32 



MOVEMENT 



Fig. 24. — Paradoxic*) 1 appearance of a 

 sphere engendered by tlie rotation of 

 a brilliant metallic thread. 



Fitr. 24 was obtained by allowing a semicircular arc of 

 polished brass to rotate round a vertical axis. There 

 is for each position of rotation only one particular 



spot on the polished 

 metal arc from which the 

 incidental rays of the sun 

 can be reflected into 

 the body of the camera. 

 Now, as this bright spot 

 changes its position, it 

 is sometimes on the con- 

 vex side and sometimes 

 on the concave side of 

 the metal arc. It is the 

 displacement of this lumi- 

 nous spot which traces the 

 complete rings on the opposite portions of the sphere. 

 In order more clearly to understand the causation of this 

 peculiar appearance, we must have recourse to stereo- 

 scopic figures, with an intermittent series of images. 

 Such figures show that in each position certain portions 

 of the surface of the semicircular arc are dark, that 

 is to say, do not transmit light in the direction of the 

 photographic apparatus; on the contrary, other points 

 are brilliantly illuminated, because in this position 

 they are " set," so to speak, so as to reflect the sun's 

 rays. These curious effects can never be caused by a 

 real body. The form and position of the illuminated 

 spots on the sphere can be varied at will by changing 

 the direction of incidence of the luminous rays. 



The mathematical study of these diverse effects 

 would be, perhaps, rather complicated ; in any case, it 

 would afford a very limited amount of interest. It 

 was necessary, however, to mention them, because in 

 the course of our studies, we shall meet with analogous 

 forms, produced by the movement of certain bodies. 



