SECT. XIX. FRAUNHOFER'S LINES. 157 



visible. This experiment may also be made, but in an 

 imperfect manner, by viewing a narrow slit between two 

 nearly closed window-shutters through a very excellent 

 glass prism held close to the eye, with its refracting 

 angle parallel to the line of light. The rayless lines in 

 the red portion of the spectrum become most visible as 

 the sun approaches the horizon, while those in the blu 

 extremity are most obvious in the middle of the day. 

 AVhen the spectrum is formed by the sun's rays, either 

 direct or indirect as from the sky, clouds, rainbow, moon, 

 or planets the black bands are always found to be in 

 the same parts of the spectrum, and under all circum- 

 stances to maintain 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 inde- 

 pendently 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 stand- 

 ard 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 refrangi- 

 bility of seven of the principal rays in each often differ- 

 ent substances solid and liquid. The refraction increased 

 in all from the red ta the violet end of the spectrum ; 

 but so irregularly for each ray and in each medium, that 

 no law (ioukl be discovered. The rays that are wanting 

 in the solar spectrum which occasion the dark lines, 

 O 



