The value or effectiveness of a selenium bridge depends upon several factors; namely: 1. The resistance of the bridge. 2. Its permanence, or stability. 3. Its sensitiveness, i.e., the ratio of the resistance of the bridge in the dark to that in the light. 4. The shape of the wave-length-sensibility curve. This paper summarizes the results of an investigation of the conditions governing the production of selenium bridges of certain types
