162 NEW COLOUES. SECT. XIX. 



the spectrum ; however, there appears to be still a doubt as to 

 the real character of the phenomena presented by certain absorbing 

 substances. 



In addition to the seven colours of the Newtonian spectrum, 

 Sir John Herschel has discovered a set of very dark red rays 

 beyond the red extremity of the spectrum which can only be seen 

 when the eye is defended from the glare of the other colours by a 

 dark blue cobalt glass. He has also found that beyond the 

 extreme violet there are visible rays of a lavender gray colour, 

 which may be seen by throwing the spectrum on a sheet of paper 

 moistened by the carbonate of soda. The illuminating power of 

 the different rays of the spectrum varies with the colour. The 

 most intense light is in the mean yellow ray, or, according to M. 

 Fraunhofer, at the boundary of the orange and yellow. 



When the prism is very perfect and the sunbeam small, so 

 that the spectrum may be received on a sheet of white paper in 

 its utmost state of purity, it presents the appearance of a riband 

 shaded with all the prismatic colours, having its breadth irregu- 

 larly striped or subdivided by an indefinite number of dark, and 

 sometimes black lines. The greater number of these rayless lines 

 are so extremely narrow that it is impossible to see them in ordi- 

 nary circumstances. The best method is to receive the spectrum 

 on the object-glass of a telescope, so as to magnify them suffi- 

 ciently to render them 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 ex- 

 cellent 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 blue extremity are most obvious in 

 the middle of the day. When 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 circumstances 

 to maintain the same relative positions. 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. Dr. Wollaston and M. Fraunhofer, of Munich, dis- 

 covered these lines deficient of rays independently of each other. 



