face. The explanation of this deflection is that light travels more slowly in water than in air, Such deffection, or “refraction,” occurs also in the atmosphere, because light travels more slowly in the lower, denser strata of the asmosphere than in its higher layers. Now then, when a light ray ‘emitted by a star reaches the eyes of an observer, its path will show a more or less complicated curve, due to the different degrees of refraction in the various atmospheric layers (untess the star happens to be exactly in the zenith) . ‘This curve is fully determined by the following simple law: ‘Out of all the possible paths leading from the star to the eye of the observer, light will always follow the one which it can cover in the shortest time, allowance being made for the differences in its velocity in different atmospheric layers. Thus, the photons which constitute a ray of light behave like intelligent beings: Out of all the possible curves they always select the one which will take them most quickly to their goal. ‘This principle permits a large-scale generaliza- tion. According to all that we know about the laws relating to the processes taking place in any physical structure, we can characterize in all its