64 LECTURES TO SCIENCE TEACHERS. 



it does not subside in any reasonable length of time, and 

 gives you what looks like a pretty bright solution, but 

 which really contains suspended matter. In such cases if 

 you introduce a beam of condensed sunlight, the path of the 

 beam is marked by light, because these motes reflect the 

 light which falls upon them, but that light is reflected and 

 is polarised by reflection, and this origin of the light 

 is known by its polarisation. Moreover, being simply 

 reflected it sends back the light which falls upon it un- 

 changed in kind, whereas the truly dispersed light differs 

 altogether in its nature from the light which falls upon the 

 solution, glass, or crystal that shows it. 



Now as I say the phenomenon being referred to the 

 cause just explained in the one case, you may expect that 

 in other cases something similar would be perceived, and 

 I will take now the green solution obtained from leaves. 

 I will suppose the experiment exactly the same, but the 

 result is different in appearance although the nature of the 

 phenomenon is the same. The path of the rays within the 

 green fluid is marked by a blood-red light, which in 

 different parts of the spectrum penetrates to a greater 

 or less distance into the fluid. In this case the phenomenon 

 begins very near the red end of the spectrum, somewhere 

 about the line B of Fraunhofer. You first have a dart 

 of red light extending right across the vessel. Then 

 you come to a region of the spectrum for which the fluid 

 is excessively opaque, and the red light, which is produced 

 by the action of the incident light on the fluid in some 

 way or other, cannot therefore extend very far into the fluid. 

 Then you come to a region where the fluid although still 

 opaque is comparatively transparent, and the red is trace- 

 able further inwards, and so on. It is noticed, however, 

 after close observation, that where the incident rays are 

 quickly used up the red light is very copious, and where 

 they are more slowly used up the red light is not so 

 strong. 



In the figure some of the red bands are slightly shaded 

 in the middle, to indicate that the red light is not so strong 

 there as elsewhere. In the case of this fluid the effect is 

 produced mainly by the visible spectrum, but it extends 

 beyond that into the invisible region beyond the violet ; 

 so that the solution of quinine, and the alcoholic solution 



