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"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to photograph Katie when she came outside, and to photograph anything also inside the cabinet, whenever the curtain was withdrawn for the purpose.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.

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  • The curtain is withdrawn for the purpose of Katie coming out. The cameras then photograph that act and anything else that can be seen (that was previously hidden behind the curtain). – Weather Vane Jun 13 '20 at 12:14
  • @Weather Vane and does exposure means "exposing of film to light" or something else? – user116597 Jun 13 '20 at 12:22
  • In the days of Arthur Conan Doyle (yet again) a camera had a glass plate inserted, which was covered in a light sensitive film of a silver compound (hence the word "film"). A single exposure to light could be obtained, which darkened the chemical where the light was strongest. That was why a negative image was obtained, which had to be used as a filter in the developing process. – Weather Vane Jun 13 '20 at 12:27
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The closed curtains are for the purpose of taking photographs inside the cabinet.

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to photograph Katie when she came outside, and to photograph anything also inside the cabinet, whenever the curtain was withdrawn for the purpose.

It seems obvious that curtains would not be for the purpose of photographing outside, so it can't refer to "Katie when she came outside".

Also, you have the word separating the outside photography from the photography inside the cabinet, and the remark about the curtain is clearly linked to that.

"Exposure" in this context is a noun and is photography terminology for a single photograph on a piece of film. So the cameras contained photographs of plates.

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  • i think that Weather Vane is right. i could not understand your explanation about "for the purpose". sorry. – user116597 Jun 13 '20 at 14:13

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