SECT, xix.] THE SOLAR SPECTRUM. 181 



point of the spectrum above what is necessary to form white 

 light, such white light will appear at that point as never mor- 

 tal eye looked upon before this experiment, since it possesses 

 the remarkable property of remaining the same after any 

 number of refractions, and of being capable of decomposition 

 by absorption alone. 



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. 



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 

 irregularly 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 ordinary circumstances. The best method is 

 to receive the spectrum on the object-glass of a telescope, so as 

 to magnify them sufficiently 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 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 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 



