I have just been watching a documentary called Filthy Cities on BBC2. This week it was New York. One reformer and photographer, Jacob Riis,played a significant part in bringing the conditions of the slums and the corruption of the authorities to the attention of America in his book 'How the Other Half Lives' 1890. Real street photography.....
Slide show
Wikipedia Entry
(Scroll down to the section on Photography for a description of the processes used)