Instruction in Photography

which the density of a negative or the depth of a print may be measured with great exactitude and in a very simple manner. Fig. 4. Suppose F to be a half-plate ferrotype or a piece of blackened card of the same size. In either of them pierce a Square aperture A, and cover it and a similar square surface, B, with white Saxe paper, and surround these two squares with black paper. If now a candle or small lamp be placed on the opposite side of the screen held vertically, the square A will be illuminated when viewed from the front. If, in a darkened room, another small candle or lamp be placed in front of the screen, both A and B will be illuminated. If, however, a rod of the diameter of B be so placed that its shadow is cast on A, both squares will be still illuminated, A by the lamp at the back, and B by the lamp in the front. By moving the lamp in front nearer to or further from the screen, the two squares can be made equally bright to the eye. If, close behind A, we place a bit of the negativewhose transparency we wish to measure, we can again make the brightness of the two squares equal; and if we have measured the distance from the screen in the two cases, we can at once find out the light coming through the piece of negative. Thus, suppose in the first case the light had to be 36 inches from the square, and in the second only 9 inches, we should know that only -jV of light was passing through the negative, as the brightness would be inversely as the squa...