srACE 25 



of threads between metal armatures, This is the 

 method : — 



A vertical metal rod furnished with two transverse 

 arms, Avhieh are exactly opposite to one another, is 

 allowed to rotate in front of a dark screen. This 

 metal frame must be blackened with the smoke of a 

 candle, and thus rendered as nearly invisible as 

 possible. The two cross-bars must be of equal length, 

 and their free extremities connected by a white thread, 

 which is stretched vertically between them. This 

 thread, by means of the rotatory motion which is 

 imparted to the two cross-bars, moves in a circular 

 manner round the axis, and describes in space the 

 circular outline of a cylinder. 



A photographic camera directed towards this thread, 

 and kept permanently open, would receive an image, 

 which would be the projection of a cylinder on a plane 

 surface. But this continuous trajectory would not 

 show its method of production with sufficient clear- 

 ness. In order that one may see that such a picture 

 results entirely from the movement of the thread, 

 which at every moment adds a new component- 

 element to the surface of the cylinder, separate images, 

 at successive intervals of time, must be obtained. 

 That is to say, the light must be admitted in an inter- 

 mittent manner. Fig. 15 was thus obtained.* 



If the thread, instead of lying parallel to the axis 

 round which it rotates, is directed obliquely towards 

 it, the figure described will be a hyperboloid by 

 revolution (Fig. 16). And, finally, if the thread is 



* In this diagram the central axis appe <rs white, because, although 

 the amount of light reflected from such a blackened surface is ex- 

 tremely feeble, nevertheless, this light is always reflected on to the 

 plate every time the objective is uncovered. 



Now, since the axis remains permanently in one place, each 

 separate impression, however feebl", is superimpos d on the same 

 part of the s^nsitizt d plate, and ends by being clearly defined. 



