PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 465 



also easy to explain by the undulatory theory the coloration of the fringes 

 when ordinary light is used in this experiment ; the rays of different 

 colours being produced by luminous undulations of different lengths, 

 the points of coincidence and of opposition are more or less separated 



according to the length of the undulations. Thus if the distances 

 between the circles in the diagram (Fig. 208) be taken as correspond- 

 ing to the wave-lengths for violet light, other series of circles more 

 widely separated will represent red light, and the hyperbolas F/ would 

 be found more widely separated. 



The actual shadow of a body projected by a luminous point extends 

 beyond the " geometrical shadow," that is, beyond the space which is 

 bounded by lines drawn through the point tangential to the body. 

 From this Fresnel concluded that the- secondary waves arising at the 

 margins of the body were retarded by half a wave-length, because if 

 they started with the same phase as the original waves, there would be 

 a perfect agreement between the phases of the direct and of those of 

 the secondary undulations in the tangent plane, which, on the contrary, 



SO 



