562 Mb potter, on A NEW CORRECTION 



11.5 . , 



y, = -^ inches, 



= .47916 feet, 

 /, = .03125, 

 ^, = .083, 

 5m' = .03 nearly, from Sir D. Brewster's tables. 



With these we obtain, by substituting in the formula before inves- 

 tigated, as follows : 



5(Z)-Z)')=y, . 5m' {a . 0009555 + t . 00006055} 

 = .000000501759375. 



To find the diameter of the least circle of dispersion. 



Let RiMs be as in the last figure, (see Fig. 5.) qB,q' the angle of 



residual dispersion 



= 5(Z>-Z)'); 



then ab, the diameter of the least circle into which all the coloured rays 

 are collected, 



'^ qMi 



,.,,,., „ , , , qHs ■ sin qE^q' 



and in the triangle qB^q, we have qq = — . „ ,%,, 



Sill JX/ntl jlrjf n 



q'R. 



.-. uh = qR, x5(Z)-Z)') 



= focal length X 5(JD-Z)'); 

 and tlnis, in our example, the diameter of circle of residual dispersion 



= .000009533 of a foot. 



The angle which this subtends at the object-glass is 1{D-D')\ and, 

 measured in seconds of a degree, becomes 



_ .0000005017 

 ~ .0000048481 



= 0".103 nearly. 



