Toronto, When light sensitive media were very slow and insensitive to the lower end of the spectrum (reds, oranges), the photographer’s eye could judge a fully developed image by subdued light or reddish illumination. About a century ago, flexible roll film made alternative means more practical. As sensitivity increased, and especially when film went panchromatic, and then colour, development HAD to be done in total darkness and another means had to be found to replace the educated eye.
This was the tried and true “Temperature and Time” method. Film began to be sold with a small bit of paper specifying the ideal time, temperature (usually 68 degrees Fahrenheit), and agitation (usually a slow spin of the spool once each minute). The trouble was, that the developers listed were only those sold by the film maker. This opened the door for books like the Photo-Lab Index which listed many formulae and the time/temperature to be used for a variety of film materials.
Under-developed film tended to be flat, lacking in contrast, and possibly with little or no shadow detail. On the other hand, over-developed film was very contrasty with the risk of little or no highlight detail. Developer too hot or too cold affected the film as did the development duration and agitation. Thermometers gave decent temperature settings, Timers (or watches with big second hands) allowed for good timing while attention to agitation kept that variable in line too.
NB. The post title is a takeoff on a Beatles song “Why Don’t We Do It in the Road“. The post photo is my two dial thermometers taken at 8mp hand held with my iPod Touch camera and cropped a bit. AirDrop transferred the image to my computer painlessly – no chemicals, no long delays, no sweat …