Toronto. In the days of film, exposure created a latent image, invisible to the eye. A chemical reaction to a hand full of chemicals, including at least one which converted and clumped silver halide molecules exposed to light into metallic silver brought out a visible image.
Since the light exposed areas turned black and vice versa, the result was a negative image that had to be exposed once again to another sensitive medium to make a right way around or positive image.
This second sensitive media was usually on the surface of a special paper. It was insensitive to red/orange light leading to the common red light lit darkroom of the day. Film was usually red insensitive too (or orthochromatic to use the correct name) and could be developed under a red light too. Until flexible rolls became common, tanks were long (deep), Weighted film could later be immersed in the tank in a developer solution.
Once panchromatic film became common, so called daylight loading tanks became popular. Some tanks were loaded in the dark, then processed in daylight. A special rubber lined bag held the film and tank while the photographer could stand outside the bag in subdued daylight and carefully load the exposed film on to a reel before cutting off the cassette and adding the tank lid to block daylight exposure. A baffle allowed chemicals to enter and exit the tank without light exposure.
NB. The title of this post is a parody that comes from a song by The Fall Out Boy.