of viewing Newton's Rings. 



249 



the two plates. If there are any of these, they cause white 

 specks in the black area, and in its reflexion in B, and will 

 obviously make it easy for the eye to follow the black conti- 

 nuations of the primaries across the reflexion of the spot. 

 The writer is of opinion that the interference is not complete. 

 It seems worth notice that when the plates are very clean the 

 darkest area of the black spot has a sharply defined edge, 

 recalling the character of the black film of a soap-bubble. If 

 there is a species of welding together of the molecules of glass 

 at the black spot, with molecules of the gases of air mecha- 

 nically entangled, the blackness would be explained, since the 

 light can pass through without leaving the glass medium; 

 and, on the other hand, the entangled air being, as it were, 

 pulverized, would reflect irregularly. The fact that the edge 

 of the spot is very sharply defined shows that it is not only the 

 effect of the difference of wave-length. As the black spot is 

 approached in the plane of the rings, the light does fall off 

 gradually up to a certain point, at which the shade is a dark 

 grey, but then passes "per saltum" to the velvety black 



before alluded to. 



Fijr. 6. 



To explain how it is the continuations of the primaries 

 are seen in B (vide fig. 6), see fig. 7, where Im is one 



