28 MOVEMENT 



a semicircular wire of white metal rotates round a 

 vertical axis, which is also its diameter. Fig. 19 is 

 the projection of such a sphere on a plane surface. 

 (The imperfections of the figure are due to the fact 

 that it is impossible to impart perfect regularity of 

 curvature to the wire which constitutes the semicircle.) 

 It is unnecessary to multiply examples of figures 



Fi:. 20.— Sphere engendered bv th n rotation of a semi-annular tbrcai. 

 (Steri-osco, ic images ) 



Fig. 21. — Hyperboloid and its asymptotic cone. (Stereoscopic images.) 



generated by the movement of curves, such as ellip- 

 soids, paraboloids by revolution, etc. 



In order that these figures of three dimensions may 

 be rendered more intelligible, we have reproduced 

 them in the form of stereoscopic pictures by the 

 following proceeding. 



Stereoscopic Pictures of Figures of Three Dimensions. — 

 We must provide ourselves with a stereoscopic camera 



