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MATHEMATICS AND PHYSICS. 



On a new case of Interference of Light. By Prof. Powell, F.R.S. 



The author observed, that when a pvisra of one substance was op- 

 posed to another slightly differing in dispersive power, (as plate-glass 

 and oil of sassafras,) so as to produce a partial achromatism, in the co- 

 loured edges which appeared on either side of the white image of a nar- 

 row line of light, when viewed through a small telescope, there were 

 formed dark hands, about four or five being visible on each edge, pa- 

 rallel to the line of light. 



The explanation is easy, when we consider that of the pencil of each 

 primary ray which enters the eye, (in breadth equal to the aperture of 

 the pupil,) the rays which have traversed greater thicknesses of the 

 first prism, pass through less thicknesses of the second ; and thus have 

 their retardations so compensated as to be in a condition to interfere 

 and produce the dark bands observed. 



On the explanation of some Optical Phcenomena observed hy Sir David 

 Brewster. By Prof. Powell, F.R.S. 



The experiments of Sir D. Brewster here referred to were those stated 

 at two preceding meetings of the Association, in which, on looking at 

 the spectrum, partly through, and partly over, the edge of a plate of 

 any transparent substance, it is seen marked by numerous dark bands. 

 If a plate of mica or selenite be cut with an edge sloping at an angle, 



VOL. VIII. 1839. B 



