of Edinhurgli, Session 1882-83. 171 
therefore 
= ^2 + iAD = ^2 + AC, 
C being the centre of the circle, because AD is a diameter. In fact 
the construction reduces itself practically to the determination of 
the centre C of the circumference passing through A, A'A" ; the 
axis of rotation being then a line passing through C and being 
perpendicular to the plane which is parallel to rj and f the normals 
of the planes of symmetry. 
