1. Voigtlander and Lohmann: German patent 289,066, 1915.
2. Voigtlander and Lohmann: German patent 292,583, 1916.
3. Voigtlander and Lohmann: German patent 295,565, 1916.
4. Voigtlander and Lohmann: German patent 295,726, 1916.
5. K. Schröter: U.S. Patent 1,549,615, 1925.