1. IN A METHOD OF PREPARING A MEDIUM FOR USE IN MAKING CAMERA COPY, THE STEPS WHICH CONSISTS IN APPLYING TO A NON-MOISTURE-PROOF SURFACE, A SOLUTION CONTAINING AT LEAST TWO INGREDIENTS, ONE OF WHICH REMAINS SUPERFICIALLY ON SAID SURFACE AS A CHEMICALLY-DEVELOPABLE PATTERN COMPRISING UNIFORMLY SPACED DOTS, AND THE OTHER OF WHICH IMPREGNATES SAID SURFACE AND SPREADS UNIFORMLY IN ALL DIRECTIONS TO FORM A SECOND PATTERN COMPRISING SPACED DOTS LARGER THAN SAID FIRST-NAMED DOTS AND IN REGISTRATION WITH SAID FIRST-NAMED DOTS, THE INGREDIENT FORMING THE FIRST-NAMED DOTS BEING MERCUROUS CHLORIDE, AND THE INGREDIENTS FORMING THE LARGER DOTS BEING COBALTOUS CHLORIDE.