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8 LANTERN-SLIDE MAKING AND EXHIBITING. experiment that a certain exposure (for example 20 seconds) gives with a standard developer a certain result, we know that by working under precisely similar conditions we shall always be able to secure approximately identical results. If the reader will keep these facts steadfastly before him in making his early experiments in slide-making, half the initial difficulties and troubles which ordinarily beset the beginner will never appear. To assist in securing uniformity, an The Exposure exposure board should be employed. This may be simply a piece of smooth deal, 4 feet long, with an upright piece at one end to support the frame, the source of light being placed at a marked position at the other end. The light may be either a gas flame, an oil lamp, an incandes- cent electric light of not more than 16 c.p., or an ordinary candle. The board should have lines plainly ruled across it at right angles to its length, and 6 inches apart. A slot should run down the centre of the board, so that the support may be adjusted at any distance from the light. A candle is perhaps the most satisfactory illuminant for the beginner, because of its uniformity. If an exposure of, say. 60 seconds proves too little, we have two courses open : either to reduce the distance between the light and the printing frame, or to increase the ex- posure. If the negative be dense, and as the light is a somewhat feeble one, we will adopt the former plan, and place the frame at the two-foot mark. Now in so doing we bear in mind the well-known rule in physics that the intensity of light varies as the square of the distance. If, therefore, we expose a second plate, again carefully counting 60 seconds,