A Manual Of Photography

THE DYNACTINOMETEB. 161 covered, in order to be represented black on the daguerreotype plate and mark the zero of intensity : the second is exposed to light during l', the third during 2", the fourth during 4", the fifth during 8", the sixth during I6", the seventh during 32", and the eighth during 64". This series, which could be extended by dividing the circle into a greater number of segments, is quite sufficient for all observations intended for practically measuring the intensity of the photogenic light, and for comparing the power of object-glasses. The instrument is made to move by applying the hand on a handle fixed on the back at the extremity of the axis on which the disc revolves. An operator accustomed to count seconds by memory, or by following a seconds’ beater, can perform the experiment with sufficient regularity ; but in order to render the instrument more exact and more complete, it can be made to revolve by clock-work, which gives it at will either the arith- metical or the geometrical progression. This last movement presented some difficulty ; but the inventor has been able to obtain it without much complication in the machinery, and the apparatus is within the reach of the greater number of operators having establishments on a complete footing. For the instrument moving by hand, it is necessary that a second person should open and shut the object-glass at a given signal. But in adapting before the object-glass a flap connected with a cord and pulley, the operator, holding the cord in the left hand, can open the flap at the moment that with the right hand he makes the disc revolve, and shut the apparatus when the revolution is complete. When the instrument acts by clock-work, the object-glass may be opened and shut by the same means, at the signal given by a bell which strikes at the commencement and at the end of the revolution. If a daguerreotype plate receive the image of the dynactino- meter during its revolution, it is obvious that each segment in- dicates an effect in proportion to the intensity of light and to the time that it lias remained uncovered; also that the number of seconds marked on the first segment visible is the measure of the intensity of light at the moment of the experiment; the effect of each segment being in reality the degree of intensity whicli can be obtained during the corresponding time. When we want to compare two object-glasses, they are adapted to two cameræ obscuræ placed before the dynactinometer. After having set the focus of the two apparatus, they are charged each witli a daguerreotype plate or a photogenic paper. When al1 is ready, the flaps are opened at the moment that the