330 ON LIGHT. 



screen illuminated by ligbt emanating from a single 

 bright point o (Fig. lo), from which is propagated a series 

 of equidistant spherical waves corresponding to light of 

 any one refrangibility, and therefore distant from each 

 other by one entire undulation of such light (say, to fix 

 our ideas, a 50,000th of an inch). If o p be joined, inter- 

 secting the surface of any one such wave, at a given 

 distance, oa from o in a ; pa will be the shortest line 

 that can be drawn from p to that surface. Suppose, 

 nov,', we take on either side of a a series of points b, b ; 

 c, c ; D, d^ &c., progressively more distant (by pairs) 

 from p than a is, by i, 2, 3, &c., hundred-thousandths 

 of an inch, or semi-undulations of the light under con- 

 sideration ; and let the whole figure be conceived as 

 turned round on o p as an axis. Then these points will 

 mark off on the spherical surface of the wave, a central 

 circular area (call it the area a), and a series of con- 

 centric rings or rather zones of the waves (call them in 

 succession b, c, d, (Sec), surrounding it, like those repre- 

 sented in Fisj. 7, from every point in each one of which 

 the light sent to p will reach it in more or less discord- 

 ance of phase, with that which reaches it from the next 

 in succession. Thus if all the vibrations propagated 

 from the central circle (a) arrive at P in a phase of com- 

 pression, all these siinultaneoiisly reaching it from the 

 zone (b) will arrive in a phase of expansion, all from 

 (c) again in one of compression, and so on alternately. 

 Now if the distance a p of p from the wave be anytlnng 

 considerable, suppose a few feet or even inches, it 

 will be enormously great in proportion to one semi- 



