2l8 KANSAS UNIVERSITY (QUARTERLY. 



In case that b=c, a^-i-b^^i, and the involution becomes 



Xi = ax t by, 



Yi^bx— ay, 



I 

 z = , 

 z 



Geometrically this means that every straight line parallel to the 

 z-axis is first reflected on the xz-plane, so that x=xi, y^y^ z=zi 

 and then rotated about the z-axis, 



Zj=axi — by^, 



yj=bxi+ayS 

 which results into the involution 



x,=ax + by, 



y^=bx— ay, 

 I 



