656 



Mb. stokes, on THE FORMATION OF THE CENTRAL SPOT 



The column for i^y may be continued with sufficient accuracy, by taking / to vary inversely as 

 the square of the number in the first column. 



27. I have seen the ring very distinctly by viewing the light transmitted at an angle of inci- 

 dence a little greater than the critical angle. In what follows, in speaking of angles of position, I 

 shall consider those positive which are measured in the direction of motion of the hands of a watch, 

 to a person looking at the light. The plane of incidence being about 45" to the positive side of the 

 plane of primitive polarization, the appearance presented as the analyzer, (a NicoFs prism,) was 

 turned, in the positive direction, through the position in which the light from the centre was extin- 

 guished, was as follows. On approacliing that position, in addition to the general darkening of the 

 spot, a dark ring was observed to separate itself from the dark field about the spot, and to move 

 towards the centre, where it formed a broad dark patch, surrounded by a rather faint ring of light. 

 On continuing to turn, the ring got brighter, and the central patch ceased to be quite black. The 



