It is now necessary to explain at once the value and the danger of light in photography. The image must, so to speak, be imprisoned in the dark chamber before it can be caught on the sensitive plate. The admission of a single ray of daylight other than the rays composing the image would prove fatal to the operation... For the same reason the plate must be manipulated in darkness throughout, or in a light such as shall not affect its action. - John Thomson
from Taking a Photograph by John Thomson, from Science for All, Vol 1. London: Cassell, Petter, Galpin & Co., ca. 1879.