Atmosphere is the medium through which we see all things. In order, therefore, to see them in their true value on a photograph, as we do in Nature, atmosphere must be there. Atmosphere softens all lines; it graduates the transition from light to shade; it is essential to the reproduction of the sense of distance. That dimness of outline which is characteristic for distant objects is due to atmosphere. Now, what atmosphere is to Nature, tone is to a picture.

In: 'A Plea for Art Photography in America', Alfred Stieglitz, in 'Photographic Mosaics,' Vol 28, 1892: About Pictorialism.