FLUORESCENCE. 



63 



of examining the ref rangibilit j of the light which is emitted 

 in this manner. This part is supposed to represent what 

 would be seen if you place a screen to receive the incident 

 rays, but which is not seen because you do not have a screen 

 there. Take a small portion of the spectrum, and condense 

 it further with a very small lens fixed in a blackened box, 

 so as to get a very condensed beam. This is supposed to 

 be a vessel containing one of these fluids, and the appearance 

 you get is this, but so long as you are in the visible spec- 

 trum there is, generally speaking, an image of the double 

 cone due to the light rejected from motes, which it is 

 practically impossible to get rid of ; that is followed at a 

 certain interval by a beam extending over a greater or less 

 width of the spectrum, and which is heterogeneous, that is 



FIG 2. 



to say contains lights of various degrees of refrangibility. 

 And if you analyse the light by a ISTicol's prism or double- 

 image prism, you will see that this speckled image is almost 

 wholly polarised in the plane of reflection, indicating that 

 it is merely due to reflected light, whereas the other, in the 

 case of a solution, is wholly unpolarised. 



The phenomenon being thus explained, so far as to make 

 out the immediate nature of it, it was to be expected that 

 something of the same kind would be observed in other 

 instances of what Sir David Brewster called internal 

 dispersion, but I may mention that under that term he 

 classed together two phenomena which in reality are utterly 

 different as to their nature. In certain cases you get 

 what is virtually a powder in fine suspension, so fine that 



