SECT. ii. RAYLESS SPACES IN SOLAR SPECTRUM. 59 



is crossed at right angles to its length by innumerable 

 dark rayless lines, differing in breadth and intensity. 

 Sir John Herschel discovered vacant spaces in the 

 extra-luminous part of the heat spectrum, and more 

 recently M. Edouard Becquerel, by throwing the solar 

 spectrum upon a daguerreotype plate, discovered that 

 the chemical spectrum given by a glass prism, from its 

 beginning in the yellow to its extreme point beyond the 

 violet, is crossed by rayless lines, and that the lines in 

 the part passing through the visible spectrum coincide 

 exactly with the rayless lines in the luminous part. 

 This coincidence was confirmed by the independent 

 researches of Dr. Draper at New York. By means of 

 the rayless spaces or black lines in the visible spectrum, 

 M. Kirchhoff has proved that thirteen terrestrial sub- 

 stances are constituents of the sun's atmosphere. 



The length of the undulations of the ether which 

 produce the impression of the extreme violet rays of 

 the solar spectrum on our eyes, is the seventeen millionth 

 part of an inch ; the length of the ethereal undulation 

 that produces the sensation of the extreme red is the 

 twenty-six millionth part of an inch ; the ethereal 

 undulations beyond these limits are invisible to -human 

 eyes. Nevertheless certain substances have the power 

 of increasing the length of the vibrations, and reducing 

 the rays of the spectrum to a lower grade in the scale 

 of refrangibility, so that the invisible rays of the 

 chemical spectrum have thus been brought within the 

 limit of human vision. 



For example, the chemical rays shine as visible light 

 when they fall on glass tinged with the oxide of uranium. 

 When these dark rays fall upon the glass, they put the 

 whole of its molecules into vibrations, the same with 

 their own, while at the same time they give a moro 

 sapid vibration to a certain number of the same mole- 

 cules. The whole of the molecules restore their vibra- 



