185 



ordinary circumstances. If though the rays of light be 

 intercepted by a solution of sulphate of quinine or of horse 

 chestnut bark, or by a crystal of fluor spar, these extra 

 spectral rays are rendered apparent. These rays, which 

 were unknown to Newton, have been investigated by Prof. 

 Stokes, who has named them the fluorescent rays. 



They are luminous probably under all circumstances to 

 those animals whose eyes are adjusted, as are the eyes of 

 most of the night roaming creatures, to admit the rays of 

 the highest refrangibility and to vibrate in unison with their 

 vibrations ; but unless peculiar conditions be established, 

 the fluorescent rays are not sensible to the human eye. 



Such, then, is the amount of our knowledge respecting 

 the luminous principle of the sunbeam. 



It must be remembered that these rays vary considerably 

 in the intensity of their illuminating power. The maxi- 

 mum exists in the yellow ray, and it diminishes as we 

 recede from it towards either end of the spectrum. 



The least refrangible, or the red rays, give a modified 

 amount of light, but the maximum of heat exists in them. 

 The most refrangible or the blue end of the spectrum is 

 less luminous, but the maximum of chemical action is 

 fixed at this extremity. The fluorescent rays beyond the 

 spectrum of Newton, being only visible under the peculiar 

 circumstances already mentioned. 



If now we examine these beautifully colored bands of 

 light when well defined upon a screen, with a small tele- 

 scope, a new set of phenomena will become apparent. The 

 spectrum is then seen crossed by a number of black lines. 

 Every ray, even the most brilliant, will be found to have 

 spaces in which there is an entire absence of light. 



It was Dr. Wollaston who first observed these non-lumin- 

 ous spaces in the prismatic spectrum. Fraunhofer, how- 

 ever, was the first to make a full investigation of these lines 

 and to publish a map of them, and they have hence generally 

 been called Frauuhofer's lines. 



