WATKINS' " STANDARD " EXPOSURE METER.
In addition to a simple actinometer for testing the light, it has a chain pendulum for counting seconds or half-seconds, which may be used either for camera exposure or for taking the actinometer time. The cal culating numbers are set out upon brass rings revolving round a brass cylinder, each ring being labelled with the letter corre sponding with the exposure factor, as will be seen by referring to Fig. 177. The chain of the pendulum is kept inside the cylinder and is enclosed by a cap which forms the weight of the pendulum. At the other end is a similar cap containing a circular win dow of blue glass, and beneath it the sensi tive bromide paper is uncoiled and passed out through a slot in the cap. The object of the blue glass is to ensure the tint being more easily and more accurately matched.
How TO USE THE " STANDARD " METER.
To use this instrument, unfasten the ]id of the pendulum box, then pull forward about I in. of bromide paper, so as to have a new portion exposed beneath the win dow. Cover the window with the thumb,
and, holding it opposite the deepest shadows of the picture in which detail is required, set the pendulum swinging and remove the thumb. Now count the num ber of seconds required to darken this to match the deeper tint at the side of the pre pared portion of bromide paper ; this gives the actinometer time. The speed of the plate must next be found, and for this pur pose the instrument is provided with a set of speed numbers, showing the proportion ate sensitiveness of all well-known brands. These numbers are to the extreme left of the instrument. Revolve ring P until it is over the number indicated by the speed of the plate ; next revolve D until it is against the stop which is to be used ; then revolve A until it is against the actinometer time, when the number against E which revolves with it will be the correct exposure.