I was recently led to consider this subject by Professor A. Dickson, who showed me some of his beautifully-mounted specimens, and explained to me the method he employs for the determination of the divergence, and of the successive leaves of the fundamental spiral or spirals. He referred me to two terribly elaborate papers by Bravais, and I have since met with another of a similar character by Naumann. These papers certainly cannot be supposed to present the subject from the simplest point of view.