26 



MOVEMENT 



placed still more obliquely— in fact, if it touches the 

 axis at one point — the figure described will be a 

 cone. 



Photography, with a dark background, is especially 

 adapted for demonstrating the construction of cones 

 and hyperboloids ; and, further, it clearly shows the 

 relations which these two kinds of figures bear to 

 one another. 



An indefinite number of images can be taken on 



Fig. 15.— Cylinder engendered by the 

 displacement of a white thread mov- 

 ing round a central axis. 



Fig. 16. — Hyperboloid by revolution: 

 a s-ingle web engendered by the 

 revolut'on of a thread stt obliquely 

 to the axis. 



the same plate. No sooner has one image been taken 

 than another can be superimposed ; the second im- 

 pression is just as good as the first. This method was 

 employed in the production of Fig. 17. After having 

 taken a photograph of a hyperboloid by revolution, 

 the dark slide was closed, and the thread arranged so 

 as to describe a cone ; the slide was then opened again, 

 and the image of the cone obtained. The two pictures 



