122 DIFFRACTION. 



homogeneous light are so pure and unmixed in the spectra, 

 that the fixed lines may be discerned. These lines, then, are 

 wholly independent of refraction, and exist in the parts of 

 the solar beam before they are separated by the prism. The 

 phenomenon, when thus exhibited, is however distinguished 

 by a remarkable peculiarity. The distances of the fixed lines 

 in the diffracted spectrum are always proportional, whatever 

 be the diffracting substance ; while the ratios of the intervals 

 of the fixed lines (or of the breadths of the coloured spaces), 

 in the spectra formed by refraction, vary with the dispersive 

 powers of the prisms. In fact, the angle 9 being small, we 

 may make sin = 6 sin V ; so that 



*.Ti 



= 



sin 1' 



Hence, if 0i, 2 , 3 , denote the deviations corresponding to 

 any three definite points in the spectrum, and Ai, A 2 , A 3 , the 

 corresponding wave-lengths, it follows that 



02 0i A 2 Ai 



9 3 Oi X 3 - AI ' 



or the intervals of the fixed lines of the spectrum are as the 

 differences of the corresponding wave-lengths, and are there- 

 fore in an invariable ratio. The difference in the disposition 

 of the fixed lines, in the spectra formed by diffraction and by 

 refraction, will be seen in the diagrams of Art. (136), in 

 which the points B, C, D, &c., of the horizontal line BH, 

 represent the relative positions of the principal fixed lines, in 

 the spectrum formed by a prism of flint-glass, and in the 

 diffracted spectrum, respectively. 



(135) The formula of Art. (133) suggests a very simple 

 method of determining the length of the wave corresponding 

 to any given ray of the spectrum. The value of f, or the in- 



