metry. It would take us too far to expound such a construction 
here, it can be found in any book on descriptive geometry. For 
explanation see Pl. 3; 3a shows a three dimensional representation 
conform to Pl. 2 in which the projections of the points a, 6 and c 
are also indicated, whilst in Pl. 34 the projections and the passages 
have been put down in one plane. 
a Fig. 3. b 
Projections of the points of definition and of the passages of a semicircular canal. 
The passages of a second plane of a semicircular canal in the 
same system of projection having been determined according to this 
method, the angle between these two planes of semicircular canals 
can be construed and measured in the way usual in descriptive 
geometry. Pl. 4a and 44, reproduce the mode of construction of the angle 
