`   :Sorry, new here, I really need more information on the Mathematical phenomenon ``Piper (1901) calls this phenomenon ``consecutive depths of field`` and shows how to test the idea easily``. there is no citation, I've searched thoroughly for his work, and I need to see how he tested the idea urgently. If this is the wrong place to ask this can you tell me? in that case message me Hans.thm@gmail.com thanks.`