Wilson's quarter century in photography : a collection of hints on practical photography which form a complete text-book of the art . ry in photography. exposing shutter. With the pneumatic holder in his hand he may learn tospell out the riddle tolerably, and gradually grow up to the mastery. Or, he to measure hundredths or thousandths of seconds, instruments of high precision areindispensable. To measure by approximation such small fractions of seconds seems tous as foolish as to weigh milligrammes with gramme weights. If we take up these ques-tions it is necessary to do so with all possible
Wilson's quarter century in photography : a collection of hints on practical photography which form a complete text-book of the art . ry in photography. exposing shutter. With the pneumatic holder in his hand he may learn tospell out the riddle tolerably, and gradually grow up to the mastery. Or, he to measure hundredths or thousandths of seconds, instruments of high precision areindispensable. To measure by approximation such small fractions of seconds seems tous as foolish as to weigh milligrammes with gramme weights. If we take up these ques-tions it is necessary to do so with all possible precision, and it is to carry out this orderof ideas that I have devised the following method. Take a registering cylinder put in motion by a Foucault regulator; on its edge affix abrilliant point—the head of a nickel-plated nail, for example. The point is carried^ forward by the regulator at the same time as the cylinder of which it forms a part; it isits displacement that we will photograph. It moves behind a graduated dial-platepierced with a circular opening (Fig. 234). The dial is black; the divisions are white Fig. Dial Method. A, brilliant point. The cylinder is covered with a smoked paper, upon which vibrates an electric diapasonfurnished with a small style. A camera is now directed toward the dial, the regulatoris put in motion, the style is made to touch the diapason, and the stop is lifted. EXPOSURE, OR THE QUESTION OF TIME. 233 may use a photometer or sensitometer to determine the sensibility of his plates,and expose according to what they indicate. We give here the reproduction of the result of an experiment. We see the divideddial and the track A B left by the point. Light commenced to act at A and finished atB. We have now to see on our sinusoid to what places these two points correspond,and what time has elapsed between A and B\ nothing is more simple. With the handthe cylinder is made to revolve until the brilliant point reaches A. This is the com-mence
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Keywords: ., bookcentury1800, bookdecade1880, bookidwilsonsquart, bookyear1887
