FLUORESCENCE. 



BY PROFESSOR STOKES. 



LECTURE II. 



THE subject which I am about to bring before you to-day 

 is one which has attracted a great deal of attention for some 

 years back, and in which 1 have myself had a considerable 

 share. One of the first phenomena discovered in connec- 

 tion with those I have to bring before you was that which Sir 

 David Brewster called the internal dispersion of light, which 

 he first noticed in an alcoholic solution of the green colouring 

 matter of leaves, as mentioned in a paper read before the 

 Pvoyal Society of Edinburgh in 1833, 1 and fully described 

 in a later paper read before the same Society in 1846. 2 I 

 have here a solution of the green colouring matter of leaves 

 which I used in my lecture on absorption, but which I am 

 now going to use for a different purpose. Brewster had 

 occasion to pass a beam of sunlight through this green fluid, 

 and he was surprised to observe the whole of the path of 

 the beam exhibiting a blood-red light. The figure which I 

 have here^ is intended to represent what could be seen, and 

 I will endeavour to show it presently. This represents a 

 vessel filled with the green fluid and placed on a white ground 

 such as paper, with a glass bottom so that you can see the light 

 through. In looking through you see the green colour of 

 the solution, and there is supposed to be a board standing 

 vertically on its edge containing a lens. The sun's light is 

 reflected horizontally and sent through that lens so as to 

 form a condensed beam, the focus of which lies within the 



1 Edinburgh Transactions, xii. 541. 



2 Ibid. xvi. Ill ; or Phil. Mag. for June, 1848. 



3 The figures referred to in this lecture are not reproduced, except 

 in two cases, as they are mostly coloured, and would lose much of 

 their significance if merely represented by black and white. 



