256 



Prof. Maxwell on Reciprocal Figures 



capable of two degrees of variability, and is subject to two con- 

 ditions. 



Fig. 5. 



**.. 



y ,- 



Fig. V. 



The conditions are that the four intersections of corresponding 

 sides of opposite quadrilaterals in fig. V. shall lie in one straight 

 line, parallel to the line joining the opposite points of fig. 5 which 

 correspond to these quadrilaterals. There are three such lines 

 marked x, y, z, and four points of intersection lie on each line. 



We may express this condition also by saying that fig. V. must 

 be a perspective projection of a plane-sided polyhedron, the in- 

 tersections of opposite planes being the lines w } y } z. 



