Out of Doc's clothing came a tiny metal device, the principal gadget of which was a small reservoir filled with a liquid the color of coagulated blood. Doc held the paper pad over this and flicked the lever, causing the apparatus to give off a vapor. From Fear Cay by Lester Dent writing as Kenneth Robeson.
The device releases a iodine vapor that caused the written impressions from the previous sheet of paper to appear and be faintly legible. Doc Savage used this technique to gain a clue as to what his enemies were doing in Fear Cay.