Kodak film tank
This is the system proposed by Kodak to the amateur photographer develop his own pictures even without a darkroom. That was in the beginning of last century from 1907 to 1920. The idea can be understood like this: the film in its original spool is protected from light. But it can't be developed in that state because it is thighed spooled. This device transfers it to another spool light protected by the same principle (one layer film and one layer opaque material), but in a loose way so chemicals can get in and touch evenly the emulsion.