3i8 OPTICAL PROJECTION 



the fluorescent rays, being now taken up in the cell, no longer 

 have the power to excite fluorescence on the screen. This is 

 further illustrated by the fact, that in all tank experiments the 

 fluorescent glow rapidly becomes leebler as the light passes 

 deeper into the fluid, and that in any of the following experi- 

 ments, the fluorescence may be stopped, or nearly so (de- 

 pending upon there being sufficient absorption) by interposing 

 a tank of the same substance in solution. But it is not so 

 with different substances. Thus, if we have a tank of quinine 

 solution, with a fluorescent cone of light traversing it, we 

 stop this fluorescence by interposing another tank of the 

 quinine, which, in becoming fluorescent, robs the beam of the 

 power to similarly affect the second tank. But if we inter- 

 pose a cell of uranine solution, its brilliant green fluorescence 

 does not do so, but the quinine cell is fluorescent as before. 



Amongst substances which fluoresce well in the electric 

 light or magnesium, are turmeric in castor-oil (green), tinc- 

 ture of stramonium, fustic steeped in alum solution, camwood 

 steeped in castor-oil (this oil, though not fluorescent itself, 

 seems able to excite the property in several other substances) 

 and sesculin. Of the latter a grain in powder should be 

 shaken into a confectioner's jar filled with slightly ammoniated 

 water, placed in the lantern beam : beautiful blue fluorescence 

 will be seen by reflected light. Even a few bruised fragments 

 of horse-chestnut bark thrown on the water will do the same. 



There are plenty of substances which fluoresce magnifi- 

 cently by the ordinary lime-light, however. Even quinine 

 will show fairly in a tank, with a good jet. A solution of 

 nettle or any other green leaves (chlorophyll) in ether, 

 methylated spirit, or benzol, which is green, fluoresces blood- 

 red. A cube of greenish-yellow uranium glass fluoresces a 

 bright and lovely green. Fluorescein, or any of its deriva- 

 tives, such as eosin, or uranine, fluoresces brilliantly in even 

 gas-light, and it is a beautiful experiment to scatter as much 

 as is ta.ken on the very end of a penknife, on the surface of a 



