Z 1 {\displaystyle Z_{1}} is arbitrary point on the surface of the first sphere k 1 {\displaystyle k_{1}} , that is not on the intersection circle k s {\displaystyle k_{s}} . The extended cone created by k s {\displaystyle k_{s}} and Z 1 {\displaystyle Z_{1}} intersects the second sphere k 2 {\displaystyle k_{2}} in a circle. The length of the diameter of this circle is constant, that is it does not depend on the location of Z 1 {\displaystyle Z_{1}} on k 1 {\displaystyle k_{1}} .