"for the purpose" refer to what? does "exposure" mean "the action of exposing a photographic film to light or other radiation?
My library was used as a dark cabinet. It has folding doors opening into the laboratory; one of these doors was taken off its hinges, and a curtain suspended in its place to enable Katie to pass in and out easily. Those of our friends who were present were seated in the laboratory facing the curtain, and the cameras were placed a little behind them, ready to photograph Katie when she came outside, and to photograph anything also inside the cabinet, whenever the curtain was withdrawn for the purpose. Each evening there were three or four exposures of plates in the five cameras, giving at least fifteen separate pictures at each séance; some of these were spoilt in the developing, and some in regulating the amount of light. Altogether I have forty-four negatives, some inferior, some indifferent, and some excellent.