From Roy R. Behrens, CAMOUPEDIA: A Compendium of Research on Art, Architecture and Camouflage (Bobolink Books, 2009), p. 79—
Cagoules were paint-splattered hooded cloaks devised by the first French camoufleurs, to prevent themselves from being seen by aerial observers as they manned long-range artillery. A wonderful painting of this [shown above] was published on the cover of Scientific American on September 29, 1917. Some of these cloaks still exist, including prototypes designed by French camoufleur Eugene Corbin, and reproduced in Blechman• (2004:126-127) and Newark•• (2007:83). They are also described in Kahn••• (1984:148), who states the following: "Hidden beneath camouflaged cagoules, hooded uniforms…was a ghoulish image of the modern soldier, whose finely fitted and brilliant red and blue clothing was replaced by an amorphous costume of drab greens and browns that turned the individual into a frightening form."
A photograph of a group of hooded artillerymen was also published in the Popular Science Monthly, Vol 93 No 3, September 1918, p. 335, as shown below. The headline reads "Not Monks, But Camouflaged Artillerymen."
Amazingly, full-color French photographs of comparable outfits have also survived (see adjusted example below), and can be found online by searching "camouflage" at the Bibliotheque nationale de France.
•Hardy Blechman, ed. (2004), DPM: An Encyclopedia of Camouflage. London: DPM. ••Tim Newark (2007), Camouflage. London: Thames and Hudson. •••Elizabeth Louise Kahn (1984), The Neglected Majority: "Les Camoufleurs," Art History, and World War I. Lanham MD: University Press of America.