Complex sentence: The new art and the public's reactions to it were very vitalizing to Stieglitz; it gave him a brand new set of admirers and followers at a time when he was feeling less and less connected to his old colleagues at the Photo-Secession.
Split into simpler sentences: The new art and the public's reactions to it were very stimulating to Stieglitz. Moreover, it gave him a brand new set of admirers and followers at a time when he was feeling less and less connected to his old colleagues at the Photo-Secession.