30 MOVEMENT 



surface left white ; when such a band rotates, a figure 

 such as Fig. 22 will be produced. Except for certain 

 breaches of continuity, due to the intermittent character 

 of the illumination, the surface thus produced will 

 resemble that of a solid sphere, which is illuminated 

 from the left and from above, while the opposite side 

 is only feebly lighted by reflection. This appearance 

 is easily explained. The strip of paper, as it travels 

 over successive meridians of this imaginary circle, is 

 placed under exactly the same conditions of illumina- 

 tion as would be the case if the meridians were those 

 of a real sphere in the same position. 



Paradoxical Effect produced by Certain Conditions of 

 Illumination. — Instead of the strip of paper used in the 

 preceding experiment, and from which light was only 

 reflected from the convex surface, let us take a strip 

 of similar board, only white on both surfaces. We shall 

 thus obtain a peculiar effect (Fig. 23) which can only be 

 understood when viewed under stereoscopic conditions. 



The inner and outer surfaces of this sphere can be 

 seen at one and the same time. This is because the 

 arc of Bristol board is white within as well as without, 

 and consequently reflects the light, sometimes from 

 one surface, sometimes from the other, according to 

 the position of rotation. When the convex arc faces 

 the light, i.e. is directed upwards and towards the left, 

 the corresponding portion of the engendered sphere 

 is clearly visible. When the arc is in an exactly 

 opposite phase of rotation, it receives the light on its 

 concave aspect ; that is to say, the interior of the 

 sphere below and on the left is the part illuminated. 



At first sight this figure appears to be transparent, 

 but on the one hand we know that it has been formed 

 by an opaque substance, and on the other that all 

 known transparent media reflect light in a totally 

 different manner. 



