130 



Journal of the Mitchell Society. 



{Dec. 



The right hand column of co-ordinates was made out for 

 convenience in the construction, the edge of the tetrahedron 

 ABCD (supposed regular) being- taken equal to 100 units. 

 Whenever a point lies on an edge of the tetrahedron, the sum 

 of its co-ordinates is equal to 100, and when it lies in one of 

 the faces of the tetrahedron, the sum of its co-ordinates is 

 equal to 86.6, the altitude of the equilateral triangle forming 

 the face. 



The argument leading to the conclusion that the projection 

 of the two triads of planes a, b, c; f, g, h from the point D 

 upon the plane ABC {yo = 0) gives the Pascalian configura- 

 tion proceeds as follows. Consider the six planes «, b, c,f, g, 

 h and the point of projection D, and the plane of projection 

 ABC — then the line of intersection ab of the planes a and b 



