^* We use here the convention that θ is to be measured in the positive (anti-colock-wise) sense about A from AB (though the line through A may well lie outside the triangle, and θ need not be restricted to lie in the range 0 ⩽ θ < 2π), and similarly for ϕ, ψ. The senses of the lines are, of course, irrelevant, since (2) is unaltered if we Increase any of the angles θ, ϕ, ψ by a π (or, indeed, by any multiple of π), while the condition of concurrency is also unaffected.

page 55 note * Vol. 34 (1950), note 2144, pp. 129-133.