A′
at an angle of 5° to it. This is to allow the necessary freedom for the dart when entering the crescent; from
A′
we draw a line through the center of the ruby pin. We do not show it in the drawing, as it would be indiscernible, coming very close to
A′ X
. This line will also pass through the center of the crescent. At the point of intersection of
A′
b
with the safety roller we have one of the edges of the crescent. By placing our compass at the center of the crescent on the periphery of the roller and on the edge which we have just found, it follows that our compass will span the radius of the crescent. We now sweep the arc for the latter, thus also drawing in the remaining half of the crescent on the other side of
A′ X