These instruments test the activity of the light by the length of time it takes a piece of sensitive paper to assume a given colour, and the exposure is made accordingly. Fig. 36 illustrates one of the watch-shape meters. In the small circle A on the dial there are three divisions, the two outer ones are tints, the centre one is occupied by the sensitive paper. In the outer circle, B, there is one set of figures representing the speed numbers of the plates, and on the inner circle, C, another set of time numbers.
One tint in A is four times darker than the other and is for out-door work. The paler one is for interiors. The working of the instrument depends upon the darker tint, therefore the result given by the paler one requires to be multiplied by four. With the actinometer or exposure meter is issued a chart giving the various speed numbers of the different makes of plates. The instrument is worked by moving round the back part to bring a fresh piece of sensitive paper into the middle of A. The time this takes to assume the same colour as the standard or darker tint is carefully noted. When this is ascertained the speed number of the plate is turned and brought into line with the time number. All that remains to be done is to look for the diaphragm number at which the exposure is to be made, and the exposure time will be in a line with it. Thus, supposing it requires four seconds for the sensitive paper to colour when the instrument is held in the shadow of the body away from the sun and the speed number of the plate is 45, the time number 4 is brought into a line with 45. If the exposure is to be made with the diaphragm of the lens closed down to 32, the figure in the time circle C will be 2, and the required time for the exposure will be 2 seconds.
These instruments are exceedingly useful, and quickly save their cost by reducing the number of waste plates from faulty exposing.