A Johnson's actinometer is very useful to time the exposure.
We can, however, by chemical means arrive at an exact estimate of the active power, and for this purpose an actinometer is used.
You may sometimes find that the actinometer indicates a very different exposure from what the eye would lead you to expect.
The actinometer indicated an exposure of thirty seconds where in good light one would be right.
For judging long exposures, the use of an actinometer (issued in many inexpensive forms) is helpful.
A very suitable instrument for timing the exposure of carbon tissue is Sawyer's actinometer.
We then shift the paper in the actinometer so as to get a fresh portion under the tissue strips, or we substitute a new piece.
Now refer to the actinometer and see what has taken place on the silver paper which we put into it.
It is a good plan to employ an actinometer to judge the correct exposure.