as an experimentum 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. 
Th is conclusion is also borne out by the phaenoinena 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 
effect 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. 
Fig. 2. 
same letters refer to the same parts : c, is a cylinder of 
wood about three inches in diameter, and seven inches in 
length from a to with pivots as in the figures. In fio-. 2 
this cylinder is seen placed in a rectangular aperture cutm a 
thick piece of board in which it exactly fits, and is retained in 
