44 DISPERSION. 



light in their position, &c. ; so that each species of flame, and 

 the light of each fixed star, has its own system of bands, 

 which remains unaltered under all circumstances, and 

 which, therefore, is a distinct physical characteristic of the 

 species of light to which it belongs. Thus the light of the 

 electric spark has bright bands, instead of dark ones. The 

 flames of oil, hydrogen, and alcohol, have each a brilliant line 

 between the red and the yellow. The red flames coloured by 

 nitrate of strontian exhibit a brilliant blue line, which is de- 

 tached from the rest of the spectrum ; and the salts of potash 

 give rise to a remarkable red ray, beyond the limits of the 

 ordinary red of the spectrum, and separated from it by a dark 

 interval. On the other hand, the spectrum of the flame of 

 cyanogen exhibits great regularity, as well in the distribution 

 of the dark bands, as in the intensity of the intervening 

 luminous spaces. 



These bands depend on the rapidity of the combustion. 

 Thus sulphur, when burning slowly, exhibits blue and green 

 bands in the spectrum ; in rapid combustion, its light is 

 nearly homogeneous. 



(54) These fixed lines, as they are called, were first 

 noticed by Wollaston, in the year 1802. They have since 

 been much more fully examined by Fraunhofer, who distin- 

 guished 590 in the solar spectrum, of which he has delineated 

 354. Of these he has selected seven principal ones, to serve 

 as standards of comparison, and has designated them by the 

 letters B, C, D, E, F, G and H. Of these, B and C are 

 single lines in the red portion of the spectrum, the former 

 near to its extremity ; D is a double line, at the confines of 

 the orange and yellow ; E is a group of fine lines in the 

 green ; F is a strongly marked black line in the blue ; Gr is a 

 group of fine lines in the indigo ; and H is a similar group in 

 the violet, clustered round one much stronger line. They 



