SPACE 23 



Movements of a Straight Line in Space. Photography 

 of the Solid Figures formed thereby : Cylinders, Hyper- 

 boloids, Cones, etc. — A straight line, which is moved 

 in various ways on a plane surface, covers the latter 

 with a variety of configurations, or else, if attention 

 is paid only to the various positions which it occupies 

 at certain moments, these positions can be expressed 



Fig. 14.— Stereoscopic trajectory of a brilliant point pliced at the level of ihe lumbar 

 vertebra; of a man walking a\\\iy from tue photographic camera. 



as geometrical figures, which can be transferred to 

 paper ; but if this straight line moves in the three 

 directions of space it describes surfaces, the projec- 

 tions of which can only be represented by perspective 

 drawing. In such cases recourse must be had to 

 diagrams in relief, which have the advantage of giving 

 beginners a clearer notion of solid forms. By means 

 of threads stretched between metal armatures, one can 



