actinometer

For judging long exposures, the use of an actinometer (issued in many inexpensive forms) is helpful.

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.

The mystery was not a new type of infernal machine as they imagined but merely a home-made actinometer!

A very suitable instrument for timing the exposure of carbon tissue is Sawyer's actinometer.