Figure 1. H. Fol and E. Sarasin Photometers. 1. 1884 model.
2. and 3. 1885 model. 4. and 5. 1886 model as published
in Fol and Sarasin, 1887. Photometers are instruments to measure light. They
are used to study how far light penetrates into the water, sometimes the
color of the water, and, also in modern instruments, bioluminescence.