as an experimentmn crucis as to the nature of Light. 385 



From the full investigation, I thus maintain that the normal 

 result of this fundamental experiment in interferences, is, that 

 the central band is black, and at variance with the conse- 

 quences of all wave theories of light, and therefore the undu- 

 latory theory of light is not the physical theory. 



This conclusion is also borne out by the phaenomena of 

 the rainbow, (see Lon, and Edin. Phil. Mag. for July 1838, vol. 

 xiii. p. 9.) the actual primary and secondary bows being much 

 more distant than they should be from all calculations accord- 

 ing to the undulatory theory of light, the position calculated by 

 that theory to be bright being actually found to be dark. In 

 the artificial methods of producing the analogous effect with 

 a minute stream of water,, there is no doubt but that the 

 elfect calculated from the undulatory theory might be ob- 

 tained, by using a line of light in a previous state of inter- 

 ference. 



The apparatus is represented in figs. 1 and 2, in which the 



Fig. 1. 



same letters refer to the same parts : a, h, c, ^ is a cylinder of 

 wood about three inches in diameter, and seven inches in 

 length from a to 5, with pivots as in the figures. In fig. 2 

 this cylinder is seen placed in a rectangular aperture cut in a 

 thick piece of board in which it exactly fits, and is retained in 



