182 FRAUNHOFER'S LINES. [SKCT. xix. 



planets the black bands are always found to be in the same 

 parts of the spectrum, and under all circumstances to main- 

 tain the same relative positions, breadths, and intensities. 

 Similar dark lines are also seen in the light of the stars, in 

 the electric light, and in the flame of combustible substances, 

 though differently arranged, each star and each flame having 

 a system of dark lines peculiar to itself, which remains the 

 same under every circumstance. Dr. Wollaston and M. 

 Fraunhofer of Munich discovered these lines deficient of 

 rays independently of each other. M. Fraunhofer found 

 that their number extends to nearly six hundred. There 

 are bright lines in the solar spectrum which also maintain 

 a fixed position. Among the dark lines, M. Fraunhofer 

 selected seven of the most remarkable, and determined their 

 distances so accurately, that they now form standard and 

 invariable points of reference for measuring the refractive 

 powers of different media on the rays of light, which renders 

 this department of optics as exact as any of the physical 

 sciences. These lines are designated by the letters of the 

 alphabet, beginning with B, which is in the red near the end 

 of the spectrum ; c is farther advanced in the red ; D is in 

 the orange ; E, in the green ; F, in the blue ; a, in the indigo ; 

 and H, in the violet. By means of these fixed points, M. 

 Fraunhofer has ascertained from prismatic observation the 

 refrangibility of seven of the principal rays in each of ten 

 different substances solid and liquid. The refraction in- 

 creased in all from the red to the violet end of the spectrum ; 

 but so irregularly for each ray and in each medium, that no 

 law could be discovered. The rays that are wanting in the 

 solar spectrum, which occasion the dark lines, were supposed 

 to be absorbed by the atmosphere of the sun. If they were 

 absorbed by the earth's atmosphere, the very same rays 

 would be wanting in the spectra from the light of the fixed 

 stars, which is not the case ; for it has already been stated 

 that the position of the dark lines is not the same in spectra 

 from star-light and from the light of the sun. The solar 



