FLUORESCENCE. 69 



most cases answer the purpose sufficiently well. Suppose 

 then you have a dark glass on a window-shutter and you 

 have in front of it a substance to be examined, and it gives 

 out this beautiful phosphorescent light. In general this 

 may be at once distinguished* from mere scattered light ; 

 but to make sure of it use your yellow glass, and place 

 it between the blue and the substance you are examining. 

 If it is well chosen it will cut off almost all the effect. 

 Then place it between your eye and tjie medium which 

 is shining with this phosphorescent light and you will 

 see it quite plainly. The difference of effect with this 

 additional glass in the two positions proves that you 

 have really to deal with this peculiar phenomenon, and 

 enables you to at once distinguish it from some appearances 

 which at the first glance resemble it very much. If you 

 put a minute quantity of a solution of proto- chloride of tin 

 into a large quantity of common water, the mixture will 

 have a bluish look by reflected light, and if you condense 

 sunlight upon it you will get a beam somewhat like what 

 you do when you receive a beam on the solution of sulphate 

 of quinine. This however is merely scattered light ; and 

 that it is so is shown at once when you come to make 

 experiments upon it in which you strain the incident light. 

 For example, if you place the mixture inside your darkened 

 chamber in lieu of the solution of quinine, the difference 

 will appear in a moment. The mixture will merely give 

 out a little light of a deep blue colour, which is scattered 

 light, whereas the solution of quinine will be lighted up 

 with this beautiful light that you see. I may mention a 

 very simple and pretty experiment which can be made in 

 that way. Take a bit of common horse-chestnut bark, 

 float it in a glass of still water in which a drop of ammonia 

 had been mixed ; the peculiar substances contained in 

 the bark will begin to be dissolved, the solution will descend, 

 and you will see streams of descending blue light. If you 

 can obtain specimens of these substances esculin and 

 fraxin, a minute quantity of the two thrown together on 

 the water instead of the bark looks very pretty. The 

 substances will form little luminous specks here and there 

 on the surface, which will give rise to descending streams 

 of blue and greenish light. 



Now there is another way of testing the change of 



