What does H. J. Slijper's canon pertain to in falconry?

H. J. Slijper's canon refers specifically to a geometric formula for designing hoods used in falconry. This concept is significant because the proper design and fit of a hood are essential for the comfort and effectiveness of the bird during training and hunting. Slijper's work emphasizes the importance of accurate measurements and shapes to ensure that the hood fulfills its function of keeping the bird calm and focused. Understanding this formula allows falconers to create hoods that accommodate the unique shapes of different bird species, which is vital for their welfare and performance.