424 



ON THE THEORY OF COMPOUND COLOURS. 



Now observe the position of two of Fraunhofer's fixed lines with respect to 

 the dark bands, and let n,, n, be their positions expressed in the number of 

 bands, whole or fractional, reckoning from zero. Let X,, X, be the wave-lengths 

 of these fixed lines as determined by Fraunhofer, then 



R = (N+n l )\ = (N+n t )^ ) (7); 



whence ^^^i^^^^-n, (8), 



A! A, Aj A, 



and . J R = ^X 1 A S (9). 



Having thus found N and R, we may find the wave-length corresponding to 

 the dark band n from the formula 



In my experiments the line D corresponded with the seventh dark band, and 

 F was between the 15th and 16th, so that n,= 157. Here then for D, 



n, = 7, X, = 2175l . ,, , , . . 



t P r, 'in rraunhoiers measure (H), 



and for F, w,= 157, X,= 1794J 



whence we find N=U, 72=89175 (12). 



There were 22 bands visible, corresponding to 22 different positions on the 

 scale AS, as determined 4th August, 1859. 



n = 



Band. Scale. 

 17 

 19 

 21* 



2 

 3 

 4 

 6 

 6 

 7 

 8 



26 

 28} 

 31 

 33} 



TABLE I. 



Band. Scale. 

 n= 9 36 

 10 39 

 42 

 45 

 48 



11 

 12 

 13 

 14 

 15 



51 



54 



Band. Scale. 



n= 16 

 17 

 18 

 19 

 20 

 21 

 22 



57 

 61 

 65 

 69 

 73 

 77 

 82 



Sixteen equidistant points on the scale were chosen for standard colours 

 in the experiments to be described. The following Table gives the reading on 

 the scale AB, the value of N+n, and the calculated wave-length for each of 

 these: 



