. The American annual of photography. rcle that it would draw in one revolution is, if the 36 THE AMERICAN ANNUAL OF PHOTOGRAPHY revolution of the wheel be at the rate of sixty revolutions aminute, the fraction of a second at which the shutter is work-ing. Thus if the line drawn is one-twentieth of a completecircumference, the exposure was one-twentieth of a second. It may be supposed that any small white object—say, a spotof white paint—would do as well as a mirror ball. This isnot so, however, and for two reasons. In the first place, thereflection of the sun in the small mirror ball is, prac
. The American annual of photography. rcle that it would draw in one revolution is, if the 36 THE AMERICAN ANNUAL OF PHOTOGRAPHY revolution of the wheel be at the rate of sixty revolutions aminute, the fraction of a second at which the shutter is work-ing. Thus if the line drawn is one-twentieth of a completecircumference, the exposure was one-twentieth of a second. It may be supposed that any small white object—say, a spotof white paint—would do as well as a mirror ball. This isnot so, however, and for two reasons. In the first place, thereflection of the sun in the small mirror ball is, practically, amathematical point, and the length of the line drawn by it is,therefore, quite definite—a thing that would not be the casewith a white spot. Secondly, the brightness of this minutespot is the same as that of the sun itself, less a small percent-age due to imperfect reflection, assuming the ball to be aperfect sphere. Practically it is so intensely bright that itwill draw a line at whatever speed the wheel is
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishernewyo, bookyear1892
