Here follows an extract from the book, Whistler Landscapes and Seascapes by Donald Holden:
Whistler's method of painting the nocturnes was as unconventional as the paintings themselves.
T.R Way, who printed many of Whistlers lithographs, describes a walk with the artist, who stopped suddenly and "pointing to a group of buildings in the distance, an old public house at the corner of the road, with windows and shops showing golden lights through the gathering mist of the twilight said, 'Look!' As he did not seem to have anything to sketch or make notes on, I offered him my note book. 'No, no, be quiet,' was the answer; and after a long pause he turned and walked back a few yards; then, with his back to the scene at which I was looking, he said, 'Now see if I have learned it,' and repeated a full description of the scene, even as one might repeat a poem one had learned by heart"
Whistler was methodically training his memory according to the principles of a great and forgotten teacher, Lecoq de Boisbaudran. Whistler had not studied with him, but Legros and Fanin-Latour had. These two closest friends of the American's formative years in Paris must have described Lecoq's unique teaching method. "Lecoq conducted his last series of classes in the open air," says Horace Gregory, "in woods along the Seine, while models strolled through the sunlight and shade. While the models rested students were required to reconstruct from memory what they had lately seen, the walking figures, the glancing light between the leaves."
Lecoq had taught "that the imagination does no more than fuse the material furnished to it by the memory, thus producing completely new compounds." Equally important, Whistler discovered that painting from memory - with the occasional aid of a few scribbled compositional notes - radically simplified the image that emerged on the canvas. Details dropped away. The mind retained only the big, bold shapes, the abstract structure of the subject, suppressing the trivia and reducing the pictorial design to a pattern of essentials. If this abstract structure was right, neither detail nor finish mattered much. The job was to put the right shape in the right place, and Whistler was one of the first artists to acknowledge the role of the unconscious in making such decisions. Whether he knew it or not, he was also following the classic method of the Chinese landscape painters when he said, "Painting from nature should be done in the studio."