FLUORESCENCE. 



61 



on a screen if you placed a screen there to receive the rays. 

 This figure represents what is seen horizontally. The rays 

 enter the solution of quinine, and the red, orange, green, 

 and blue rays pass through it as through water, and 

 come out on the other side, and if you received them 

 on a screen, you would perceive the spectrum unmodified ; 

 but when you get to about the beginning of the violet, the 

 path of the rays within the fluid is marked by a beautiful 

 sky-blue light, which at first extends right across the 

 vessel, and then the extent falls off as you get towards the 

 end of the violet. But the light does not stop there. This 



FIG. l. 



blue light extends far beyond the violet into a region of the 

 spectrum which contains only invisible rays ; and if the 

 spectrum is pure you see not only light there, but inter- 

 ruptions which are of the same nature as the Fraunhofer 

 lines which are seen in the visible spectrum. On a screen 

 of paper placed vertically to receive the light, the Fraun- 

 hofer lines would be seen as distinct vertical lines when the 

 paper was in focus, and would be seen a little imperfectly 

 if the paper were a little out of focus. If you imagine the 

 paper placed in one position, and moved towards or from 

 the light, any particular dark line may be considered as 



