Choose an object, say the wheel of a bicycle, which may be got to make exactly one revolution per second. Fasten to one of the spokes near the tyre a disc of bright tinfoil, and focus the wheel as large as the plate will allow. When the wheel is making one revolution per second release the shutter. Now, without altering the camera, make an exposure with the wheel at rest to serve as a measuring chart. On development it will be found that the first exposure shows an arc or smudge of light. The proportion which this arc bears to the complete circle is the proportion which the shutter exposure bears to one second, so that all that remains is to measure the arc with a pair of compasses and divide the circumference by it. For a brief exposure of less, say, than one-fiftieth of a second, it is necessary to have a special arrangement by which a wheel can be rotated at a much higher speed and with greater certainty.