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

H.J. Slijper's "canon" is best described as a geometrical formula for fitting Indian hoods. This term refers to the precise calculations and measurements that ensure a proper fit for hoods used in falconry, particularly those influenced by Indian designs. Properly fitted hoods are crucial for the comfort and effectiveness of the bird during training and hunting, as they prevent the bird from becoming overly stressed or agitated by excessive light or noise.

The significance of this formulary approach underscores the importance of tailoring gear specifically to the anatomy of different bird species, optimizing their performance in the field. Fitting hoods correctly enables falconers to manage their birds effectively, ensuring that the equipment supports the bird's welfare and its hunting efficiency.