26 WONDER AXD 3IYSTEKY. 



rounded by an atmosphere so far transparent as not to 

 contain any substances capable of sensibly reflecting the 

 light, the light would go on diminishing in intensity as it 

 receded from the centre until it disappeared, and in that 

 sense the whole sphere would be illuminated that is, it 

 would befitted with a radiation of light; but in looking 

 toward it we should see nothing except the luminous point 

 in the centre, and we should not see that unless our eyes 

 were turned directly toward it. 



And now let us suppose that, instead of one luminous 

 point or candle flame, there were two, and that they were 

 placed at the distance of half a mile from each other; the 

 two illuminated spheres would then interpenetrate each 

 other, so to speak, to the extent of half their diameters. 

 The rays from A, proceeding in the direction toward B, 

 would encounter those of B coming toward A. The en- 

 counter would be direct on the line joining the two central 

 points, and in all other parts it would be indirect, and the 

 crossing would be at various angles. 



LT.CMINATEB SPHEEE8. 



In those parts of the space common to both spheres 

 which are equidistant from the two centres, the two ra- 

 diances encountering each other would be equal. In those 

 parts which were nearer to one than to the other, the light 



