it turns and makes a mark under certain conditions."
"Well, here is something put on this slab that turns also. Photography is a wonderful thing. Dr. Draper, the first great photographer, and who was also a scientist, says that every wall, or other object, which you stand before, has your photograph imprinted on it. The only question is to find some chemical which will develop the picture."
"What is meant by developing the picture?"
"You remember some time ago we talked about reagents, and the properties of certain chemicals to act on others, and in doing so, to make a change. Sometimes the change is a complete one, and makes a new product; in other cases the result is a complete change of color. Now, in photography, if a certain chemical is placed on a glass or a film, and the film is exposed, the light and dark portions of the object show on the film. The sunlight, or the actinic rays in the sunlight, affect the chemical material so that when the fixing chemical is applied it prevents a change in the condition of the chemical."
"What do you mean by the actinic ray?"
"All light is vibration; the greatest motions which are perceptible to the eye, being known as violet. Now there are still more rapid vibrations than are put forth to make the violet rays, which are called the actinic rays, and are the ones which affect the chemicals so acutely."
"Is it then possible to photograph with a light that is not perceptible to the eye?"