SECT. XIX. FRAUNHOFER'S DARK LINES. 163 



M. Fraunhofer found that their number extends to nearly six 

 hundred, but they are much more numerous. 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 ; G 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 increased in all from the red to the violet 

 end of the spectrum. 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. But the annular eclipse 

 which happened on the 15th of May, 1836, afforded Professor 

 Forbes the means of proving that the dark lines in question cannot 

 be attributed to the absorption of the solar atmosphere ; they were 

 neither broader nor more numerous in the spectrum formed during 

 that phenomenon than at any other time, though the rays came 

 only from the circumference of the sun's disc, and consequently 

 had to traverse a greater depth of his atmosphere. 



Sir David Brewster found that in certain states of the atmo- 

 sphere the obscure lines become much broader, and some of 

 them deeply black ; and he observed also, that, at the time the 

 sun was setting in a veil of red light, part of the luminous 

 spectrum was absorbed, whence he concluded that the earth's 

 atmosphere had absorbed the rays of light which occupied the 

 dark bands. By direct experiments also the atmosphere was 

 observed to act powerfully upon the rayless lines. 



When a lens is used along with a prism, longitudinal dark 

 lines of different breadths are seen to cross the bands, already 

 described, at right angles ; these M. Kagona-Scina and M. 

 Babinet believe to be lines of interference which exist in light 

 that has passed through a convex lens. 



The lines are different both in kind and number in the spectra 



