H.J. Slijper's "canon" is best described as what?

H.J. Slijper's "canon" refers to a geometrical formula specifically designed for fitting Indian hoods, which are protective headgear for birds of prey. The importance of this formula lies in its precision and the specific measurements needed to create hoods that fit properly on different bird species, thereby enhancing the comfort and effectiveness of the equipment used in falconry. This understanding is crucial for falconers, as a well-fitted hood is essential for maintaining a bird’s well-being and effectiveness in training and hunting. In contrast, the other options pertain to various different aspects of falconry equipment or methods, which, while relevant to the field, do not align with the specific definition of Slijper's "canon."