Example 264. To hit a point in the surface of a sphere of radius r we must make the errors x 1 near or far, x 2 in a direction at right angles to this direction and x 3 in a direction at right angles to each of the other directions, that is, we have to make the error https://www.w3.org/1998/Math/MathML"> r = x 1 2 + x 2 2 + x 3 2 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429294204/c59d152a-6373-4ba1-ba84-9ade22990710/content/ieq0532.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> , by Axiom 7.